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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd">
<html><head><title>C++ Standard Library Active Issues List</title></head>
<body bgcolor="#ffffff" text="#000000">
<table>
<tbody><tr>
<td align="left">Doc. no.</td>
<td align="left">N1908=05-0168</td>
</tr>
<tr>
<td align="left">Date:</td>
<td align="left">2005-10-23</td>
</tr>
<tr>
<td align="left">Project:</td>
<td align="left">Programming Language C++</td>
</tr>
<tr>
<td align="left">Reply to:</td>
<td align="left">Howard Hinnant <howard.hinnant@gmail.com></td>
</tr>
</tbody></table>
<h1>C++ Standard Library Active Issues List (Revision R39)</h1>
<p>Reference ISO/IEC IS 14882:1998(E)</p>
<p>Also see:</p>
<ul>
<li>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-toc.html">Table of Contents</a> for all library issues.</li>
<li>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-index.html">Index by Section</a> for all library issues.</li>
<li>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-status.html">Index by Status</a> for all library issues.</li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html">Library Defect Reports List</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html">Library Closed Issues List</a></li>
</ul>
<p>The purpose of this document is to record the status of issues
which have come before the Library Working Group (LWG) of the ANSI
(J16) and ISO (WG21) C++ Standards Committee. Issues represent
potential defects in the ISO/IEC IS 14882:1998(E) document. Issues
are not to be used to request new features. </p>
<p>This document contains only library issues which are actively being
considered by the Library Working Group. That is, issues which have a
status of <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Ready">Ready</a>, and <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Review">Review</a>. See
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html">Library Defect Reports List</a> for issues considered defects and
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html">Library Closed Issues List</a> for issues considered closed.</p>
<p>The issues in these lists are not necessarily formal ISO Defect
Reports (DR's). While some issues will eventually be elevated to
official Defect Report status, other issues will be disposed of in
other ways. See <a href="#Status">Issue Status</a>.</p>
<p>This document is in an experimental format designed for both
viewing via a world-wide web browser and hard-copy printing. It
is available as an HTML file for browsing or PDF file for
printing.</p>
<p>Prior to Revision 14, library issues lists existed in two slightly
different versions; a Committee Version and a Public
Version. Beginning with Revision 14 the two versions were combined
into a single version.</p>
<p>This document includes <i>[bracketed italicized notes]</i> as a
reminder to the LWG of current progress on issues. Such notes are
strictly unofficial and should be read with caution as they may be
incomplete or incorrect. Be aware that LWG support for a particular
resolution can quickly change if new viewpoints or killer examples are
presented in subsequent discussions.</p>
<p>For the most current official version of this document see
<a href="http://www.open-std.org/jtc1/sc22/wg21/">http://www.open-std.org/jtc1/sc22/wg21/</a>.
Requests for further information about this document should include
the document number above, reference ISO/IEC 14882:1998(E), and be
submitted to Information Technology Industry Council (ITI), 1250 Eye
Street NW, Washington, DC 20005.</p>
<p>Public information as to how to obtain a copy of the C++ Standard,
join the standards committee, submit an issue, or comment on an issue
can be found in the comp.std.c++ FAQ.
Public discussion of C++ Standard related issues occurs on <a href="news://comp.std.c++/">news:comp.std.c++</a>.
</p>
<p>For committee members, files available on the committee's private
web site include the HTML version of the Standard itself. HTML
hyperlinks from this issues list to those files will only work for
committee members who have downloaded them into the same disk
directory as the issues list files. </p>
<h2>Revision History</h2>
<ul>
<li>R39:
2005-10-14 post-Mont Tremblant mailing.
Added new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#526">526</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#528">528</a>.
Moved issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#280">280</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#461">461</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#464">464</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#465">465</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#467">467</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#468">468</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#474">474</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#496">496</a> from Ready to WP as per the vote from Mont Tremblant.
Moved issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#247">247</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#294">294</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#342">342</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#362">362</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#369">369</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#371">371</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#376">376</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#384">384</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#475">475</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#478">478</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#495">495</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#497">497</a> from Review to Ready.
Moved issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#498">498</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#504">504</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#506">506</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#509">509</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#510">510</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#511">511</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#512">512</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#513">513</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#514">514</a> from New to Open.
Moved issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#505">505</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#507">507</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#508">508</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#519">519</a> from New to Ready.
Moved issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#500">500</a> from New to NAD.
Moved issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#518">518</a> from New to Review.
</li>
<li>R38:
2005-07-03 pre-Mont Tremblant mailing.
Merged open TR1 issues in <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#504">504</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#522">522</a>.
Added new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#523">523</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#523">523</a>
</li>
<li>R37:
2005-06 mid-term mailing.
Added new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#498">498</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#503">503</a>.
</li>
<li>R36:
2005-04 post-Lillehammer mailing. All issues in "ready" status except
for <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#454">454</a> were moved to "DR" status, and all issues
previously in "DR" status were moved to "WP".
</li>
<li>R35:
2005-03 pre-Lillehammer mailing.
</li>
<li>R34:
2005-01 mid-term mailing. Added new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#488">488</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#494">494</a>.
</li>
<li>R33:
2004-11 post-Redmond mailing. Reflects actions taken in Redmond.
</li>
<li>R32:
2004-09 pre-Redmond mailing: reflects new proposed resolutions and
new issues received after the 2004-07 mailing. Added
new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#479">479</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#481">481</a>.
</li>
<li>R31:
2004-07 mid-term mailing: reflects new proposed resolutions and
new issues received after the post-Sydney mailing. Added
new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#463">463</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#478">478</a>.
</li>
<li>R30:
Post-Sydney mailing: reflects decisions made at the Sydney meeting.
Voted all "Ready" issues from R29 into the working paper.
Added new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#460">460</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#462">462</a>.
</li>
<li>R29:
Pre-Sydney mailing. Added new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#441">441</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#457">457</a>.
</li>
<li>R28:
Post-Kona mailing: reflects decisions made at the Kona meeting.
Added new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#432">432</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#440">440</a>.
</li>
<li>R27:
Pre-Kona mailing. Added new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#404">404</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#431">431</a>.
</li>
<li>R26:
Post-Oxford mailing: reflects decisions made at the Oxford meeting.
All issues in Ready status were voted into DR status. All issues in
DR status were voted into WP status.
</li>
<li>R25:
Pre-Oxford mailing. Added new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#390">390</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#402">402</a>.
</li>
<li>R24:
Post-Santa Cruz mailing: reflects decisions made at the Santa Cruz
meeting. All Ready issues from R23 with the exception of <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#253">253</a>, which has been given a new proposed resolution, were
moved to DR status. Added new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#383">383</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#389">389</a>. (Issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#387">387</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#389">389</a> were discussed
at the meeting.) Made progress on issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#225">225</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#226">226</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#229">229</a>: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#225">225</a> and <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#229">229</a> have been moved to Ready status, and the only remaining
concerns with <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#226">226</a> involve wording.
</li>
<li>R23:
Pre-Santa Cruz mailing. Added new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#367">367</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#382">382</a>.
Moved issues in the TC to TC status.
</li>
<li>R22:
Post-Curaçao mailing. Added new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#362">362</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#366">366</a>.
</li>
<li>R21:
Pre-Curaçao mailing. Added new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#351">351</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#361">361</a>.
</li>
<li>R20:
Post-Redmond mailing; reflects actions taken in Redmond. Added
new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#336">336</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#350">350</a>, of which issues
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#347">347</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#350">350</a> were added since Redmond, hence
not discussed at the meeting.
All Ready issues were moved to DR status, with the exception of issues
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#284">284</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#241">241</a>, and <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#267">267</a>.
Noteworthy issues discussed at Redmond include
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#120">120</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#202">202</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#226">226</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#233">233</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#270">270</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#253">253</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#254">254</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#323">323</a>.
</li>
<li>R19:
Pre-Redmond mailing. Added new issues
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#323">323</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#335">335</a>.
</li>
<li>R18:
Post-Copenhagen mailing; reflects actions taken in Copenhagen.
Added new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#312">312</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#317">317</a>, and discussed
new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#271">271</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#314">314</a>.
Changed status of issues
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#103">103</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#118">118</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#136">136</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#153">153</a>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#165">165</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#171">171</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#183">183</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#184">184</a>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#185">185</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#186">186</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#214">214</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#221">221</a>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#234">234</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#237">237</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#243">243</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#248">248</a>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#251">251</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#252">252</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#256">256</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#260">260</a>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#261">261</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#262">262</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#263">263</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#265">265</a>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#268">268</a>
to DR.
Changed status of issues
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#49">49</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#109">109</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#117">117</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#182">182</a>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#228">228</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#230">230</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#232">232</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#235">235</a>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#238">238</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#241">241</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#242">242</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#250">250</a>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#259">259</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#264">264</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#266">266</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#267">267</a>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#271">271</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#272">272</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#273">273</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#275">275</a>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#281">281</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#284">284</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#285">285</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#286">286</a>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#288">288</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#292">292</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#295">295</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#297">297</a>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#298">298</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#301">301</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#303">303</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#306">306</a>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#307">307</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#308">308</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#312">312</a>
to Ready.
Closed issues
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#111">111</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#277">277</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#279">279</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#287">287</a>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#289">289</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#293">293</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#302">302</a> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#313">313</a>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#314">314</a>
as NAD.
</li>
<li>R17:
Pre-Copenhagen mailing. Converted issues list to XML. Added proposed
resolutions for issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#49">49</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#76">76</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#91">91</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#235">235</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#250">250</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#267">267</a>.
Added new issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#278">278</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#311">311</a>.
</li>
<li>R16:
post-Toronto mailing; reflects actions taken in Toronto. Added new
issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#265">265</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#277">277</a>. Changed status of issues
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#3">3</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#8">8</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#9">9</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#19">19</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#26">26</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#31">31</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#61">61</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#63">63</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#86">86</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#108">108</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#112">112</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#114">114</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#115">115</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#122">122</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#127">127</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#129">129</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#134">134</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#137">137</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#142">142</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#144">144</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#146">146</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#147">147</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#159">159</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#164">164</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#170">170</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#181">181</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#199">199</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#208">208</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#209">209</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#210">210</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#211">211</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#212">212</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#217">217</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#220">220</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#222">222</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#223">223</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#224">224</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#227">227</a> to "DR". Reopened issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#23">23</a>. Reopened
issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#187">187</a>. Changed issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#2">2</a> and
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#4">4</a> to NAD. Fixed a typo in issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#17">17</a>. Fixed
issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#70">70</a>: signature should be changed both places it
appears. Fixed issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#160">160</a>: previous version didn't fix
the bug in enough places.
</li>
<li>R15:
pre-Toronto mailing. Added issues
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#233">233</a>-<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#264">264</a>. Some small HTML formatting
changes so that we pass Weblint tests.
</li>
<li>R14:
post-Tokyo II mailing; reflects committee actions taken in
Tokyo. Added issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#228">228</a> to <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#232">232</a>. (00-0019R1/N1242)
</li>
<li>R13:
pre-Tokyo II updated: Added issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#212">212</a> to <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#227">227</a>.
</li>
<li>R12:
pre-Tokyo II mailing: Added issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#199">199</a> to
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#211">211</a>. Added "and paragraph 5" to the proposed resolution
of issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#29">29</a>. Add further rationale to issue
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#178">178</a>.
</li>
<li>R11:
post-Kona mailing: Updated to reflect LWG and full committee actions
in Kona (99-0048/N1224). Note changed resolution of issues
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#4">4</a> and <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#38">38</a>. Added issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#196">196</a>
to <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#198">198</a>. Closed issues list split into "defects" and
"closed" documents. Changed the proposed resolution of issue
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#4">4</a> to NAD, and changed the wording of proposed resolution
of issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#38">38</a>.
</li>
<li>R10:
pre-Kona updated. Added proposed resolutions <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#83">83</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#86">86</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#91">91</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#92">92</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#109">109</a>. Added issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#190">190</a> to
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#195">195</a>. (99-0033/D1209, 14 Oct 99)
</li>
<li>R9:
pre-Kona mailing. Added issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#140">140</a> to
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#189">189</a>. Issues list split into separate "active" and
"closed" documents. (99-0030/N1206, 25 Aug 99)
</li>
<li>R8:
post-Dublin mailing. Updated to reflect LWG and full committee actions
in Dublin. (99-0016/N1193, 21 Apr 99)
</li>
<li>R7:
pre-Dublin updated: Added issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#130">130</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#131">131</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#132">132</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#133">133</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#134">134</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#135">135</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#136">136</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#137">137</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#138">138</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#139">139</a> (31 Mar 99)
</li>
<li>R6:
pre-Dublin mailing. Added issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#127">127</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#128">128</a>,
and <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#129">129</a>. (99-0007/N1194, 22 Feb 99)
</li>
<li>R5:
update issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#103">103</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#112">112</a>; added issues
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#114">114</a> to <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#126">126</a>. Format revisions to prepare
for making list public. (30 Dec 98)
</li>
<li>R4:
post-Santa Cruz II updated: Issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#110">110</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#111">111</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#112">112</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#113">113</a> added, several
issues corrected. (22 Oct 98)
</li>
<li>R3:
post-Santa Cruz II: Issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#94">94</a> to <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#109">109</a>
added, many issues updated to reflect LWG consensus (12 Oct 98)
</li>
<li>R2:
pre-Santa Cruz II: Issues <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#73">73</a> to <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#93">93</a> added,
issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#17">17</a> updated. (29 Sep 98)
</li>
<li>R1:
Correction to issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#55">55</a> resolution, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#60">60</a> code
format, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#64">64</a> title. (17 Sep 98)
</li>
</ul>
<h2>
<a name="Status"></a>Issue Status</h2>
<p><b><a name="New">New</a></b> - The issue has not yet been
reviewed by the LWG. Any <b>Proposed Resolution</b> is purely a
suggestion from the issue submitter, and should not be construed as
the view of LWG.</p>
<p><b><a name="Open">Open</a></b> - The LWG has discussed the issue
but is not yet ready to move the issue forward. There are several
possible reasons for open status:</p>
<ul>
<li>Consensus may have not yet have been reached as to how to deal
with the issue.</li>
<li>Informal consensus may have been reached, but the LWG awaits
exact <b>Proposed Resolution</b> wording for review.</li>
<li>The LWG wishes to consult additional technical experts before
proceeding.</li>
<li>The issue may require further study.</li>
</ul>
<p>A <b>Proposed Resolution</b> for an open issue is still not be
construed as the view of LWG. Comments on the current state of
discussions are often given at the end of open issues in an italic
font. Such comments are for information only and should not be given
undue importance.</p>
<p><b><a name="Dup">Dup</a></b> - The LWG has reached consensus that
the issue is a duplicate of another issue, and will not be further
dealt with. A <b>Rationale</b> identifies the duplicated issue's
issue number. </p>
<p><b><a name="NAD">NAD</a></b> - The LWG has reached consensus that
the issue is not a defect in the Standard, and the issue is ready to
forward to the full committee as a proposed record of response. A
<b>Rationale</b> discusses the LWG's reasoning.</p>
<p><b><a name="Review">Review</a></b> - Exact wording of a
<b>Proposed Resolution</b> is now available for review on an issue
for which the LWG previously reached informal consensus.</p>
<p><b><a name="Ready">Ready</a></b> - The LWG has reached consensus
that the issue is a defect in the Standard, the <b>Proposed
Resolution</b> is correct, and the issue is ready to forward to the
full committee for further action as a Defect Report (DR).</p>
<p><b><a name="DR">DR</a></b> - (Defect Report) - The full J16
committee has voted to forward the issue to the Project Editor to be
processed as a Potential Defect Report. The Project Editor reviews
the issue, and then forwards it to the WG21 Convenor, who returns it
to the full committee for final disposition. This issues list
accords the status of DR to all these Defect Reports regardless of
where they are in that process.</p>
<p><b><a name="TC">TC</a></b> - (Technical Corrigenda) - The full
WG21 committee has voted to accept the Defect Report's Proposed
Resolution as a Technical Corrigenda. Action on this issue is thus
complete and no further action is possible under ISO rules.</p>
<p><b><a name="WP">WP</a></b> - (Working Paper) - The proposed
resolution has not been accepted as a Technical Corrigendum, but
the full WG21 committee has voted to apply the Defect Report's Proposed
Resolution to the working paper.</p>
<p><b><a name="RR">RR</a></b> - (Record of Response) - The full WG21
committee has determined that this issue is not a defect in the
Standard. Action on this issue is thus complete and no further
action is possible under ISO rules.</p>
<p><b><a name="Future">Future</a></b> - In addition to the regular
status, the LWG believes that this issue should be revisited at the
next revision of the standard. It is usually paired with NAD.</p>
<p>Issues are always given the status of <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a> when
they first appear on the issues list. They may progress to
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> or <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Review">Review</a> while the LWG
is actively working on them. When the LWG has reached consensus on
the disposition of an issue, the status will then change to
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Dup">Dup</a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#NAD">NAD</a>, or <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Ready">Ready</a> as appropriate. Once the full J16 committee votes to
forward Ready issues to the Project Editor, they are given the
status of Defect Report ( <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#DR">DR</a>). These in turn may
become the basis for Technical Corrigenda (<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#TC">TC</a>),
or are closed without action other than a Record of Response
(<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#RR">RR</a> ). The intent of this LWG process is that
only issues which are truly defects in the Standard move to the
formal ISO DR status.
</p>
<h2>Active Issues</h2>
<hr>
<a name="23"><h3>23. Num_get overflow result</h3></a><p><b>Section:</b> 22.2.2.1.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-locales.html#lib.facet.num.get.virtuals"> [lib.facet.num.get.virtuals]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Nathan Myers <b>Date:</b> 6 Aug 1998</p>
<p>The current description of numeric input does not account for the
possibility of overflow. This is an implicit result of changing the
description to rely on the definition of scanf() (which fails to
report overflow), and conflicts with the documented behavior of
traditional and current implementations. </p>
<p>Users expect, when reading a character sequence that results in a
value unrepresentable in the specified type, to have an error
reported. The standard as written does not permit this. </p>
<p><b>Further comments from Dietmar:</b></p>
<p>
I don't feel comfortable with the proposed resolution to issue 23: It
kind of simplifies the issue to much. Here is what is going on:
</p>
<p>
Currently, the behavior of numeric overflow is rather counter intuitive
and hard to trace, so I will describe it briefly:
</p>
<ul>
<li>
According to 22.2.2.1.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-locales.html#lib.facet.num.get.virtuals"> [lib.facet.num.get.virtuals]</a>
paragraph 11 <tt>failbit</tt> is set if <tt>scanf()</tt> would
return an input error; otherwise a value is converted to the rules
of <tt>scanf</tt>.
</li>
<li>
<tt>scanf()</tt> is defined in terms of <tt>fscanf()</tt>.
</li>
<li>
<tt>fscanf()</tt> returns an input failure if during conversion no
character matching the conversion specification could be extracted
before reaching EOF. This is the only reason for <tt>fscanf()</tt>
to fail due to an input error and clearly does not apply to the case
of overflow.
</li>
<li>
Thus, the conversion is performed according to the rules of
<tt>fscanf()</tt> which basically says that <tt>strtod</tt>,
<tt>strtol()</tt>, etc. are to be used for the conversion.
</li>
<li>
The <tt>strtod()</tt>, <tt>strtol()</tt>, etc. functions consume as
many matching characters as there are and on overflow continue to
consume matching characters but also return a value identical to
the maximum (or minimum for signed types if there was a leading minus)
value of the corresponding type and set <tt>errno</tt> to <tt>ERANGE</tt>.
</li>
<li>
Thus, according to the current wording in the standard, overflows
can be detected! All what is to be done is to check <tt>errno</tt>
after reading an element and, of course, clearing <tt>errno</tt>
before trying a conversion. With the current wording, it can be
detected whether the overflow was due to a positive or negative
number for signed types.
</li>
</ul>
<p><b>Further discussion from Redmond:</b></p>
<p>The basic problem is that we've defined our behavior,
including our error-reporting behavior, in terms of C90. However,
C90's method of reporting overflow in scanf is not technically an
"input error". The <tt>strto_*</tt> functions are more precise.</p>
<p>There was general consensus that <tt>failbit</tt> should be set
upon overflow. We considered three options based on this:</p>
<ol>
<li>Set failbit upon conversion error (including overflow), and
don't store any value.</li>
<li>Set failbit upon conversion error, and also set <tt>errno</tt> to
indicated the precise nature of the error.</li>
<li>Set failbit upon conversion error. If the error was due to
overflow, store +-numeric_limits<T>::max() as an
overflow indication.</li>
</ol>
<p>Straw poll: (1) 5; (2) 0; (3) 8.</p>
<p><b>Proposed resolution:</b></p>
<p>Discussed at Lillehammer. General outline of what we want the
solution to look like: we want to say that overflow is an error, and
provide a way to distinguish overflow from other kinds of errors.
Choose candidate field the same way scanf does, but don't describe
the rest of the process in terms of format. If a finite input field
is too large (positive or negative) to be represented as a finite
value, then set failbit and assign the nearest representable value.
Bill will provide wording.</p>
<hr>
<a name="96"><h3>96. Vector<bool> is not a container</h3></a><p><b>Section:</b> 23.2.5 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.vector.bool"> [lib.vector.bool]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> AFNOR <b>Date:</b> 7 Oct 1998</p>
<p><tt>vector<bool></tt> is not a container as its reference and
pointer types are not references and pointers. </p>
<p>Also it forces everyone to have a space optimization instead of a
speed one.</p>
<p><b>See also:</b> 99-0008 == N1185 Vector<bool> is
Nonconforming, Forces Optimization Choice.</p>
<p><b>Proposed resolution:</b></p>
<p><i>[In Santa Cruz the LWG felt that this was Not A Defect.]</i></p>
<p><i>[In Dublin many present felt that failure to meet Container
requirements was a defect. There was disagreement as to whether
or not the optimization requirements constituted a defect.]</i></p>
<p><i>[The LWG looked at the following resolutions in some detail:
<br>
* Not A Defect.<br>
* Add a note explaining that vector<bool> does not meet
Container requirements.<br>
* Remove vector<bool>.<br>
* Add a new category of container requirements which
vector<bool> would meet.<br>
* Rename vector<bool>.<br>
<br>
No alternative had strong, wide-spread, support and every alternative
had at least one "over my dead body" response.<br>
<br>
There was also mention of a transition scheme something like (1) add
vector_bool and deprecate vector<bool> in the next standard. (2)
Remove vector<bool> in the following standard.]</i></p>
<p><i>[Modifying container requirements to permit returning proxies
(thus allowing container requirements conforming vector<bool>)
was also discussed.]</i></p>
<p><i>[It was also noted that there is a partial but ugly workaround in
that vector<bool> may be further specialized with a customer
allocator.]</i></p>
<p><i>[Kona: Herb Sutter presented his paper J16/99-0035==WG21/N1211,
vector<bool>: More Problems, Better Solutions. Much discussion
of a two step approach: a) deprecate, b) provide replacement under a
new name. LWG straw vote on that: 1-favor, 11-could live with, 2-over
my dead body. This resolution was mentioned in the LWG report to the
full committee, where several additional committee members indicated
over-my-dead-body positions.]</i></p>
<p>Discussed at Lillehammer. General agreement that we should
deprecate vector<bool> and introduce this functionality under
a different name, e.g. bit_vector. This might make it possible to
remove the vector<bool> specialization in the standard that comes
after C++0x. There was also a suggestion that
in C++0x we could additional say that it's implementation defined
whether vector<bool> refers to the specialization or to the
primary template, but there wasn't general agreement that this was a
good idea.</p>
<p>We need a paper for the new bit_vector class.</p>
<hr>
<a name="201"><h3>201. Numeric limits terminology wrong</h3></a><p><b>Section:</b> 18.2.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-support.html#lib.limits"> [lib.limits]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Stephen Cleary <b>Date:</b> 21 Dec 1999</p>
<p>
In some places in this section, the terms "fundamental types" and
"scalar types" are used when the term "arithmetic types" is intended.
The current usage is incorrect because void is a fundamental type and
pointers are scalar types, neither of which should have
specializations of numeric_limits.
</p>
<p><b>Proposed resolution:</b></p>
<p><i>[Lillehammer: it remains true that numeric_limits is using
imprecise language. However, none of the proposals for changed
wording are clearer. A redesign of numeric_limits is needed, but this
is more a task than an open issue.]</i></p>
<hr>
<a name="233"><h3>233. Insertion hints in associative containers</h3></a><p><b>Section:</b> 23.1.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.associative.reqmts"> [lib.associative.reqmts]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Andrew Koenig <b>Date:</b> 30 Apr 2000</p>
<p>
If <tt>mm</tt> is a multimap and <tt>p</tt> is an iterator
into the multimap, then <tt>mm.insert(p, x)</tt> inserts
<tt>x</tt> into <tt>mm</tt> with <tt>p</tt> as a hint as
to where it should go. Table 69 claims that the execution time is
amortized constant if the insert winds up taking place adjacent to
<tt>p</tt>, but does not say when, if ever, this is guaranteed to
happen. All it says it that <tt>p</tt> is a hint as to where to
insert.
</p>
<p>
The question is whether there is any guarantee about the relationship
between <tt>p</tt> and the insertion point, and, if so, what it
is.
</p>
<p>
I believe the present state is that there is no guarantee: The user
can supply <tt>p</tt>, and the implementation is allowed to
disregard it entirely.
</p>
<p><b>Additional comments from Nathan:</b><br>
The vote [in Redmond] was on whether to elaborately specify the use of
the hint, or to require behavior only if the value could be inserted
adjacent to the hint. I would like to ensure that we have a chance to
vote for a deterministic treatment: "before, if possible, otherwise
after, otherwise anywhere appropriate", as an alternative to the
proposed "before or after, if possible, otherwise [...]".
</p>
<p><b>Proposed resolution:</b></p>
<p>In table 69 "Associative Container Requirements" in 23.1.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.associative.reqmts"> [lib.associative.reqmts]</a>, in the row for <tt>a.insert(p, t)</tt>,
change</p>
<blockquote>
iterator p is a hint pointing to where the insert
should start to search.
</blockquote>
<p>to</p>
<blockquote>
insertion adjacent to iterator p is preferred if
more than one insertion point is valid.
</blockquote>
<p>and change</p>
<blockquote>
logarithmic in general, but amortized constant if
t is inserted right after p.
</blockquote>
<p>to</p>
<blockquote>
logarithmic in general, but amortized constant if
t is inserted adjacent to iterator p.
</blockquote>
<p><i>[Toronto: there was general agreement that this is a real defect:
when inserting an element x into a multiset that already contains
several copies of x, there is no way to know whether the hint will be
used. The proposed resolution was that the new element should always
be inserted as close to the hint as possible. So, for example, if
there is a subsequence of equivalent values, then providing a.begin()
as the hint means that the new element should be inserted before the
subsequence even if a.begin() is far away. JC van Winkel supplied
precise wording for this proposed resolution, and also for an
alternative resolution in which hints are only used when they are
adjacent to the insertion point.]</i></p>
<p><i>[Copenhagen: the LWG agreed to the original proposed resolution,
in which an insertion hint would be used even when it is far from the
insertion point. This was contingent on seeing a reference
implementation showing that it is possible to implement this
requirement without loss of efficiency. John Potter provided such a
reference implementation.]</i></p>
<p><i>[Redmond: The LWG was reluctant to adopt the proposal that
emerged from Copenhagen: it seemed excessively complicated, and went
beyond fixing the defect that we identified in Toronto. PJP provided
the new wording described in this issue. Nathan agrees that we
shouldn't adopt the more detailed semantics, and notes: "we know that
you can do it efficiently enough with a red-black tree, but there are
other (perhaps better) balanced tree techniques that might differ
enough to make the detailed semantics hard to satisfy."]</i></p>
<p><i>[Curaçao: Nathan should give us the alternative wording he
suggests so the LWG can decide between the two options.]</i></p>
<p><i>[Lillehammer: The LWG previously rejected the more detailed
semantics, because it seemed more loike a new feature than like
defect fixing. We're now more sympathetic to it, but we (especially
Bill) are still worried about performance. N1780 describes a naive
algorithm, but it's not clear whether there is a non-naive
implementation. Is it possible to implement this as efficently as
the current version of insert?]</i></p>
<p><i>[Post Lillehammer: N1780 updated in post meeting mailing with
feedback from Lillehammer with more information regarding performance.
]</i></p>
<hr>
<a name="247"><h3>247. <tt>vector</tt>, <tt>deque::insert</tt> complexity</h3></a><p><b>Section:</b> 23.2.4.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.vector.modifiers"> [lib.vector.modifiers]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Ready">Ready</a> <b>Submitter:</b> Lisa Lippincott <b>Date:</b> 06 June 2000</p>
<p>Paragraph 2 of 23.2.4.3 [lib.vector.modifiers] describes the complexity
of <tt>vector::insert</tt>:</p>
<blockquote>
Complexity: If first and last are forward iterators, bidirectional
iterators, or random access iterators, the complexity is linear in
the number of elements in the range [first, last) plus the distance
to the end of the vector. If they are input iterators, the complexity
is proportional to the number of elements in the range [first, last)
times the distance to the end of the vector.
</blockquote>
<p>First, this fails to address the non-iterator forms of
<tt>insert</tt>.</p>
<p>Second, the complexity for input iterators misses an edge case --
it requires that an arbitrary number of elements can be added at
the end of a <tt>vector</tt> in constant time.</p>
<p>I looked to see if <tt>deque</tt> had a similar problem, and was
surprised to find that <tt>deque</tt> places no requirement on the
complexity of inserting multiple elements (23.2.1.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.deque.modifiers"> [lib.deque.modifiers]</a>,
paragraph 3):</p>
<blockquote>
Complexity: In the worst case, inserting a single element into a
deque takes time linear in the minimum of the distance from the
insertion point to the beginning of the deque and the distance
from the insertion point to the end of the deque. Inserting a
single element either at the beginning or end of a deque always
takes constant time and causes a single call to the copy constructor
of T.
</blockquote>
<p><b>Proposed resolution:</b></p>
<p>Change Paragraph 2 of 23.2.4.3 [lib.vector.modifiers] to</p>
<blockquote>
Complexity: The complexity is linear in the number of elements
inserted plus the distance to the end of the vector.
</blockquote>
<p><i>[For input iterators, one may achieve this complexity by first
inserting at the end of the <tt>vector</tt>, and then using
<tt>rotate</tt>.]</i></p>
<p>Change 23.2.1.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.deque.modifiers"> [lib.deque.modifiers]</a>, paragraph 3, to:</p>
<blockquote>
Complexity: The complexity is linear in the number of elements
inserted plus the shorter of the distances to the beginning and
end of the deque. Inserting a single element at either the
beginning or the end of a deque causes a single call to the copy
constructor of T.
</blockquote>
<p><b>Rationale:</b></p>
<p>This is a real defect, and proposed resolution fixes it: some
complexities aren't specified that should be. This proposed
resolution does constrain deque implementations (it rules out the
most naive possible implementations), but the LWG doesn't see a
reason to permit that implementation.</p>
<hr>
<a name="254"><h3>254. Exception types in clause 19 are constructed from <tt>std::string</tt>
</h3></a><p><b>Section:</b> 19.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-diagnostics.html#lib.std.exceptions"> [lib.std.exceptions]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Dave Abrahams <b>Date:</b> 01 Aug 2000</p>
<p>
Many of the standard exception types which implementations are
required to throw are constructed with a const std::string&
parameter. For example:
</p>
<pre> 19.1.5 Class out_of_range [lib.out.of.range]
namespace std {
class out_of_range : public logic_error {
public:
explicit out_of_range(const string& what_arg);
};
}
1 The class out_of_range defines the type of objects thrown as excep-
tions to report an argument value not in its expected range.
out_of_range(const string& what_arg);
Effects:
Constructs an object of class out_of_range.
Postcondition:
strcmp(what(), what_arg.c_str()) == 0.
</pre>
<p>
There are at least two problems with this:
</p>
<ol>
<li>A program which is low on memory may end up throwing
std::bad_alloc instead of out_of_range because memory runs out while
constructing the exception object.</li>
<li>An obvious implementation which stores a std::string data member
may end up invoking terminate() during exception unwinding because the
exception object allocates memory (or rather fails to) as it is being
copied.</li>
</ol>
<p>
There may be no cure for (1) other than changing the interface to
out_of_range, though one could reasonably argue that (1) is not a
defect. Personally I don't care that much if out-of-memory is reported
when I only have 20 bytes left, in the case when out_of_range would
have been reported. People who use exception-specifications might care
a lot, though.
</p>
<p>
There is a cure for (2), but it isn't completely obvious. I think a
note for implementors should be made in the standard. Avoiding
possible termination in this case shouldn't be left up to chance. The
cure is to use a reference-counted "string" implementation
in the exception object. I am not necessarily referring to a
std::string here; any simple reference-counting scheme for a NTBS
would do.
</p>
<p><b>Further discussion, in email:</b></p>
<p>
...I'm not so concerned about (1). After all, a library implementation
can add const char* constructors as an extension, and users don't
<i>need</i> to avail themselves of the standard exceptions, though this is
a lame position to be forced into. FWIW, std::exception and
std::bad_alloc don't require a temporary basic_string.
</p>
<p>
...I don't think the fixed-size buffer is a solution to the problem,
strictly speaking, because you can't satisfy the postcondition
<br>
<tt> strcmp(what(), what_arg.c_str()) == 0</tt>
<br>
For all values of what_arg (i.e. very long values). That means that
the only truly conforming solution requires a dynamic allocation.
</p>
<p><b>Further discussion, from Redmond:</b></p>
<p>The most important progress we made at the Redmond meeting was
realizing that there are two separable issues here: the const
string& constructor, and the copy constructor. If a user writes
something like <tt>throw std::out_of_range("foo")</tt>, the const
string& constructor is invoked before anything gets thrown. The
copy constructor is potentially invoked during stack unwinding.</p>
<p>The copy constructor is a more serious problem, becuase failure
during stack unwinding invokes <tt>terminate</tt>. The copy
constructor must be nothrow. <i>Curaçao: Howard thinks this
requirement may already be present.</i></p>
<p>The fundamental problem is that it's difficult to get the nothrow
requirement to work well with the requirement that the exception
objects store a string of unbounded size, particularly if you also try
to make the const string& constructor nothrow. Options discussed
include:</p>
<ul>
<li>Limit the size of a string that exception objects are required to
throw: change the postconditions of 19.1.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-diagnostics.html#lib.domain.error"> [lib.domain.error]</a> paragraph 3
and 19.1.6 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-diagnostics.html#lib.runtime.error"> [lib.runtime.error]</a> paragraph 3 to something like this:
"strncmp(what(), what_arg._str(), N) == 0, where N is an
implementation defined constant no smaller than 256".</li>
<li>Allow the const string& constructor to throw, but not the
copy constructor. It's the implementor's responsibility to get it
right. (An implementor might use a simple refcount class.)</li>
<li>Compromise between the two: an implementation is not allowed to
throw if the string's length is less than some N, but, if it doesn't
throw, the string must compare equal to the argument.</li>
<li>Add a new constructor that takes a const char*</li>
</ul>
<p>(Not all of these options are mutually exclusive.)</p>
<p><b>Proposed resolution:</b></p>
<p><b>Rationale:</b></p>
<p>Throwing a bad_alloc while trying to construct a message for another
exception-derived class is not necessarily a bad thing. And the
bad_alloc constructor already has a no throw spec on it (18.4.2.1).</p>
<p><b>Future:</b></p>
<p>All involved would like to see const char* constructors added, but
this should probably be done for C++0X as opposed to a DR.</p>
<p>I believe the no throw specs currently decorating these functions
could be improved by some kind of static no throw spec checking
mechanism (in a future C++ language). As they stand, the copy
constructors might fail via a call to unexpected. I think what is
intended here is that the copy constructors can't fail.</p>
<p><i>[Pre-Sydney: reopened at the request of Howard Hinnant.
Post-Redmond: James Kanze noticed that the copy constructors of
exception-derived classes do not have nothrow clauses. Those
classes have no copy constructors declared, meaning the
compiler-generated implicit copy constructors are used, and those
compiler-generated constructors might in principle throw anything.]</i></p>
<hr>
<a name="258"><h3>258. Missing allocator requirement</h3></a><p><b>Section:</b> 20.1.5 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-utilities.html#lib.allocator.requirements"> [lib.allocator.requirements]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Matt Austern <b>Date:</b> 22 Aug 2000</p>
<p>
>From lib-7752:
</p>
<p>
I've been assuming (and probably everyone else has been assuming) that
allocator instances have a particular property, and I don't think that
property can be deduced from anything in Table 32.
</p>
<p>
I think we have to assume that allocator type conversion is a
homomorphism. That is, if x1 and x2 are of type X, where
X::value_type is T, and if type Y is X::template
rebind<U>::other, then Y(x1) == Y(x2) if and only if x1 == x2.
</p>
<p>
Further discussion: Howard Hinnant writes, in lib-7757:
</p>
<p>
I think I can prove that this is not provable by Table 32. And I agree
it needs to be true except for the "and only if". If x1 != x2, I see no
reason why it can't be true that Y(x1) == Y(x2). Admittedly I can't
think of a practical instance where this would happen, or be valuable.
But I also don't see a need to add that extra restriction. I think we
only need:
</p>
<blockquote>
if (x1 == x2) then Y(x1) == Y(x2)
</blockquote>
<p>
If we decide that == on allocators is transitive, then I think I can
prove the above. But I don't think == is necessarily transitive on
allocators. That is:
</p>
<p>
Given x1 == x2 and x2 == x3, this does not mean x1 == x3.
</p>
<p>Example:</p>
<blockquote>
<p>
x1 can deallocate pointers from: x1, x2, x3 <br>
x2 can deallocate pointers from: x1, x2, x4 <br>
x3 can deallocate pointers from: x1, x3 <br>
x4 can deallocate pointers from: x2, x4
</p>
<p>
x1 == x2, and x2 == x4, but x1 != x4
</p>
</blockquote>
<p><b>Proposed resolution:</b></p>
<p><i>[Toronto: LWG members offered multiple opinions. One
opinion is that it should not be required that <tt>x1 == x2</tt>
implies <tt>Y(x1) == Y(x2)</tt>, and that it should not even be
required that <tt>X(x1) == x1</tt>. Another opinion is that
the second line from the bottom in table 32 already implies the
desired property. This issue should be considered in light of
other issues related to allocator instances.]</i></p>
<hr>
<a name="290"><h3>290. Requirements to for_each and its function object</h3></a><p><b>Section:</b> 25.1.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-algorithms.html#lib.alg.foreach"> [lib.alg.foreach]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Angelika Langer <b>Date:</b> 03 Jan 2001</p>
<p>The specification of the for_each algorithm does not have a
"Requires" section, which means that there are no
restrictions imposed on the function object whatsoever. In essence it
means that I can provide any function object with arbitrary side
effects and I can still expect a predictable result. In particular I
can expect that the function object is applied exactly last - first
times, which is promised in the "Complexity" section.
</p>
<p>I don't see how any implementation can give such a guarantee
without imposing requirements on the function object.
</p>
<p>Just as an example: consider a function object that removes
elements from the input sequence. In that case, what does the
complexity guarantee (applies f exactly last - first times) mean?
</p>
<p>One can argue that this is obviously a nonsensical application and
a theoretical case, which unfortunately it isn't. I have seen
programmers shooting themselves in the foot this way, and they did not
understand that there are restrictions even if the description of the
algorithm does not say so.
</p>
<p><b>Proposed resolution:</b></p>
<p><i>[Lillehammer: This is more general than for_each. We don't want
the function object in transform invalidiating iterators
either. There should be a note somewhere in clause 17 (17, not 25)
saying that user code operating on a range may not invalidate
iterators unless otherwise specified. Bill will provide wording.]</i></p>
<hr>
<a name="294"><h3>294. User defined macros and standard headers</h3></a><p><b>Section:</b> 17.4.3.1.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-intro.html#lib.macro.names"> [lib.macro.names]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Ready">Ready</a> <b>Submitter:</b> James Kanze <b>Date:</b> 11 Jan 2001</p>
<p>Paragraph 2 of 17.4.3.1.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-intro.html#lib.macro.names"> [lib.macro.names]</a> reads: "A
translation unit that includes a header shall not contain any macros
that define names declared in that header." As I read this, it
would mean that the following program is legal:</p>
<pre> #define npos 3.14
#include <sstream>
</pre>
<p>since npos is not defined in <sstream>. It is, however, defined
in <string>, and it is hard to imagine an implementation in
which <sstream> didn't include <string>.</p>
<p>I think that this phrase was probably formulated before it was
decided that a standard header may freely include other standard
headers. The phrase would be perfectly appropriate for C, for
example. In light of 17.4.4.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-intro.html#lib.res.on.headers"> [lib.res.on.headers]</a> paragraph 1, however,
it isn't stringent enough.</p>
<p><b>Proposed resolution:</b></p>
<p>For 17.4.3.1.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-intro.html#lib.macro.names"> [lib.macro.names]</a>, replace the current wording, which reads:</p>
<blockquote>
<p>Each name defined as a macro in a header is reserved to the
implementation for any use if the translation unit includes
the header.168)</p>
<p>A translation unit that includes a header shall not contain any
macros that define names declared or defined in that header. Nor shall
such a translation unit define macros for names lexically
identical to keywords.</p>
<p>168) It is not permissible to remove a library macro definition by
using the #undef directive.</p>
</blockquote>
<p>with the wording:</p>
<blockquote>
<p>A translation unit that includes a standard library header shall not
#define or #undef names declared in any standard library header.</p>
<p>A translation unit shall not #define or #undef names lexically
identical to keywords.</p>
</blockquote>
<p><i>[Lillehammer: Beman provided new wording]</i></p>
<hr>
<a name="299"><h3>299. Incorrect return types for iterator dereference</h3></a><p><b>Section:</b> 24.1.4 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iterators.html#lib.bidirectional.iterators"> [lib.bidirectional.iterators]</a>, 24.1.5 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iterators.html#lib.random.access.iterators"> [lib.random.access.iterators]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> John Potter <b>Date:</b> 22 Jan 2001</p>
<p>
In section 24.1.4 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iterators.html#lib.bidirectional.iterators"> [lib.bidirectional.iterators]</a>,
Table 75 gives the return type of *r-- as convertible to T. This is
not consistent with Table 74 which gives the return type of *r++ as
T&. *r++ = t is valid while *r-- = t is invalid.
</p>
<p>
In section 24.1.5 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iterators.html#lib.random.access.iterators"> [lib.random.access.iterators]</a>,
Table 76 gives the return type of a[n] as convertible to T. This is
not consistent with the semantics of *(a + n) which returns T& by
Table 74. *(a + n) = t is valid while a[n] = t is invalid.
</p>
<p>
Discussion from the Copenhagen meeting: the first part is
uncontroversial. The second part, operator[] for Random Access
Iterators, requires more thought. There are reasonable arguments on
both sides. Return by value from operator[] enables some potentially
useful iterators, e.g. a random access "iota iterator" (a.k.a
"counting iterator" or "int iterator"). There isn't any obvious way
to do this with return-by-reference, since the reference would be to a
temporary. On the other hand, <tt>reverse_iterator</tt> takes an
arbitrary Random Access Iterator as template argument, and its
operator[] returns by reference. If we decided that the return type
in Table 76 was correct, we would have to change
<tt>reverse_iterator</tt>. This change would probably affect user
code.
</p>
<p>
History: the contradiction between <tt>reverse_iterator</tt> and the
Random Access Iterator requirements has been present from an early
stage. In both the STL proposal adopted by the committee
(N0527==94-0140) and the STL technical report (HPL-95-11 (R.1), by
Stepanov and Lee), the Random Access Iterator requirements say that
operator[]'s return value is "convertible to T". In N0527
reverse_iterator's operator[] returns by value, but in HPL-95-11
(R.1), and in the STL implementation that HP released to the public,
reverse_iterator's operator[] returns by reference. In 1995, the
standard was amended to reflect the contents of HPL-95-11 (R.1). The
original intent for operator[] is unclear.
</p>
<p>
In the long term it may be desirable to add more fine-grained
iterator requirements, so that access method and traversal strategy
can be decoupled. (See "Improved Iterator Categories and
Requirements", N1297 = 01-0011, by Jeremy Siek.) Any decisions
about issue 299 should keep this possibility in mind.
</p>
<p>Further discussion: I propose a compromise between John Potter's
resolution, which requires <tt>T&</tt> as the return type of
<tt>a[n]</tt>, and the current wording, which requires convertible to
<tt>T</tt>. The compromise is to keep the convertible to <tt>T</tt>
for the return type of the expression <tt>a[n]</tt>, but to also add
<tt>a[n] = t</tt> as a valid expression. This compromise "saves" the
common case uses of random access iterators, while at the same time
allowing iterators such as counting iterator and caching file
iterators to remain random access iterators (iterators where the
lifetime of the object returned by <tt>operator*()</tt> is tied to the
lifetime of the iterator).
</p>
<p>
Note that the compromise resolution necessitates a change to
<tt>reverse_iterator</tt>. It would need to use a proxy to support
<tt>a[n] = t</tt>.
</p>
<p>
Note also there is one kind of mutable random access iterator that
will no longer meet the new requirements. Currently, iterators that
return an r-value from <tt>operator[]</tt> meet the requirements for a
mutable random access iterartor, even though the expression <tt>a[n] =
t</tt> will only modify a temporary that goes away. With this proposed
resolution, <tt>a[n] = t</tt> will be required to have the same
operational semantics as <tt>*(a + n) = t</tt>.
</p>
<p><b>Proposed resolution:</b></p>
<p>
In section 24.1.4 [lib.bidirectdional.iterators], change the return
type in table 75 from "convertible to <tt>T</tt>" to
<tt>T&</tt>.
</p>
<p>
In section 24.1.5 [lib.random.access.iterators], change the
operational semantics for <tt>a[n]</tt> to " the r-value of
<tt>a[n]</tt> is equivalent to the r-value of <tt>*(a +
n)</tt>". Add a new row in the table for the expression <tt>a[n] = t</tt>
with a return type of convertible to <tt>T</tt> and operational semantics of
<tt>*(a + n) = t</tt>.
</p>
<p><i>[Lillehammer: Real problem, but should be addressed as part of
iterator redesign]</i></p>
<hr>
<a name="342"><h3>342. seek and eofbit</h3></a><p><b>Section:</b> 27.6.1.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iostreams.html#lib.istream.unformatted"> [lib.istream.unformatted]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Ready">Ready</a> <b>Submitter:</b> Howard Hinnant <b>Date:</b> 09 Oct 2001</p>
<p>I think we have a defect.</p>
<p>According to lwg issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#60">60</a> which is now a dr, the
description of seekg in 27.6.1.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iostreams.html#lib.istream.unformatted"> [lib.istream.unformatted]</a> paragraph 38 now looks
like:</p>
<blockquote>
Behaves as an unformatted input function (as described in 27.6.1.3,
paragraph 1), except that it does not count the number of characters
extracted and does not affect the value returned by subsequent calls to
gcount(). After constructing a sentry object, if fail() != true,
executes rdbuf()>pubseekpos( pos).
</blockquote>
<p>And according to lwg issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#243">243</a> which is also now a dr,
27.6.1.3, paragraph 1 looks like:</p>
<blockquote>
Each unformatted input function begins execution by constructing an
object of class sentry with the default argument noskipws (second)
argument true. If the sentry object returns true, when converted to a
value of type bool, the function endeavors to obtain the requested
input. Otherwise, if the sentry constructor exits by throwing an
exception or if the sentry object returns false, when converted to a
value of type bool, the function returns without attempting to obtain
any input. In either case the number of extracted characters is set to
0; unformatted input functions taking a character array of non-zero
size as an argument shall also store a null character (using charT())
in the first location of the array. If an exception is thrown during
input then ios::badbit is turned on in *this'ss error state. If
(exception()&badbit)!= 0 then the exception is rethrown. It also counts
the number of characters extracted. If no exception has been thrown it
ends by storing the count in a member object and returning the value
specified. In any event the sentry object is destroyed before leaving
the unformatted input function.
</blockquote>
<p>And finally 27.6.1.1.2/5 says this about sentry:</p>
<blockquote>
If, after any preparation is completed, is.good() is true, ok_ != false
otherwise, ok_ == false.
</blockquote>
<p>
So although the seekg paragraph says that the operation proceeds if
!fail(), the behavior of unformatted functions says the operation
proceeds only if good(). The two statements are contradictory when only
eofbit is set. I don't think the current text is clear which condition
should be respected.
</p>
<p><b>Further discussion from Redmond:</b></p>
<p>PJP: It doesn't seem quite right to say that <tt>seekg</tt> is
"unformatted". That makes specific claims about sentry that
aren't quite appropriate for seeking, which has less fragile failure
modes than actual input. If we do really mean that it's unformatted
input, it should behave the same way as other unformatted input. On
the other hand, "principle of least surprise" is that seeking from EOF
ought to be OK.</p>
<p><b>Proposed resolution:</b></p>
<p>Change 27.6.1.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iostreams.html#lib.istream.unformatted"> [lib.istream.unformatted]</a> to:</p>
<blockquote>
Behaves as an unformatted input function (as described in 27.6.1.3,
paragraph 1), except that it does not count the number of characters
extracted, does not affect the value returned by subsequent calls to
gcount(), and does not examine the value returned by the sentry
object. After constructing a sentry object, if <tt>fail() !=
true</tt>, executes <tt>rdbuf()->pubseekpos(pos)</tt>. In
case of success, the function calls clear().
In case of failure, the function calls <tt>setstate(failbit)</tt>
(which may throw <tt>ios_base::failure</tt>).
</blockquote>
<p><i>[Lillehammer: Matt provided wording.]</i></p>
<p><b>Rationale:</b></p>
<p>In C, fseek does clear EOF. This is probably what most users would
expect. We agree that having eofbit set should not deter a seek,
and that a successful seek should clear eofbit. Note
that <tt>fail()</tt> is true only if <tt>failbit</tt>
or <tt>badbit</tt> is set, so using <tt>!fail()</tt>, rather
than <tt>good()</tt>, satisfies this goal.</p>
<hr>
<a name="362"><h3>362. bind1st/bind2nd type safety</h3></a><p><b>Section:</b> 20.3.6.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-utilities.html#lib.bind.1st"> [lib.bind.1st]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Ready">Ready</a> <b>Submitter:</b> Andrew Demkin <b>Date:</b> 26 Apr 2002</p>
<p>
The definition of bind1st() (20.3.6.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-utilities.html#lib.bind.1st"> [lib.bind.1st]</a>) can result in
the construction of an unsafe binding between incompatible pointer
types. For example, given a function whose first parameter type is
'pointer to T', it's possible without error to bind an argument of
type 'pointer to U' when U does not derive from T:
</p>
<pre> foo(T*, int);
struct T {};
struct U {};
U u;
int* p;
int* q;
for_each(p, q, bind1st(ptr_fun(foo), &u)); // unsafe binding
</pre>
<p>
The definition of bind1st() includes a functional-style conversion to
map its argument to the expected argument type of the bound function
(see below):
</p>
<pre> typename Operation::first_argument_type(x)
</pre>
<p>
A functional-style conversion (5.2.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/expr.html#expr.type.conv"> [expr.type.conv]</a>) is defined to be
semantically equivalent to an explicit cast expression (5.4 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/expr.html#expr.cast"> [expr.cast]</a>), which may (according to 5.4, paragraph 5) be interpreted
as a reinterpret_cast, thus masking the error.
</p>
<p>The problem and proposed change also apply to 20.3.6.4 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-utilities.html#lib.bind.2nd"> [lib.bind.2nd]</a>.</p>
<p><b>Proposed resolution:</b></p>
<p>Add this sentence to the end of 20.3.6 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-utilities.html#lib.binders"> [lib.binders]</a>/1:
"Binders <tt>bind1st</tt> and <tt>bind2nd</tt> are deprecated in
favor of <tt>std::tr1::bind</tt>."</p>
<p>(Notes to editor: (1) when and if tr1::bind is incorporated into
the standard, "std::tr1::bind" should be changed to "std::bind". (2)
20.3.6 should probably be moved to Annex D.</p>
<p><b>Rationale:</b></p>
<p>There is no point in fixing bind1st and bind2nd. tr1::bind is a
superior solution. It solves this problem and others.</p>
<hr>
<a name="369"><h3>369. io stream objects and static ctors</h3></a><p><b>Section:</b> 27.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iostreams.html#lib.iostream.objects"> [lib.iostream.objects]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Ready">Ready</a> <b>Submitter:</b> Ruslan Abdikeev <b>Date:</b> 8 Jul 2002</p>
<p>
Is it safe to use standard iostream objects from constructors of
static objects? Are standard iostream objects constructed and are
their associations established at that time?
</p>
<p>Surpisingly enough, Standard does NOT require that.</p>
<p>
27.3/2 [lib.iostream.objects] guarantees that standard iostream
objects are constructed and their associations are established before
the body of main() begins execution. It also refers to ios_base::Init
class as the panacea for constructors of static objects.
</p>
<p>
However, there's nothing in 27.3 [lib.iostream.objects],
in 27.4.2 [lib.ios.base], and in 27.4.2.1.6 [lib.ios::Init],
that would require implementations to allow access to standard
iostream objects from constructors of static objects.
</p>
<p>Details:</p>
<p>Core text refers to some magic object ios_base::Init, which will
be discussed below:</p>
<blockquote>
"The [standard iostream] objects are constructed, and their
associations are established at some time prior to or during
first time an object of class basic_ios<charT,traits>::Init
is constructed, and in any case before the body of main
begins execution." (27.3/2 [lib.iostream.objects])
</blockquote>
<p>
The first <i>non-normative</i> footnote encourages implementations
to initialize standard iostream objects earlier than required.
</p>
<p>However, the second <i>non-normative</i> footnote makes an explicit
and unsupported claim:</p>
<blockquote>
"Constructors and destructors for static objects can access these
[standard iostream] objects to read input from stdin or write output
to stdout or stderr." (27.3/2 footnote 265 [lib.iostream.objects])
</blockquote>
<p>
The only bit of magic is related to that ios_base::Init class. AFAIK,
the rationale behind ios_base::Init was to bring an instance of this
class to each translation unit which #included <iostream> or
related header. Such an inclusion would support the claim of footnote
quoted above, because in order to use some standard iostream object it
is necessary to #include <iostream>.
</p>
<p>
However, while Standard explicitly describes ios_base::Init as
an appropriate class for doing the trick, I failed to found a
mention of an _instance_ of ios_base::Init in Standard.
</p>
<p><b>Proposed resolution:</b></p>
<p>Add to 27.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iostreams.html#lib.iostream.objects"> [lib.iostream.objects]</a>, p2, immediately before the last sentence
of the paragraph, the following two sentences:</p>
<blockquote>
If a translation unit includes <iostream>, or explicitly
constructs an ios_base::Init object, these stream objects shall
be constructed before dynamic initialization of non-local
objects defined later in that translation unit, and these stream
objects shall be destroyed after the destruction of dynamically
initialized non-local objects defined later in that translation unit.
</blockquote>
<p><i>[Lillehammer: Matt provided wording.]</i></p>
<p><i>[Mont Tremblant: Matt provided revised wording.]</i></p>
<p><b>Rationale:</b></p>
<p>
The original proposed resolution unconditionally required
implementations to define an ios_base::Init object of some
implementation-defined name in the header <iostream>. That's an
overspecification. First, defining the object may be unnecessary
and even detrimental to performance if an implementation can
guarantee that the 8 standard iostream objects will be initialized
before any other user-defined object in a program. Second, there
is no need to require implementations to document the name of the
object.</p>
<p>
The new proposed resolution gives users guidance on what they need to
do to ensure that stream objects are constructed during startup.</p>
<hr>
<a name="371"><h3>371. Stability of multiset and multimap member functions</h3></a><p><b>Section:</b> 23.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.container.requirements"> [lib.container.requirements]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Ready">Ready</a> <b>Submitter:</b> Frank Compagner <b>Date:</b> 20 Jul 2002</p>
<p>
The requirements for multiset and multimap containers (23.1
[lib.containers.requirements], 23.1.2 [lib.associative.reqmnts],
23.3.2 [lib.multimap] and 23.3.4 [lib.multiset]) make no mention of
the stability of the required (mutating) member functions. It appears
the standard allows these functions to reorder equivalent elements of
the container at will, yet the pervasive red-black tree implementation
appears to provide stable behaviour.
</p>
<p>This is of most concern when considering the behaviour of erase().
A stability requirement would guarantee the correct working of the
following 'idiom' that removes elements based on a certain predicate
function.
</p>
<pre> multimap<int, int> m;
multimap<int, int>::iterator i = m.begin();
while (i != m.end()) {
if (pred(i))
m.erase (i++);
else
++i;
}
</pre>
<p>
Although clause 23.1.2/8 guarantees that i remains a valid iterator
througout this loop, absence of the stability requirement could
potentially result in elements being skipped. This would make
this code incorrect, and, furthermore, means that there is no way
of erasing these elements without iterating first over the entire
container, and second over the elements to be erased. This would
be unfortunate, and have a negative impact on both performance and
code simplicity.
</p>
<p>
If the stability requirement is intended, it should be made explicit
(probably through an extra paragraph in clause 23.1.2).
</p>
<p>
If it turns out stability cannot be guaranteed, i'd argue that a
remark or footnote is called for (also somewhere in clause 23.1.2) to
warn against relying on stable behaviour (as demonstrated by the code
above). If most implementations will display stable behaviour, any
problems emerging on an implementation without stable behaviour will
be hard to track down by users. This would also make the need for an
erase_if() member function that much greater.
</p>
<p>This issue is somewhat related to LWG issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#130">130</a>.</p>
<p><b>Proposed resolution:</b></p>
<p>Add the following to the end of 23.1.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.associative.reqmts"> [lib.associative.reqmts]</a> paragraph 4:
"For <tt>multiset</tt> and <tt>multimap</tt>, <tt>insert</tt>and <tt>erase</tt>
are <i>stable</i>: they preserve the relative ordering of equivalent
elements.</p>
<p><i>[Lillehammer: Matt provided wording]</i></p>
<p><i>[Joe Gottman points out that the provided wording does not address
multimap and multiset. N1780 also addresses this issue and suggests
wording.]</i></p>
<p><i>[Mont Tremblant: Changed set and map to multiset and multimap.]</i></p>
<p><b>Rationale:</b></p>
<p>The LWG agrees that this guarantee is necessary for common user
idioms to work, and that all existing implementations provide this
property. Note that this resolution guarantees stability for
multimap and multiset, not for all associative containers in
general.</p>
<hr>
<a name="376"><h3>376. basic_streambuf semantics</h3></a><p><b>Section:</b> 27.7.1.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iostreams.html#lib.stringbuf.virtuals"> [lib.stringbuf.virtuals]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Ready">Ready</a> <b>Submitter:</b> Ray Lischner <b>Date:</b> 14 Aug 2002</p>
<p>
In Section 27.7.1.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iostreams.html#lib.stringbuf.virtuals"> [lib.stringbuf.virtuals]</a>, Table 90, the implication is that
the four conditions should be mutually exclusive, but they are not.
The first two cases, as written, are subcases of the third.</p>
<p>
As written, it is unclear what should be the result if cases 1 and 2
are both true, but case 3 is false.
</p>
<p><b>Proposed resolution:</b></p>
<p>Rewrite these conditions as:</p>
<blockquote>
<p>
(which & (ios_base::in|ios_base::out)) == ios_base::in
</p>
<p>
(which & (ios_base::in|ios_base::out)) == ios_base::out
</p>
<p>
(which & (ios_base::in|ios_base::out)) ==
(ios_base::in|ios_base::out)
and way == either ios_base::beg or ios_base::end
</p>
<p>Otherwise</p>
</blockquote>
<p><b>Rationale:</b></p>
<p>It's clear what we wanted to say, we just failed to say it. This
fixes it.</p>
<hr>
<a name="382"><h3>382. codecvt do_in/out result</h3></a><p><b>Section:</b> 22.2.1.5 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-locales.html#lib.locale.codecvt"> [lib.locale.codecvt]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 30 Aug 2002</p>
<p>
It seems that the descriptions of codecvt do_in() and do_out() leave
sufficient room for interpretation so that two implementations of
codecvt may not work correctly with the same filebuf. Specifically,
the following seems less than adequately specified:
</p>
<ol>
<li>
the conditions under which the functions terminate
</li>
<li>
precisely when the functions return ok
</li>
<li>
precisely when the functions return partial
</li>
<li>
the full set of conditions when the functions return error
</li>
</ol>
<ol>
<li>
22.2.1.5.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-locales.html#lib.locale.codecvt.virtuals"> [lib.locale.codecvt.virtuals]</a>, p2 says this about the effects of the
function: ...Stops if it encounters a character it cannot
convert... This assumes that there *is* a character to
convert. What happens when there is a sequence that doesn't form a
valid source character, such as an unassigned or invalid UNICODE
character, or a sequence that cannot possibly form a character
(e.g., the sequence "\xc0\xff" in UTF-8)?
</li>
<li>
Table 53 says that the function returns codecvt_base::ok
to indicate that the function(s) "completed the conversion."
Suppose that the source sequence is "\xc0\x80" in UTF-8,
with from pointing to '\xc0' and (from_end==from + 1).
It is not clear whether the return value should be ok
or partial (see below).
</li>
<li>
Table 53 says that the function returns codecvt_base::partial
if "not all source characters converted." With the from pointers
set up the same way as above, it is not clear whether the return
value should be partial or ok (see above).
</li>
<li>
Table 53, in the row describing the meaning of error mistakenly
refers to a "from_type" character, without the symbol from_type
having been defined. Most likely, the word "source" character
is intended, although that is not sufficient. The functions
may also fail when they encounter an invalid source sequence
that cannot possibly form a valid source character (e.g., as
explained in bullet 1 above).
</li>
</ol>
<p>
Finally, the conditions described at the end of 22.2.1.5.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-locales.html#lib.locale.codecvt.virtuals"> [lib.locale.codecvt.virtuals]</a>, p4 don't seem to be possible:
</p>
<blockquote>
"A return value of partial, if (from_next == from_end),
indicates that either the destination sequence has not
absorbed all the available destination elements, or that
additional source elements are needed before another
destination element can be produced."
</blockquote>
<p>
If the value is partial, it's not clear to me that (from_next
==from_end) could ever hold if there isn't enough room
in the destination buffer. In order for (from_next==from_end) to
hold, all characters in that range must have been successfully
converted (according to 22.2.1.5.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-locales.html#lib.locale.codecvt.virtuals"> [lib.locale.codecvt.virtuals]</a>, p2) and since there are no
further source characters to convert, no more room in the
destination buffer can be needed.
</p>
<p>
It's also not clear to me that (from_next==from_end) could ever
hold if additional source elements are needed to produce another
destination character (not element as incorrectly stated in the
text). partial is returned if "not all source characters have
been converted" according to Table 53, which also implies that
(from_next==from) does NOT hold.
</p>
<p>
Could it be that the intended qualifying condition was actually
(from_next != from_end), i.e., that the sentence was supposed
to read
</p>
<blockquote>
"A return value of partial, if (from_next != from_end),..."
</blockquote>
<p>
which would make perfect sense, since, as far as I understand it,
partial can only occur if (from_next != from_end)?
</p>
<p><b>Proposed resolution:</b></p>
<p><i>[Lillehammer: Defer for the moment, but this really needs to be
fixed. Right now, the description of codecvt is too vague for it to
be a useful contract between providers and clients of codecvt
facets. (Note that both vendors and users can be both providers and
clients of codecvt facets.) The major philosophical issue is whether
the standard should only describe mappings that take a single wide
character to multiple narrow characters (and vice versa), or whether
it should describe fully general N-to-M conversions. When the
original standard was written only the former was contemplated, but
today, in light of the popularity of utf8 and utf16, that doesn't
seem sufficient for C++0x. Bill supports general N-to-M conversions;
we need to make sure Martin and Howard agree.]</i></p>
<hr>
<a name="384"><h3>384. equal_range has unimplementable runtime complexity</h3></a><p><b>Section:</b> 25.3.3.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-algorithms.html#lib.equal.range"> [lib.equal.range]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Ready">Ready</a> <b>Submitter:</b> Hans Bos <b>Date:</b> 18 Oct 2002</p>
<p>
Section 25.3.3.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-algorithms.html#lib.equal.range"> [lib.equal.range]</a>
states that at most 2 * log(last - first) + 1
comparisons are allowed for equal_range.
</p>
<p>It is not possible to implement equal_range with these constraints.</p>
<p>In a range of one element as in:</p>
<pre> int x = 1;
equal_range(&x, &x + 1, 1)
</pre>
<p>it is easy to see that at least 2 comparison operations are needed.</p>
<p>For this case at most 2 * log(1) + 1 = 1 comparison is allowed.</p>
<p>I have checked a few libraries and they all use the same (nonconforming)
algorithm for equal_range that has a complexity of</p>
<pre> 2* log(distance(first, last)) + 2.
</pre>
<p>I guess this is the algorithm that the standard assumes for equal_range.</p>
<p>
It is easy to see that 2 * log(distance) + 2 comparisons are enough
since equal range can be implemented with lower_bound and upper_bound
(both log(distance) + 1).
</p>
<p>
I think it is better to require something like 2log(distance) + O(1) (or
even logarithmic as multiset::equal_range).
Then an implementation has more room to optimize for certain cases (e.g.
have log(distance) characteristics when at most match is found in the range
but 2log(distance) + 4 for the worst case).
</p>
<p><b>Proposed resolution:</b></p>
<p>In 25.3.3.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-algorithms.html#lib.lower.bound"> [lib.lower.bound]</a>/4, change <tt>log(last - first) + 1</tt>
to <tt>log<sub>2</sub>(last - first) + <i>O</i>(1)</tt>.</p>
<p>In 25.3.3.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-algorithms.html#lib.upper.bound"> [lib.upper.bound]</a>/4, change <tt>log(last - first) + 1</tt>
to <tt>log<sub>2</sub>(last - first) + <i>O</i>(1)</tt>.</p>
<p>In 25.3.3.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-algorithms.html#lib.equal.range"> [lib.equal.range]</a>/4, change <tt>2*log(last - first) + 1</tt>
to <tt>2*log<sub>2</sub>(last - first) + <i>O</i>(1)</tt>.</p>
<p><i>[Matt provided wording]</i></p>
<p><b>Rationale:</b></p>
<p>The LWG considered just saying <i>O</i>(log n) for all three, but
Ê decided that threw away too much valuable information.Ê The fact
Ê that lower_bound is twice as fast as equal_range is important.
Ê However, it's better to allow an arbitrary additive constant than to
Ê specify an exact count.Ê An exact count would have to
Ê involve <tt>floor</tt> or <tt>ceil</tt>.Ê It would be too easy to
Ê get this wrong, and don't provide any substantial value for users.</p>
<hr>
<a name="385"><h3>385. Does call by value imply the CopyConstructible requirement?</h3></a><p><b>Section:</b> 17 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-intro.html#lib.library"> [lib.library]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Matt Austern <b>Date:</b> 23 Oct 2002</p>
<p>
Many function templates have parameters that are passed by value;
a typical example is <tt>find_if</tt>'s <i>pred</i> parameter in
25.1.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-algorithms.html#lib.alg.find"> [lib.alg.find]</a>. Are the corresponding template parameters
(<tt>Predicate</tt> in this case) implicitly required to be
CopyConstructible, or does that need to be spelled out explicitly?
</p>
<p>
This isn't quite as silly a question as it might seem to be at first
sight. If you call <tt>find_if</tt> in such a way that template
argument deduction applies, then of course you'll get call by value
and you need to provide a copy constructor. If you explicitly provide
the template arguments, however, you can force call by reference by
writing something like <tt>find_if<my_iterator,
my_predicate&></tt>. The question is whether implementation
are required to accept this, or whether this is ill-formed because
my_predicate& is not CopyConstructible.
</p>
<p>
The scope of this problem, if it is a problem, is unknown. Function
object arguments to generic algorithms in clauses 25 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-algorithms.html#lib.algorithms"> [lib.algorithms]</a>
and 26 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-numerics.html#lib.numerics"> [lib.numerics]</a> are obvious examples. A review of the whole
library is necessary.
</p>
<p><b>Proposed resolution:</b></p>
<p><i>[
This is really two issues. First, predicates are typically passed by
value but we don't say they must be Copy Constructible. They should
be. Second: is specialization allowed to transform value arguments
into references? References aren't copy constructible, so this should
not be allowed.
]</i></p>
<hr>
<a name="387"><h3>387. std::complex over-encapsulated</h3></a><p><b>Section:</b> 26.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-numerics.html#lib.complex.numbers"> [lib.complex.numbers]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Gabriel Dos Reis <b>Date:</b> 8 Nov 2002</p>
<p>
The absence of explicit description of std::complex<T> layout
makes it imposible to reuse existing software developed in traditional
languages like Fortran or C with unambigous and commonly accepted
layout assumptions. There ought to be a way for practitioners to
predict with confidence the layout of std::complex<T> whenever T
is a numerical datatype. The absence of ways to access individual
parts of a std::complex<T> object as lvalues unduly promotes
severe pessimizations. For example, the only way to change,
independently, the real and imaginary parts is to write something like
</p>
<pre>complex<T> z;
// ...
// set the real part to r
z = complex<T>(r, z.imag());
// ...
// set the imaginary part to i
z = complex<T>(z.real(), i);
</pre>
<p>
At this point, it seems appropriate to recall that a complex number
is, in effect, just a pair of numbers with no particular invariant to
maintain. Existing practice in numerical computations has it that a
complex number datatype is usually represented by Cartesian
coordinates. Therefore the over-encapsulation put in the specification
of std::complex<> is not justified.
</p>
<p><b>Proposed resolution:</b></p>
<p>Add the following requirements to 26.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-numerics.html#lib.complex.numbers"> [lib.complex.numbers]</a> as 26.2/4:</p>
<blockquote>
<p>If z is an lvalue expression of type cv std::complex<T> then</p>
<ul>
<li>the expression reinterpret_cast<cv T(&)[2]>(z)
is well-formed; and</li>
<li>reinterpret_cast<cvT(&)[2]>(z)[0]designates the
real part of z; and</li>
<li>reinterpret_cast<cvT(&)[2]>(z)[1]designates the
imaginary part of z.</li>
</ul>
<p>
Moreover, if a is an expression of pointer type cv complex<T>*
and the expression a[i] is well-defined for an integer expression
i then:
</p>
<ul>
<li>reinterpret_cast<cvT*>(a)[2+i] designates the real
part of a[i]; and</li>
<li>reinterpret_cast<cv T*>(a)[2+i+1] designates the
imaginary part of a[i].</li>
</ul>
</blockquote>
<p>In the header synopsis in 26.2.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-numerics.html#lib.complex.synopsis"> [lib.complex.synopsis]</a>, replace</p>
<pre> template<class T> T real(const complex<T>&);
template<class T> T imag(const complex<T>&);
</pre>
<p>with</p>
<pre> template<class T> const T& real(const complex<T>&);
template<class T> T& real( complex<T>&);
template<class T> const T& imag(const complex<T>&);
template<class T> T& imag( complex<T>&);
</pre>
<p>In 26.2.7 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-numerics.html#lib.complex.value.ops"> [lib.complex.value.ops]</a> paragraph 1, change</p>
<pre> template<class T> T real(const complex<T>&);
</pre>
<p>to</p>
<pre> template<class T> const T& real(const complex<T>&);
template<class T> T& real( complex<T>&);
</pre>
<p>and change the <b>Returns</b> clause to "<b>Returns:</b> The real
part of <i>x</i></p>.
<p>In 26.2.7 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-numerics.html#lib.complex.value.ops"> [lib.complex.value.ops]</a> paragraph 2, change</p>
<pre> template<class T> T imag(const complex<T>&);
</pre>
<p>to</p>
<pre> template<class T> const T& imag(const complex<T>&);
template<class T> T& imag( complex<T>&);
</pre>
<p>and change the <b>Returns</b> clause to "<b>Returns:</b> The imaginary
part of <i>x</i></p>.
<p><i>[Kona: The layout guarantee is absolutely necessary for C
compatibility. However, there was disagreement about the other part
of this proposal: retrieving elements of the complex number as
lvalues. An alternative: continue to have real() and imag() return
rvalues, but add set_real() and set_imag(). Straw poll: return
lvalues - 2, add setter functions - 5. Related issue: do we want
reinterpret_cast as the interface for converting a complex to an
array of two reals, or do we want to provide a more explicit way of
doing it? Howard will try to resolve this issue for the next
meeting.]</i></p>
<p><i>[pre-Sydney: Howard summarized the options in n1589.]</i></p>
<p><b>Rationale:</b></p>
<p>The LWG believes that C99 compatibility would be enough
justification for this change even without other considerations. All
existing implementations already have the layout proposed here.</p>
<hr>
<a name="394"><h3>394. behavior of formatted output on failure</h3></a><p><b>Section:</b> 27.6.2.5.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iostreams.html#lib.ostream.formatted.reqmts"> [lib.ostream.formatted.reqmts]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 27 Dec 2002</p>
<p>
There is a contradiction in Formatted output about what bit is
supposed to be set if the formatting fails. On sentence says it's
badbit and another that it's failbit.
</p>
<p>
27.6.2.5.1, p1 says in the Common Requirements on Formatted output
functions:
</p><pre> ... If the generation fails, then the formatted output function
does setstate(ios::failbit), which might throw an exception.
</pre>
<p></p>
<p>
27.6.2.5.2, p1 goes on to say this about Arithmetic Inserters:
</p>
<p>
... The formatting conversion occurs as if it performed the
following code fragment:
</p>
<p>
</p><pre> bool failed =
use_facet<num_put<charT,ostreambuf_iterator<charT,traits>
> >
(getloc()).put(*this, *this, fill(), val). failed();
... If failed is true then does setstate(badbit) ...
</pre>
<p></p>
<p>
The original intent of the text, according to Jerry Schwarz (see
c++std-lib-10500), is captured in the following paragraph:
</p>
<p>
In general "badbit" should mean that the stream is unusable because
of some underlying failure, such as disk full or socket closure;
"failbit" should mean that the requested formatting wasn't possible
because of some inconsistency such as negative widths. So typically
if you clear badbit and try to output something else you'll fail
again, but if you clear failbit and try to output something else
you'll succeed.
</p>
<p>
In the case of the arithmetic inserters, since num_put cannot
report failure by any means other than exceptions (in response
to which the stream must set badbit, which prevents the kind of
recoverable error reporting mentioned above), the only other
detectable failure is if the iterator returned from num_put
returns true from failed().
</p>
<p>
Since that can only happen (at least with the required iostream
specializations) under such conditions as the underlying failure
referred to above (e.g., disk full), setting badbit would seem
to be the appropriate response (indeed, it is required in
27.6.2.5.2, p1). It follows that failbit can never be directly
set by the arithmetic (it can only be set by the sentry object
under some unspecified conditions).
</p>
<p>
The situation is different for other formatted output functions
which can fail as a result of the streambuf functions failing
(they may do so by means other than exceptions), and which are
then required to set failbit.
</p>
<p>
The contradiction, then, is that ostream::operator<<(int) will
set badbit if the disk is full, while operator<<(ostream&,
char) will set failbit under the same conditions. To make the behavior
consistent, the Common requirements sections for the Formatted output
functions should be changed as proposed below.
</p>
<p><b>Proposed resolution:</b></p>
<p><i>[Kona: There's agreement that this is a real issue. What we
decided at Kona: 1. An error from the buffer (which can be detected
either directly from streambuf's member functions or by examining a
streambuf_iterator) should always result in badbit getting set.
2. There should never be a circumstance where failbit gets set.
That represents a formatting error, and there are no circumstances
under which the output facets are specified as signaling a
formatting error. (Even more so for string output that for numeric
because there's nothing to format.) If we ever decide to make it
possible for formatting errors to exist then the facets can signal
the error directly, and that should go in clause 22, not clause 27.
3. The phrase "if generation fails" is unclear and should be
eliminated. It's not clear whether it's intended to mean a buffer
error (e.g. a full disk), a formatting error, or something else.
Most people thought it was supposed to refer to buffer errors; if
so, we should say so. Martin will provide wording.]</i></p>
<p><b>Rationale:</b></p>
<hr>
<a name="396"><h3>396. what are characters zero and one</h3></a><p><b>Section:</b> 23.3.5.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.bitset.cons"> [lib.bitset.cons]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 5 Jan 2003</p>
<p>
23.3.5.1, p6 [lib.bitset.cons] talks about a generic character
having the value of 0 or 1 but there is no definition of what
that means for charT other than char and wchar_t. And even for
those two types, the values 0 and 1 are not actually what is
intended -- the values '0' and '1' are. This, along with the
converse problem in the description of to_string() in 23.3.5.2,
p33, looks like a defect remotely related to DR 303.
</p>
<p>
http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html#303
</p>
<pre>23.3.5.1:
-6- An element of the constructed string has value zero if the
corresponding character in str, beginning at position pos,
is 0. Otherwise, the element has the value one.
</pre>
<pre>23.3.5.2:
-33- Effects: Constructs a string object of the appropriate
type and initializes it to a string of length N characters.
Each character is determined by the value of its
corresponding bit position in *this. Character position N
?- 1 corresponds to bit position zero. Subsequent decreasing
character positions correspond to increasing bit positions.
Bit value zero becomes the character 0, bit value one becomes
the character 1.
</pre>
<p>
Also note the typo in 23.3.5.1, p6: the object under construction
is a bitset, not a string.
</p>
<p><b>Proposed resolution:</b></p>
<p>Change the constructor's function declaration immediately before
23.3.5.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.bitset.cons"> [lib.bitset.cons]</a> p3 to:</p>
<pre> template <class charT, class traits, class Allocator>
explicit
bitset(const basic_string<charT, traits, Allocator>& str,
typename basic_string<charT, traits, Allocator>::size_type pos = 0,
typename basic_string<charT, traits, Allocator>::size_type n =
basic_string<charT, traits, Allocator>::npos,
charT zero = charT('0'), charT one = charT('1'))
</pre>
<p>Change the first two sentences of 23.3.5.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.bitset.cons"> [lib.bitset.cons]</a> p6 to: "An
element of the constructed string has value 0 if the corresponding
character in <i>str</i>, beginning at position <i>pos</i>,
is <i>zero</i>. Otherwise, the element has the value 1.</p>
<p>Change the text of the second sentence in 23.3.5.1, p5 to read:
"The function then throws invalid_argument if any of the rlen
characters in str beginning at position pos is other than <i>zero</i>
or <i>one</i>. The function uses traits::eq() to compare the character
values."
</p>
<p>Change the declaration of the <tt>to_string</tt> member function
immediately before 23.3.5.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.bitset.members"> [lib.bitset.members]</a> p33 to:</p>
<pre> template <class charT, class traits, class Allocator>
basic_string<charT, traits, Allocator>
to_string(charT zero = charT('0'), charT one = charT('1')) const;
</pre>
<p>Change the last sentence of 23.3.5.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.bitset.members"> [lib.bitset.members]</a> p33 to: "Bit
value 0 becomes the character <tt><i>zero</i></tt>, bit value 1 becomes the
character <tt><i>one</i></tt>.</p>
<p>Change 23.3.5.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.bitset.operators"> [lib.bitset.operators]</a> p8 to:</p>
<p><b>Returns</b>:</p>
<pre> os << x.template to_string<charT,traits,allocator<charT> >(
use_facet<ctype<charT> >(<i>os</i>.getloc()).widen('0'),
use_facet<ctype<charT> >(<i>os</i>.getloc()).widen('1'));
</pre>
<p><b>Rationale:</b></p>
<p>There is a real problem here: we need the character values of '0'
and '1', and we have no way to get them since strings don't have
imbued locales. In principle the "right" solution would be to
provide an extra object, either a ctype facet or a full locale,
which would be used to widen '0' and '1'. However, there was some
discomfort about using such a heavyweight mechanism. The proposed
resolution allows those users who care about this issue to get it
right.</p>
<p>We fix the inserter to use the new arguments. Note that we already
fixed the analogous problem with the extractor in issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#303">303</a>.</p>
<hr>
<a name="397"><h3>397. ostream::sentry dtor throws exceptions</h3></a><p><b>Section:</b> 27.6.2.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iostreams.html#lib.ostream::sentry"> [lib.ostream::sentry]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 5 Jan 2003</p>
<p>
17.4.4.8, p3 prohibits library dtors from throwing exceptions.
</p>
<p>
27.6.2.3, p4 says this about the ostream::sentry dtor:
</p>
<pre> -4- If ((os.flags() & ios_base::unitbuf) && !uncaught_exception())
is true, calls os.flush().
</pre>
<p>
27.6.2.6, p7 that describes ostream::flush() says:
</p>
<pre> -7- If rdbuf() is not a null pointer, calls rdbuf()->pubsync().
If that function returns ?-1 calls setstate(badbit) (which
may throw ios_base::failure (27.4.4.3)).
</pre>
<p>
That seems like a defect, since both pubsync() and setstate() can
throw an exception.
</p>
<p><b>Proposed resolution:</b></p>
<p><i>[
The contradiction is real. Clause 17 says destructors may never
throw exceptions, and clause 27 specifies a destructor that does
throw. In principle we might change either one. We're leaning
toward changing clause 17: putting in an "unless otherwise specified"
clause, and then putting in a footnote saying the sentry destructor
is the only one that can throw. PJP suggests specifying that
sentry::~sentry() should internally catch any exceptions it might cause.
]</i></p>
<hr>
<a name="398"><h3>398. effects of end-of-file on unformatted input functions</h3></a><p><b>Section:</b> 27.6.2.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iostreams.html#lib.ostream::sentry"> [lib.ostream::sentry]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 5 Jan 2003</p>
<p>
While reviewing unformatted input member functions of istream
for their behavior when they encounter end-of-file during input
I found that the requirements vary, sometimes unexpectedly, and
in more than one case even contradict established practice (GNU
libstdc++ 3.2, IBM VAC++ 6.0, STLPort 4.5, SunPro 5.3, HP aCC
5.38, Rogue Wave libstd 3.1, and Classic Iostreams).
</p>
<p>
The following unformatted input member functions set eofbit if they
encounter an end-of-file (this is the expected behavior, and also
the behavior of all major implementations):
</p>
<p>
</p><pre> basic_istream<charT, traits>&
get (char_type*, streamsize, char_type);
</pre>
<p></p>
<p>
Also sets failbit if it fails to extract any characters.
</p>
<p>
</p><pre> basic_istream<charT, traits>&
get (char_type*, streamsize);
</pre>
<p></p>
<p>
Also sets failbit if it fails to extract any characters.
</p>
<p>
</p><pre> basic_istream<charT, traits>&
getline (char_type*, streamsize, char_type);
</pre>
<p></p>
<p>
Also sets failbit if it fails to extract any characters.
</p>
<p>
</p><pre> basic_istream<charT, traits>&
getline (char_type*, streamsize);
</pre>
<p></p>
<p>
Also sets failbit if it fails to extract any characters.
</p>
<p>
</p><pre> basic_istream<charT, traits>&
ignore (int, int_type);
</pre>
<p></p>
<p>
</p><pre> basic_istream<charT, traits>&
read (char_type*, streamsize);
</pre>
<p></p>
<p>
Also sets failbit if it encounters end-of-file.
</p>
<p>
</p><pre> streamsize readsome (char_type*, streamsize);
</pre>
<p></p>
<p>
The following unformated input member functions set failbit but
not eofbit if they encounter an end-of-file (I find this odd
since the functions make it impossible to distinguish a general
failure from a failure due to end-of-file; the requirement is
also in conflict with all major implementation which set both
eofbit and failbit):
</p>
<p>
</p><pre> int_type get();
</pre>
<p></p>
<p>
</p><pre> basic_istream<charT, traits>&
get (char_type&);
</pre>
<p></p>
<p>
These functions only set failbit of they extract no characters,
otherwise they don't set any bits, even on failure (I find this
inconsistency quite unexpected; the requirement is also in
conflict with all major implementations which set eofbit
whenever they encounter end-of-file):
</p>
<p>
</p><pre> basic_istream<charT, traits>&
get (basic_streambuf<charT, traits>&, char_type);
</pre>
<p></p>
<p>
</p><pre> basic_istream<charT, traits>&
get (basic_streambuf<charT, traits>&);
</pre>
<p></p>
<p>
This function sets no bits (all implementations except for
STLport and Classic Iostreams set eofbit when they encounter
end-of-file):
</p>
<p>
</p><pre> int_type peek ();
</pre>
<p></p>
<p><b>Proposed resolution:</b></p>
<p>Informally, what we want is a global statement of intent saying
that eofbit gets set if we trip across EOF, and then we can take
away the specific wording for individual functions. A full review
is necessary. The wording currently in the standard is a mishmash,
and changing it on an individual basis wouldn't make things better.
Dietmar will do this work.</p>
<hr>
<a name="401"><h3>401. incorrect type casts in table 32 in lib.allocator.requirements</h3></a><p><b>Section:</b> 20.1.5 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-utilities.html#lib.allocator.requirements"> [lib.allocator.requirements]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Markus Mauhart <b>Date:</b> 27 Feb 2003</p>
<p>
I think that in par2 of 20.1.5 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-utilities.html#lib.allocator.requirements"> [lib.allocator.requirements]</a> the last two
lines of table 32 contain two incorrect type casts. The lines are ...
</p>
<pre> a.construct(p,t) Effect: new((void*)p) T(t)
a.destroy(p) Effect: ((T*)p)?->~T()
</pre>
<p>
.... with the prerequisits coming from the preceding two paragraphs, especially
from table 31:
</p>
<pre> alloc<T> a ;// an allocator for T
alloc<T>::pointer p ;// random access iterator
// (may be different from T*)
alloc<T>::reference r = *p;// T&
T const& t ;
</pre>
<p>
For that two type casts ("(void*)p" and "(T*)p") to be well-formed
this would require then conversions to T* and void* for all
alloc<T>::pointer, so it would implicitely introduce extra
requirements for alloc<T>::pointer, additionally to the only
current requirement (being a random access iterator).
</p>
<p><b>Proposed resolution:</b></p>
<p>
"(void*)p" should be replaced with "(void*)&*p" and that
"((T*)p)?->" should be replaced with "(*p)." or with
"(&*p)->".
</p>
<p>
Note: Actually I would prefer to replace "((T*)p)?->dtor_name" with
"p?->dtor_name", but AFAICS this is not possible cause of an omission
in 13.5.6 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/over.html#over.ref"> [over.ref]</a> (for which I have filed another DR on 29.11.2002).
</p>
<p><i>[Kona: The LWG thinks this is somewhere on the border between
Open and NAD. The intend is clear: <tt>construct</tt> constructs an
object at the location <i>p</i>. It's reading too much into the
description to think that literally calling <tt>new</tt> is
required. Tweaking this description is low priority until we can do
a thorough review of allocators, and, in particular, allocators with
non-default pointer types.]</i></p>
<hr>
<a name="408"><h3>408. Is vector<reverse_iterator<char*> > forbidden?</h3></a><p><b>Section:</b> 24.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iterators.html#lib.iterator.requirements"> [lib.iterator.requirements]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Nathan Myers <b>Date:</b> 3 June 2003</p>
<p>
I've been discussing iterator semantics with Dave Abrahams, and a
surprise has popped up. I don't think this has been discussed before.
</p>
<p>
24.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iterators.html#lib.iterator.requirements"> [lib.iterator.requirements]</a> says that the only operation that can be performed on "singular"
iterator values is to assign a non-singular value to them. (It
doesn't say they can be destroyed, and that's probably a defect.)
Some implementations have taken this to imply that there is no need
to initialize the data member of a reverse_iterator<> in the default
constructor. As a result, code like
</p>
<blockquote>
std::vector<std::reverse_iterator<char*> > v(7);
v.reserve(1000);
</blockquote>
<p>
invokes undefined behavior, because it must default-initialize the
vector elements, and then copy them to other storage. Of course many
other vector operations on these adapters are also left undefined,
and which those are is not reliably deducible from the standard.
</p>
<p>
I don't think that 24.1 was meant to make standard-library iterator
types unsafe. Rather, it was meant to restrict what operations may
be performed by functions which take general user- and standard
iterators as arguments, so that raw pointers would qualify as
iterators. However, this is not clear in the text, others have come
to the opposite conclusion.
</p>
<p>
One question is whether the standard iterator adaptors have defined
copy semantics. Another is whether they have defined destructor
semantics: is
</p>
<blockquote>
{ std::vector<std::reverse_iterator<char*> > v(7); }
</blockquote>
<p>
undefined too?
</p>
<p>
Note this is not a question of whether algorithms are allowed to
rely on copy semantics for arbitrary iterators, just whether the
types we actually supply support those operations. I believe the
resolution must be expressed in terms of the semantics of the
adapter's argument type. It should make clear that, e.g., the
reverse_iterator<T> constructor is actually required to execute
T(), and so copying is defined if the result of T() is copyable.
</p>
<p>
Issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#235">235</a>, which defines reverse_iterator's default
constructor more precisely, has some relevance to this issue.
However, it is not the whole story.
</p>
<p>
The issue was whether
</p>
<blockquote>
reverse_iterator() { }
</blockquote>
<p>
is allowed, vs.
</p>
<blockquote>
reverse_iterator() : current() { }
</blockquote>
<p>
The difference is when T is char*, where the first leaves the member
uninitialized, and possibly equal to an existing pointer value, or
(on some targets) may result in a hardware trap when copied.
</p>
<p>
8.5 paragraph 5 seems to make clear that the second is required to
satisfy DR <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#235">235</a>, at least for non-class Iterator argument
types.
</p>
<p>
But that only takes care of reverse_iterator, and doesn't establish
a policy for all iterators. (The reverse iterator adapter was just
an example.) In particular, does my function
</p>
<blockquote>
template <typename Iterator>
void f() { std::vector<Iterator> v(7); }
</blockquote>
<p>
evoke undefined behavior for some conforming iterator definitions?
I think it does, now, because vector<> will destroy those singular
iterator values, and that's explicitly disallowed.
</p>
<p>
24.1 shouldn't give blanket permission to copy all singular iterators,
because then pointers wouldn't qualify as iterators. However, it
should allow copying of that subset of singular iterator values that
are default-initialized, and it should explicitly allow destroying any
iterator value, singular or not, default-initialized or not.
</p>
<p>Related issue: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#407">407</a></p>
<p><b>Proposed resolution:</b></p>
<p><i>[
We don't want to require all singular iterators to be copyable,
because that is not the case for pointers. However, default
construction may be a special case. Issue: is it really default
construction we want to talk about, or is it something like value
initialization? We need to check with core to see whether default
constructed pointers are required to be copyable; if not, it would be
wrong to impose so strict a requirement for iterators.
]</i></p>
<hr>
<a name="416"><h3>416. definitions of XXX_MIN and XXX_MAX macros in climits</h3></a><p><b>Section:</b> 18.2.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-support.html#lib.c.limits"> [lib.c.limits]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 18 Sep 2003</p>
<p>
Given two overloads of the function foo(), one taking an argument of type
int and the other taking a long, which one will the call foo(LONG_MAX)
resolve to? The expected answer should be foo(long), but whether that
is true depends on the #defintion of the LONG_MAX macro, specifically
its type. This issue is about the fact that the type of these macros
is not actually required to be the same as the the type each respective
limit.
<br>
Section 18.2.2 of the C++ Standard does not specify the exact types of
the XXX_MIN and XXX_MAX macros #defined in the <climits> and <limits.h>
headers such as INT_MAX and LONG_MAX and instead defers to the C standard.
<br>
Section 5.2.4.2.1, p1 of the C standard specifies that "The values [of
these constants] shall be replaced by constant expressions suitable for use
in #if preprocessing directives. Moreover, except for CHAR_BIT and MB_LEN_MAX,
the following shall be replaced by expressions that have the same type as
would an expression that is an object of the corresponding type converted
according to the integer promotions."
<br>
The "corresponding type converted according to the integer promotions" for
LONG_MAX is, according to 6.4.4.1, p5 of the C standard, the type of long
converted to the first of the following set of types that can represent it:
int, long int, long long int. So on an implementation where (sizeof(long)
== sizeof(int)) this type is actually int, while on an implementation where
(sizeof(long) > sizeof(int)) holds this type will be long.
<br>
This is not an issue in C since the type of the macro cannot be detected
by any conforming C program, but it presents a portability problem in C++
where the actual type is easily detectable by overload resolution.
</p>
<p><b>Proposed resolution:</b></p>
<p><i>[Kona: the LWG does not believe this is a defect. The C macro
definitions are what they are; we've got a better
mechanism, <tt>std::numeric_limits</tt>, that is specified more
precisely than the C limit macros. At most we should add a
nonnormative note recommending that users who care about the exact
types of limit quantities should use <limits> instead of
<climits>.]</i></p>
<hr>
<a name="417"><h3>417. what does ctype::do_widen() return on failure</h3></a><p><b>Section:</b> 22.2.1.1.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-locales.html#lib.locale.ctype.virtuals"> [lib.locale.ctype.virtuals]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 18 Sep 2003</p>
<p>
The Effects and Returns clauses of the do_widen() member function of
the ctype facet fail to specify the behavior of the function on failure.
That the function may not be able to simply cast the narrow character
argument to the type of the result since doing so may yield the wrong value
for some wchar_t encodings. Popular implementations of ctype<wchar_t> that
use mbtowc() and UTF-8 as the native encoding (e.g., GNU glibc) will fail
when the argument's MSB is set. There is no way for the the rest of locale
and iostream to reliably detect this failure.
</p>
<p><b>Proposed resolution:</b></p>
<p><i>[Kona: This is a real problem. Widening can fail. It's unclear
what the solution should be. Returning WEOF works for the wchar_t
specialization, but not in general. One option might be to add a
default, like <i>narrow</i>. But that's an incompatible change.
Using <i>traits::eof</i> might seem like a good idea, but facets
don't have access to traits (a recurring problem). We could
have <i>widen</i> throw an exception, but that's a scary option;
existing library components aren't written with the assumption
that <i>widen</i> can throw.]</i></p>
<hr>
<a name="418"><h3>418. exceptions thrown during iostream cleanup</h3></a><p><b>Section:</b> 27.4.2.1.6 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iostreams.html#lib.ios::Init"> [lib.ios::Init]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 18 Sep 2003</p>
<p>
The dtor of the ios_base::Init object is supposed to call flush() on the
6 standard iostream objects cout, cerr, clog, wcout, wcerr, and wclog.
This call may cause an exception to be thrown.
</p>
<p>
17.4.4.8, p3 prohibits all library destructors from throwing exceptions.
</p>
<p>
The question is: What should this dtor do if one or more of these calls
to flush() ends up throwing an exception? This can happen quite easily
if one of the facets installed in the locale imbued in the iostream
object throws.
</p>
<p><b>Proposed resolution:</b></p>
<p><i>[Kona: We probably can't do much better than what we've got, so
the LWG is leaning toward NAD. At the point where the standard
stream objects are being cleaned up, the usual error reporting
mechanism are all unavailable. And exception from flush at this
point will definitely cause problems. A quality implementation
might reasonably swallow the exception, or call abort, or do
something even more drastic.]</i></p>
<hr>
<a name="419"><h3>419. istream extractors not setting failbit if eofbit is already set</h3></a><p><b>Section:</b> 27.6.1.1.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iostreams.html#lib.istream::sentry"> [lib.istream::sentry]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 18 Sep 2003</p>
<p>
27.6.1.1.2, p2 says that istream::sentry ctor prepares for input if is.good()
is true. p4 then goes on to say that the ctor sets the sentry::ok_ member to
true if the stream state is good after any preparation. 27.6.1.2.1, p1 then
says that a formatted input function endeavors to obtain the requested input
if the sentry's operator bool() returns true.
Given these requirements, no formatted extractor should ever set failbit if
the initial stream rdstate() == eofbit. That is contrary to the behavior of
all implementations I tested. The program below prints out
eof = 1, fail = 0
eof = 1, fail = 1
on all of them.
</p>
<pre>
#include <sstream>
#include <cstdio>
int main()
{
std::istringstream strm ("1");
int i = 0;
strm >> i;
std::printf ("eof = %d, fail = %d\n",
!!strm.eof (), !!strm.fail ());
strm >> i;
std::printf ("eof = %d, fail = %d\n",
!!strm.eof (), !!strm.fail ());
}
</pre>
<p>
<br>
Comments from Jerry Schwarz (c++std-lib-11373):
<br>
Jerry Schwarz wrote:
<br>
I don't know where (if anywhere) it says it in the standard, but the
formatted extractors are supposed to set failbit if they don't extract
any characters. If they didn't then simple loops like
<br>
while (cin >> x);
<br>
would loop forever.
<br>
Further comments from Martin Sebor:
<br>
The question is which part of the extraction should prevent this from happening
by setting failbit when eofbit is already set. It could either be the sentry
object or the extractor. It seems that most implementations have chosen to
set failbit in the sentry [...] so that's the text that will need to be
corrected.
</p>
<p><b>Proposed resolution:</b></p>
<p>Kona: Possibly NAD. If eofbit is set then good() will return false. We
then set <i>ok</i> to false. We believe that the sentry's
constructor should always set failbit when <i>ok</i> is false, and
we also think the standard already says that. Possibly it could be
clearer.</p>
<hr>
<a name="421"><h3>421. is basic_streambuf copy-constructible?</h3></a><p><b>Section:</b> 27.5.2.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iostreams.html#lib.streambuf.cons"> [lib.streambuf.cons]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 18 Sep 2003</p>
<p>
The reflector thread starting with c++std-lib-11346 notes that the class
template basic_streambuf, along with basic_stringbuf and basic_filebuf,
is copy-constructible but that the semantics of the copy constructors
are not defined anywhere. Further, different implementations behave
differently in this respect: some prevent copy construction of objects
of these types by declaring their copy ctors and assignment operators
private, others exhibit undefined behavior, while others still give
these operations well-defined semantics.
</p>
<p>
Note that this problem doesn't seem to be isolated to just the three
types mentioned above. A number of other types in the library section
of the standard provide a compiler-generated copy ctor and assignment
operator yet fail to specify their semantics. It's believed that the
only types for which this is actually a problem (i.e. types where the
compiler-generated default may be inappropriate and may not have been
intended) are locale facets. See issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#439">439</a>.
</p>
<p><b>Proposed resolution:</b></p>
<p>
27.5.2 [lib.streambuf]: Add into the synopsis, public section, just above the destructor declaration:
</p>
<blockquote>
<pre>basic_streambuf(const basic_streambuf& sb);
basic_streambuf& operator=(const basic_streambuf& sb);
</pre>
</blockquote>
<p>Insert after 27.5.2.1, paragraph 2:</p>
<blockquote>
<pre>basic_streambuf(const basic_streambuf& sb);
</pre>
<p>Constructs a copy of sb.</p>
<p>Postcondtions:</p>
<pre> eback() == sb.eback()
gptr() == sb.gptr()
egptr() == sb.egptr()
pbase() == sb.pbase()
pptr() == sb.pptr()
epptr() == sb.epptr()
getloc() == sb.getloc()
</pre>
<pre>basic_streambuf& operator=(const basic_streambuf& sb);
</pre>
<p>Assigns the data members of sb to this.</p>
<p>Postcondtions:</p>
<pre> eback() == sb.eback()
gptr() == sb.gptr()
egptr() == sb.egptr()
pbase() == sb.pbase()
pptr() == sb.pptr()
epptr() == sb.epptr()
getloc() == sb.getloc()
</pre>
<p>Returns: *this.</p>
</blockquote>
<p>27.7.1 [lib.stringbuf]:</p>
<b>Option A:</b>
<blockquote>
<p>Insert into the basic_stringbuf synopsis in the private section:</p>
<pre>basic_stringbuf(const basic_stringbuf&); // not defined
basic_stringbuf& operator=(const basic_stringbuf&); // not defined
</pre>
</blockquote>
<b>Option B:</b>
<blockquote>
<p>Insert into the basic_stringbuf synopsis in the public section:</p>
<pre>basic_stringbuf(const basic_stringbuf& sb);
basic_stringbuf& operator=(const basic_stringbuf& sb);
</pre>
<p>27.7.1.1, insert after paragraph 4:</p>
<pre>basic_stringbuf(const basic_stringbuf& sb);</pre>
<p>
Constructs an independent copy of sb as if with sb.str(), and with the openmode that sb was constructed with.
</p>
<p>Postcondtions: </p>
<pre> str() == sb.str()
gptr() - eback() == sb.gptr() - sb.eback()
egptr() - eback() == sb.egptr() - sb.eback()
pptr() - pbase() == sb.pptr() - sb.pbase()
getloc() == sb.getloc()
</pre>
<p>
Note: The only requirement on epptr() is that it point beyond the
initialized range if an output sequence exists. There is no requirement
that epptr() - pbase() == sb.epptr() - sb.pbase().
</p>
<pre>basic_stringbuf& operator=(const basic_stringbuf& sb);</pre>
<p>After assignment the basic_stringbuf has the same state as if it
were initially copy constructed from sb, except that the
basic_stringbuf is allowed to retain any excess capacity it might have,
which may in turn effect the value of epptr().
</p>
</blockquote>
<p>27.8.1.1 [lib.filebuf]</p>
<p>Insert at the bottom of the basic_filebuf synopsis:</p>
<blockquote>
<pre>private:
basic_filebuf(const basic_filebuf&); // not defined
basic_filebuf& operator=(const basic_filebuf&); // not defined
</pre>
</blockquote>
<p><i>[Kona: this is an issue for basic_streambuf itself and for its
derived classes. We are leaning toward allowing basic_streambuf to
be copyable, and specifying its precise semantics. (Probably the
obvious: copying the buffer pointers.) We are less sure whether
the streambuf derived classes should be copyable. Howard will
write up a proposal.]</i></p>
<p><i>[Sydney: Dietmar presented a new argument against basic_streambuf
being copyable: it can lead to an encapsulation violation. Filebuf
inherits from streambuf. Now suppose you inhert a my_hijacking_buf
from streambuf. You can copy the streambuf portion of a filebuf to a
my_hijacking_buf, giving you access to the pointers into the
filebuf's internal buffer. Perhaps not a very strong argument, but
it was strong enough to make people nervous. There was weak
preference for having streambuf not be copyable. There was weak
preference for having stringbuf not be copyable even if streambuf
is. Move this issue to open for now.
]</i></p>
<p><b>Rationale:</b></p>
<p>
27.5.2 [lib.streambuf]: The proposed basic_streambuf copy constructor
and assignment operator are the same as currently implied by the lack
of declarations: public and simply copies the data members. This
resolution is not a change but a clarification of the current
standard.
</p>
<p>
27.7.1 [lib.stringbuf]: There are two reasonable options: A) Make
basic_stringbuf not copyable. This is likely the status-quo of
current implementations. B) Reasonable copy semantics of
basic_stringbuf can be defined and implemented. A copyable
basic_streambuf is arguably more useful than a non-copyable one. This
should be considered as new functionality and not the fixing of a
defect. If option B is chosen, ramifications from issue 432 are taken
into account.
</p>
<p>
27.8.1.1 [lib.filebuf]: There are no reasonable copy semantics for
basic_filebuf.
</p>
<hr>
<a name="422"><h3>422. explicit specializations of member functions of class templates</h3></a><p><b>Section:</b> 17.4.3.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-intro.html#lib.reserved.names"> [lib.reserved.names]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 18 Sep 2003</p>
<p>
It has been suggested that 17.4.3.1, p1 may or may not allow programs to
explicitly specialize members of standard templates on user-defined types.
The answer to the question might have an impact where library requirements
are given using the "as if" rule. I.e., if programs are allowed to specialize
member functions they will be able to detect an implementation's strict
conformance to Effects clauses that describe the behavior of the function
in terms of the other member function (the one explicitly specialized by
the program) by relying on the "as if" rule.
</p>
<p><b>Proposed resolution:</b></p>
<p>
Add the following sentence immediately after the text of 17.4.3.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-intro.html#lib.reserved.names"> [lib.reserved.names]</a>, p1:
</p>
<blockquote>
The behavior of a program that declares explicit specializations
of any members of class templates or explicit specializations of
any member templates of classes or class templates defined in
this library is undefined.
</blockquote>
<p><i>[Kona: straw poll was 6-1 that user programs should not be
allowed to specialize individual member functions of standard
library class templates, and that doing so invokes undefined
behavior. Post-Kona: Martin provided wording.]</i></p>
<p><i>[Sydney: The LWG agrees that the standard shouldn't permit users
to specialize individual member functions unless they specialize the
whole class, but we're not sure these words say what we want them to;
they could be read as prohibiting the specialization of any standard
library class templates. We need to consult with CWG to make sure we
use the right wording.]</i></p>
<hr>
<a name="423"><h3>423. effects of negative streamsize in iostreams</h3></a><p><b>Section:</b> 27 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iostreams.html#lib.input.output"> [lib.input.output]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 18 Sep 2003</p>
<p>
A third party test suite tries to exercise istream::ignore(N) with
a negative value of N and expects that the implementation will treat
N as if it were 0. Our implementation asserts that (N >= 0) holds and
aborts the test.
</p>
<p>
I can't find anything in section 27 that prohibits such values but I don't
see what the effects of such calls should be, either (this applies to
a number of unformatted input functions as well as some member functions
of the basic_streambuf template).
</p>
<p><b>Proposed resolution:</b></p>
<p>
I propose that we add to each function in clause 27 that takes an argument,
say N, of type streamsize a Requires clause saying that "N >= 0." The intent
is to allow negative streamsize values in calls to precision() and width()
but disallow it in calls to streambuf::sgetn(), istream::ignore(), or
ostream::write().
</p>
<p><i>[Kona: The LWG agreed that this is probably what we want. However, we
need a review to find all places where functions in clause 27 take
arguments of type streamsize that shouldn't be allowed to go
negative. Martin will do that review.]</i></p>
<hr>
<a name="424"><h3>424. normative notes</h3></a><p><b>Section:</b> 17.3.1.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-intro.html#lib.structure.summary"> [lib.structure.summary]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 18 Sep 2003</p>
<p>
The text in 17.3.1.1, p1 says:
<br>
"Paragraphs labelled "Note(s):" or "Example(s):" are informative, other
paragraphs are normative."
<br>
The library section makes heavy use of paragraphs labeled "Notes(s),"
some of which are clearly intended to be normative (see list 1), while
some others are not (see list 2). There are also those where the intent
is not so clear (see list 3).
<br>
List 1 -- Examples of (presumably) normative Notes:
<br>
20.4.1.1, p3, 20.4.1.1, p10, 21.3.1, p11, 22.1.1.2, p11, 23.2.1.3, p2,
25.3.7, p3, 26.2.6, p14a, 27.5.2.4.3, p7.
<br>
List 2 -- Examples of (presumably) informative Notes:
<br>
18.4.1.3, p3, 21.3.5.6, p14, 22.2.1.5.2, p3, 25.1.1, p4, 26.2.5, p1,
27.4.2.5, p6.
<br>
List 3 -- Examples of Notes that are not clearly either normative
or informative:
<br>
22.1.1.2, p8, 22.1.1.5, p6, 27.5.2.4.5, p4.
<br>
None of these lists is meant to be exhaustive.
</p>
<p><b>Proposed resolution:</b></p>
<p><i>[Definitely a real problem. The big problem is there's material
that doesn't quite fit any of the named paragraph categories
(e.g. <b>Effects</b>). Either we need a new kind of named
paragraph, or we need to put more material in unnamed paragraphs
jsut after the signature. We need to talk to the Project Editor
about how to do this.
]</i></p>
<hr>
<a name="427"><h3>427. stage 2 and rationale of DR 221</h3></a><p><b>Section:</b> 22.2.2.1.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-locales.html#lib.facet.num.get.virtuals"> [lib.facet.num.get.virtuals]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 18 Sep 2003</p>
<p>
The requirements specified in Stage 2 and reiterated in the rationale
of DR 221 (and echoed again in DR 303) specify that num_get<charT>::
do_get() compares characters on the stream against the widened elements
of "012...abc...ABCX+-"
</p>
<p>
An implementation is required to allow programs to instantiate the num_get
template on any charT that satisfies the requirements on a user-defined
character type. These requirements do not include the ability of the
character type to be equality comparable (the char_traits template must
be used to perform tests for equality). Hence, the num_get template cannot
be implemented to support any arbitrary character type. The num_get template
must either make the assumption that the character type is equality-comparable
(as some popular implementations do), or it may use char_traits<charT> to do
the comparisons (some other popular implementations do that). This diversity
of approaches makes it difficult to write portable programs that attempt to
instantiate the num_get template on user-defined types.
</p>
<p><b>Proposed resolution:</b></p>
<p><i>[Kona: the heart of the problem is that we're theoretically
supposed to use traits classes for all fundamental character
operations like assignment and comparison, but facets don't have
traits parameters. This is a fundamental design flaw and it
appears all over the place, not just in this one place. It's not
clear what the correct solution is, but a thorough review of facets
and traits is in order. The LWG considered and rejected the
possibility of changing numeric facets to use narrowing instead of
widening. This may be a good idea for other reasons (see issue
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#459">459</a>), but it doesn't solve the problem raised by this
issue. Whether we use widen or narrow the <tt>num_get</tt> facet
still has no idea which traits class the user wants to use for
the comparison, because only streams, not facets, are passed traits
classes. The standard does not require that two different
traits classes with the same <tt>char_type</tt> must necessarily
have the same behavior.]</i></p>
<p>Informally, one possibility: require that some of the basic
character operations, such as <tt>eq</tt>, <tt>lt</tt>,
and <tt>assign</tt>, must behave the same way for all traits classes
with the same <tt>char_type</tt>. If we accept that limitation on
traits classes, then the facet could reasonably be required to
use <tt>char_traits<charT></tt></p>.
<hr>
<a name="430"><h3>430. valarray subset operations</h3></a><p><b>Section:</b> 26.3.2.4 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-numerics.html#lib.valarray.sub"> [lib.valarray.sub]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 18 Sep 2003</p>
<p>
The standard fails to specify the behavior of valarray::operator[](slice)
and other valarray subset operations when they are passed an "invalid"
slice object, i.e., either a slice that doesn't make sense at all (e.g.,
slice (0, 1, 0) or one that doesn't specify a valid subset of the valarray
object (e.g., slice (2, 1, 1) for a valarray of size 1).
</p>
<p><b>Proposed resolution:</b></p>
<p><i>[Kona: the LWG believes that invalid slices should invoke
undefined behavior. Valarrays are supposed to be designed for high
performance, so we don't want to require specific checking. We
need wording to express this decision.]</i></p>
<hr>
<a name="431"><h3>431. Swapping containers with unequal allocators</h3></a><p><b>Section:</b> 20.1.5 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-utilities.html#lib.allocator.requirements"> [lib.allocator.requirements]</a>, 25 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-algorithms.html#lib.algorithms"> [lib.algorithms]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Matt Austern <b>Date:</b> 20 Sep 2003</p>
<p>Clause 20.1.5 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-utilities.html#lib.allocator.requirements"> [lib.allocator.requirements]</a> paragraph 4 says that implementations
are permitted to supply containers that are unable to cope with
allocator instances and that container implementations may assume
that all instances of an allocator type compare equal. We gave
implementers this latitude as a temporary hack, and eventually we
want to get rid of it. What happens when we're dealing with
allocators that <i>don't</i> compare equal?
</p>
<p>In particular: suppose that <tt>v1</tt> and <tt>v2</tt> are both
objects of type <tt>vector<int, my_alloc></tt> and that
<tt>v1.get_allocator() != v2.get_allocator()</tt>. What happens if
we write <tt>v1.swap(v2)</tt>? Informally, three possibilities:</p>
<p>1. This operation is illegal. Perhaps we could say that an
implementation is required to check and to throw an exception, or
perhaps we could say it's undefined behavior.</p>
<p>2. The operation performs a slow swap (i.e. using three
invocations of <tt>operator=</tt>, leaving each allocator with its
original container. This would be an O(N) operation.</p>
<p>3. The operation swaps both the vectors' contents and their
allocators. This would be an O(1) operation. That is:</p>
<blockquote>
<pre> my_alloc a1(...);
my_alloc a2(...);
assert(a1 != a2);
vector<int, my_alloc> v1(a1);
vector<int, my_alloc> v2(a2);
assert(a1 == v1.get_allocator());
assert(a2 == v2.get_allocator());
v1.swap(v2);
assert(a1 == v2.get_allocator());
assert(a2 == v1.get_allocator());
</pre>
</blockquote>
<p><b>Proposed resolution:</b></p>
<p><i>[Kona: This is part of a general problem. We need a paper
saying how to deal with unequal allocators in general.]</i></p>
<p><i>[pre-Sydney: Howard argues for option 3 in n1599.]</i></p>
<hr>
<a name="446"><h3>446. Iterator equality between different containers</h3></a><p><b>Section:</b> 24.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iterators.html#lib.iterator.requirements"> [lib.iterator.requirements]</a>, 23.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.container.requirements"> [lib.container.requirements]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Andy Koenig <b>Date:</b> 16 Dec 2003</p>
<p>
What requirements does the standard place on equality comparisons between
iterators that refer to elements of different containers. For example, if
v1 and v2 are empty vectors, is v1.end() == v2.end() allowed to yield true?
Is it allowed to throw an exception?
</p>
<p>
The standard appears to be silent on both questions.
</p>
<p><b>Proposed resolution:</b></p>
<p><i>[Sydney: The intention is that comparing two iterators from
different containers is undefined, but it's not clear if we say that,
or even whether it's something we should be saying in clause 23 or in
clause 24. Intuitively we might want to say that equality is defined
only if one iterator is reachable from another, but figuring out how
to say it in any sensible way is a bit tricky: reachability is defined
in terms of equality, so we can't also define equality in terms of
reachability.
]</i></p>
<hr>
<a name="454"><h3>454. basic_filebuf::open should accept wchar_t names</h3></a><p><b>Section:</b> 27.8.1.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iostreams.html#lib.filebuf.members"> [lib.filebuf.members]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Bill Plauger <b>Date:</b> 30 Jan 2004</p>
<pre> basic_filebuf *basic_filebuf::open(const char *, ios_base::open_mode);
</pre>
<p>should be supplemented with the overload:</p>
<pre> basic_filebuf *basic_filebuf::open(const wchar_t *, ios_base::open_mode);
</pre>
<p>
Depending on the operating system, one of these forms is fundamental and
the other requires an implementation-defined mapping to determine the
actual filename.
</p>
<p><i>[Sydney: Yes, we want to allow wchar_t filenames. Bill will
provide wording.]</i></p>
<p><b>Proposed resolution:</b></p>
<p>Change from:</p>
<blockquote>
<pre>basic_filebuf<charT,traits>* open(
const char* s,
ios_base::openmode mode );
</pre>
<p>
Effects: If is_open() != false, returns a null pointer.
Otherwise, initializes the filebuf as required. It then
opens a file, if possible, whose name is the NTBS s ("as if"
by calling std::fopen(s,modstr)).</p>
</blockquote>
<p>to:</p>
<blockquote>
<pre>basic_filebuf<charT,traits>* open(
const char* s,
ios_base::openmode mode );
basic_filebuf<charT,traits>* open(
const wchar_t* ws,
ios_base::openmode mode );
</pre>
<p>
Effects: If is_open() != false, returns a null pointer.
Otherwise, initializes the filebuf as required. It then
opens a file, if possible, whose name is the NTBS s ("as if"
by calling std::fopen(s,modstr)).
For the second signature, the NTBS s is determined from the
WCBS ws in an implementation-defined manner.
</p>
<p>
(NOTE: For a system that "naturally" represents a filename
as a WCBS, the NTBS s in the first signature may instead
be mapped to a WCBS; if so, it follows the same mapping
rules as the first argument to open.)
</p>
</blockquote>
<p><b>Rationale:</b></p>
<p>
Slightly controversial, but by a 7-1 straw poll the LWG agreed to move
this to Ready. The controversy was because the mapping between wide
names and files in a filesystem is implementation defined. The
counterargument, which most but not all LWG members accepted, is that
the mapping between narrow files names and files is also
implemenation defined.</p>
<p><i>[Lillehammer: Moved back to "open" status, at Beman's urging.
(1) Why just basic_filebuf, instead of also basic_fstream (and
possibly other things too). (2) Why not also constructors that take
std::basic_string? (3) We might want to wait until we see Beman's
filesystem library; we might decide that it obviates this.]</i></p>
<hr>
<a name="456"><h3>456. Traditional C header files are overspecified</h3></a><p><b>Section:</b> 17.4.1.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-intro.html#lib.headers"> [lib.headers]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Bill Plauger <b>Date:</b> 30 Jan 2004</p>
<p>The C++ Standard effectively requires that the traditional C headers
(of the form <xxx.h>) be defined in terms of the newer C++
headers (of the form <cxxx>). Clauses 17.4.1.2/4 and D.5 combine
to require that:</p>
<ul>
<li>Including the header <cxxx> declares a C name in namespace std.</li>
<li> Including the header <xxx.h> declares a C name in namespace std
(effectively by including <cxxx>), then imports it into the global
namespace with an individual using declaration.</li>
</ul>
<p>
The rules were left in this form despited repeated and heated objections
from several compiler vendors. The C headers are often beyond the direct
control of C++ implementors. In some organizations, it's all they can do
to get a few #ifdef __cplusplus tests added. Third-party library vendors
can perhaps wrap the C headers. But neither of these approaches supports
the drastic restructuring required by the C++ Standard. As a result, it is
still widespread practice to ignore this conformance requirement, nearly
seven years after the committee last debated this topic. Instead, what is
often implemented is:
</p>
<ul>
<li> Including the header <xxx.h> declares a C name in the
global namespace.</li>
<li> Including the header <cxxx> declares a C name in the
global namespace (effectively by including <xxx.h>), then
imports it into namespace std with an individual using declaration.</li>
</ul>
<p>
The practical benefit for implementors with the second approach is that
they can use existing C library headers, as they are pretty much obliged
to do. The practical cost for programmers facing a mix of implementations
is that they have to assume weaker rules:</p>
<ul>
<li> If you want to assuredly declare a C name in the global
namespace, include <xxx.h>. You may or may not also get the
declaration in namespace std.</li>
<li> If you want to assuredly declare a C name in namespace std,
include <cxxx.h>. You may or may not also get the declaration in
the global namespace.</li>
</ul>
<p>
There also exists the <i>possibility</i> of subtle differences due to
Koenig lookup, but there are so few non-builtin types defined in the C
headers that I've yet to see an example of any real problems in this
area.
</p>
<p>
It is worth observing that the rate at which programmers fall afoul of
these differences has remained small, at least as measured by newsgroup
postings and our own bug reports. (By an overwhelming margin, the
commonest problem is still that programmers include <string> and can't
understand why the typename string isn't defined -- this a decade after
the committee invented namespace std, nominally for the benefit of all
programmers.)
</p>
<p>
We should accept the fact that we made a serious mistake and rectify it,
however belatedly, by explicitly allowing either of the two schemes for
declaring C names in headers.
</p>
<p><i>[Sydney: This issue has been debated many times, and will
certainly have to be discussed in full committee before any action
can be taken. However, the preliminary sentiment of the LWG was in
favor of the change. (6 yes, 0 no, 2 abstain) Robert Klarer
suggests that we might also want to undeprecate the
C-style <tt>.h</tt> headers.]</i></p>
<p><b>Proposed resolution:</b></p>
<hr>
<a name="458"><h3>458. 24.1.5 contains unintented limitation for operator-</h3></a><p><b>Section:</b> 24.1.5 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iterators.html#lib.random.access.iterators"> [lib.random.access.iterators]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Daniel Frey <b>Date:</b> 27 Feb 2004</p>
<p>
In 24.1.5 [lib.random.access.iterators], table 76 the operational
semantics for the expression "r -= n" are defined as "return r += -n".
This means, that the expression -n must be valid, which is not the case
for unsigned types.
</p>
<p><i>[
Sydney: Possibly not a real problem, since difference type is required
to be a signed integer type. However, the wording in the standard may
be less clear than we would like.
]</i></p>
<p><b>Proposed resolution:</b></p>
<p>
To remove this limitation, I suggest to change the
operational semantics for this column to:
</p>
<code>
{ Distance m = n;
if (m >= 0)
while (m--) --r;
else
while (m++) ++r;
return r; }
</code>
<hr>
<a name="459"></a><h3><a name="459">459. Requirement for widening in stage 2 is overspecification</a></h3><p><b>Section:</b> 22.2.2.1.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-locales.html#lib.facet.num.get.virtuals"> [lib.facet.num.get.virtuals]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 16 Mar 2004</p>
<p>When parsing strings of wide-character digits, the standard
requires the library to widen narrow-character "atoms" and compare
the widened atoms against the characters that are being parsed.
Simply narrowing the wide characters would be far simpler, and
probably more efficient. The two choices are equivalent except in
convoluted test cases, and many implementations already ignore the
standard and use narrow instead of widen.</p>
<p>
First, I disagree that using narrow() instead of widen() would
necessarily have unfortunate performance implications. A possible
implementation of narrow() that allows num_get to be implemented
in a much simpler and arguably comparably efficient way as calling
widen() allows, i.e. without making a virtual call to do_narrow every
time, is as follows:
</p>
<pre> inline char ctype<wchar_t>::narrow (wchar_t wc, char dflt) const
{
const unsigned wi = unsigned (wc);
if (wi > UCHAR_MAX)
return typeid (*this) == typeid (ctype<wchar_t>) ?
dflt : do_narrow (wc, dflt);
if (narrow_ [wi] < 0) {
const char nc = do_narrow (wc, dflt);
if (nc == dflt)
return dflt;
narrow_ [wi] = nc;
}
return char (narrow_ [wi]);
}
</pre>
<p>
Second, I don't think the change proposed in the issue (i.e., to use
narrow() instead of widen() during Stage 2) would be at all
drastic. Existing implementations with the exception of libstdc++
currently already use narrow() so the impact of the change on programs
would presumably be isolated to just a single implementation. Further,
since narrow() is not required to translate alternate wide digit
representations such as those mentioned in issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#303">303</a> to
their narrow equivalents (i.e., the portable source characters '0'
through '9'), the change does not necessarily imply that these
alternate digits would be treated as ordinary digits and accepted as
part of numbers during parsing. In fact, the requirement in 22.2.1.1.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-locales.html#lib.locale.ctype.virtuals"> [lib.locale.ctype.virtuals]</a>, p13 forbids narrow() to translate an alternate
digit character, wc, to an ordinary digit in the basic source
character set unless the expression
(ctype<charT>::is(ctype_base::digit, wc) == true) holds. This in
turn is prohibited by the C standard (7.25.2.1.5, 7.25.2.1.5, and
5.2.1, respectively) for charT of either char or wchar_t.
</p>
<p><i>[Sydney: To a large extent this is a nonproblem. As long as
you're only trafficking in char and wchar_t we're only dealing with a
stable character set, so you don't really need either 'widen' or
'narrow': can just use literals. Finally, it's not even clear whether
widen-vs-narrow is the right question; arguably we should be using
codecvt instead.]</i></p>
<p><b>Proposed resolution:</b></p>
<p>Change stage 2 so that implementations are permitted to use either
technique to perform the comparison:</p>
<ol>
<li> call widen on the atoms and compare (either by using
operator== or char_traits<charT>::eq) the input with
the widened atoms, or</li>
<li> call narrow on the input and compare the narrow input
with the atoms</li>
<li> do (1) or (2) only if charT is not char or wchar_t,
respectively; i.e., avoid calling widen or narrow
if it the source and destination types are the same</li>
</ol>
<hr>
<a name="462"><h3>462. Destroying objects with static storage duration</h3></a><p><b>Section:</b> 3.6.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/basic.html#basic.start.term"> [basic.start.term]</a>, 18.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-support.html#lib.support.start.term"> [lib.support.start.term]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Bill Plauger <b>Date:</b> 23 Mar 2004</p>
<p>
3.6.3 Termination spells out in detail the interleaving of static
destructor calls and calls to functions registered with atexit. To
match this behavior requires intimate cooperation between the code
that calls destructors and the exit/atexit machinery. The former
is tied tightly to the compiler; the latter is a primitive mechanism
inherited from C that traditionally has nothing to do with static
construction and destruction. The benefits of intermixing destructor
calls with atexit handler calls is questionable at best, and <i>very</i>
difficult to get right, particularly when mixing third-party C++
libraries with different third-party C++ compilers and C libraries
supplied by still other parties.
</p>
<p>
I believe the right thing to do is defer all static destruction
until after all atexit handlers are called. This is a change in
behavior, but one that is likely visible only to perverse test
suites. At the very least, we should <i>permit</i> deferred destruction
even if we don't require it.
</p>
<p><b>Proposed resolution:</b></p>
<p><i>[If this is to be changed, it should probably be changed by CWG.
At this point, however, the LWG is leaning toward NAD. Implementing
what the standard says is hard work, but it's not impossible and
most vendors went through that pain years ago. Changing this
behavior would be a user-visible change, and would break at least
one real application.]</i></p>
<p>
</p>
<hr>
<a name="463"><h3>463. auto_ptr usability issues</h3></a><p><b>Section:</b> 20.4.5 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-utilities.html#lib.auto.ptr"> [lib.auto.ptr]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Rani Sharoni <b>Date:</b> 7 Dec 2003</p>
<p>
TC1 CWG DR #84 effectively made the template<class Y> operator auto_ptr<Y>()
member of auto_ptr (20.4.5.3/4) obsolete.
</p>
<p>
The sole purpose of this obsolete conversion member is to enable copy
initialization base from r-value derived (or any convertible types like
cv-types) case:
</p>
<pre>#include <memory>
using std::auto_ptr;
struct B {};
struct D : B {};
auto_ptr<D> source();
int sink(auto_ptr<B>);
int x1 = sink( source() ); // #1 EDG - no suitable copy constructor
</pre>
<p>
The excellent analysis of conversion operations that was given in the final
auto_ptr proposal
(http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/1997/N1128.pdf)
explicitly specifies this case analysis (case 4). DR #84 makes the analysis
wrong and actually comes to forbid the loophole that was exploited by the
auto_ptr designers.
</p>
<p>
I didn't encounter any compliant compiler (e.g. EDG, GCC, BCC and VC) that
ever allowed this case. This is probably because it requires 3 user defined
conversions and in fact current compilers conform to DR #84.
</p>
<p>
I was surprised to discover that the obsolete conversion member actually has
negative impact of the copy initialization base from l-value derived
case:</p>
<pre>auto_ptr<D> dp;
int x2 = sink(dp); // #2 EDG - more than one user-defined conversion applies
</pre>
<p>
I'm sure that the original intention was allowing this initialization using
the template<class Y> auto_ptr(auto_ptr<Y>& a) constructor (20.4.5.1/4) but
since in this copy initialization it's merely user defined conversion (UDC)
and the obsolete conversion member is UDC with the same rank (for the early
overloading stage) there is an ambiguity between them.
</p>
<p>
Removing the obsolete member will have impact on code that explicitly
invokes it:
</p>
<pre>int y = sink(source().operator auto_ptr<B>());
</pre>
<p>
IMHO no one ever wrote such awkward code and the reasonable workaround for
#1 is:
</p>
<pre>int y = sink( auto_ptr<B>(source()) );
</pre>
<p>
I was even more surprised to find out that after removing the obsolete
conversion member the initialization was still ill-formed:
int x3 = sink(dp); // #3 EDG - no suitable copy constructor
</p>
<p>
This copy initialization semantically requires copy constructor which means
that both template conversion constructor and the auto_ptr_ref conversion
member (20.4.5.3/3) are required which is what was explicitly forbidden in
DR #84. This is a bit amusing case in which removing ambiguity results with
no candidates.
</p>
<p>
I also found exception safety issue with auto_ptr related to auto_ptr_ref:
</p>
<pre>int f(auto_ptr<B>, std::string);
auto_ptr<B> source2();
// string constructor throws while auto_ptr_ref
// "holds" the pointer
int x4 = f(source2(), "xyz"); // #4
</pre>
<p>
The theoretic execution sequence that will cause a leak:
</p>
<ol>
<li>call auto_ptr<B>::operator auto_ptr_ref<B>()</li>
<li>call string::string(char const*) and throw</li>
</ol>
<p>
According to 20.4.5.3/3 and 20.4.5/2 the auto_ptr_ref conversion member
returns auto_ptr_ref<Y> that holds *this and this is another defect since
the type of *this is auto_ptr<X> where X might be different from Y. Several
library vendors (e.g. SGI) implement auto_ptr_ref<Y> with Y* as member which
is much more reasonable. Other vendor implemented auto_ptr_ref as
defectively required and it results with awkward and catastrophic code:
int oops = sink(auto_ptr<B>(source())); // warning recursive on all control
paths
</p>
<p>
Dave Abrahams noticed that there is no specification saying that
auto_ptr_ref copy constructor can't throw.
</p>
<p>
My proposal comes to solve all the above issues and significantly simplify
auto_ptr implementation. One of the fundamental requirements from auto_ptr
is that it can be constructed in an intuitive manner (i.e. like ordinary
pointers) but with strict ownership semantics which yield that source
auto_ptr in initialization must be non-const. My idea is to add additional
constructor template with sole propose to generate ill-formed, diagnostic
required, instance for const auto_ptr arguments during instantiation of
declaration. This special constructor will not be instantiated for other
types which is achievable using 14.8.2/2 (SFINAE). Having this constructor
in hand makes the constructor template<class Y> auto_ptr(auto_ptr<Y> const&)
legitimate since the actual argument can't be const yet non const r-value
are acceptable.
</p>
<p>
This implementation technique makes the "private auxiliary class"
auto_ptr_ref obsolete and I found out that modern C++ compilers (e.g. EDG,
GCC and VC) consume the new implementation as expected and allow all
intuitive initialization and assignment cases while rejecting illegal cases
that involve const auto_ptr arguments.
</p>
<p>The proposed auto_ptr interface:</p>
<pre>namespace std {
template<class X> class auto_ptr {
public:
typedef X element_type;
// 20.4.5.1 construct/copy/destroy:
explicit auto_ptr(X* p=0) throw();
auto_ptr(auto_ptr&) throw();
template<class Y> auto_ptr(auto_ptr<Y> const&) throw();
auto_ptr& operator=(auto_ptr&) throw();
template<class Y> auto_ptr& operator=(auto_ptr<Y>) throw();
~auto_ptr() throw();
// 20.4.5.2 members:
X& operator*() const throw();
X* operator->() const throw();
X* get() const throw();
X* release() throw();
void reset(X* p=0) throw();
private:
template<class U>
auto_ptr(U& rhs, typename
unspecified_error_on_const_auto_ptr<U>::type = 0);
};
}
</pre>
<p>
One compliant technique to implement the unspecified_error_on_const_auto_ptr
helper class is using additional private auto_ptr member class template like
the following:
</p>
<pre>template<typename T> struct unspecified_error_on_const_auto_ptr;
template<typename T>
struct unspecified_error_on_const_auto_ptr<auto_ptr<T> const>
{ typedef typename auto_ptr<T>::const_auto_ptr_is_not_allowed type; };
</pre>
<p>
There are other techniques to implement this helper class that might work
better for different compliers (i.e. better diagnostics) and therefore I
suggest defining its semantic behavior without mandating any specific
implementation. IMO, and I didn't found any compiler that thinks otherwise,
14.7.1/5 doesn't theoretically defeat the suggested technique but I suggest
verifying this with core language experts.
</p>
<p><b>Further changes in standard text:</b></p>
<p>Remove section 20.4.5.3</p>
<p>Change 20.4.5/2 to read something like:
Initializing auto_ptr<X> from const auto_ptr<Y> will result with unspecified
ill-formed declaration that will require unspecified diagnostic.</p>
<p>Change 20.4.5.1/4,5,6 to read:</p>
<pre>template<class Y> auto_ptr(auto_ptr<Y> const& a) throw();</pre>
<p> 4 Requires: Y* can be implicitly converted to X*.</p>
<p> 5 Effects: Calls const_cast<auto_ptr<Y>&>(a).release().</p>
<p> 6 Postconditions: *this holds the pointer returned from a.release().</p>
<p>Change 20.4.5.1/10</p>
<pre>template<class Y> auto_ptr& operator=(auto_ptr<Y> a) throw();
</pre>
<p>
10 Requires: Y* can be implicitly converted to X*. The expression delete
get() is well formed.
</p>
<p>LWG TC DR #127 is obsolete.</p>
<p>
Notice that the copy constructor and copy assignment operator should remain
as before and accept non-const auto_ptr& since they have effect on the form
of the implicitly declared copy constructor and copy assignment operator of
class that contains auto_ptr as member per 12.8/5,10:
</p>
<pre>struct X {
// implicit X(X&)
// implicit X& operator=(X&)
auto_ptr<D> aptr_;
};
</pre>
<p>
In most cases this indicates about sloppy programming but preserves the
current auto_ptr behavior.
</p>
<p>
Dave Abrahams encouraged me to suggest fallback implementation in case that
my suggestion that involves removing of auto_ptr_ref will not be accepted.
In this case removing the obsolete conversion member to auto_ptr<Y> and
20.4.5.3/4,5 is still required in order to eliminate ambiguity in legal
cases. The two constructors that I suggested will co exist with the current
members but will make auto_ptr_ref obsolete in initialization contexts.
auto_ptr_ref will be effective in assignment contexts as suggested in DR
#127 and I can't see any serious exception safety issues in those cases
(although it's possible to synthesize such). auto_ptr_ref<X> semantics will
have to be revised to say that it strictly holds pointer of type X and not
reference to an auto_ptr for the favor of cases in which auto_ptr_ref<Y> is
constructed from auto_ptr<X> in which X is different from Y (i.e. assignment
from r-value derived to base).
</p>
<p><b>Proposed resolution:</b></p>
<p><i>[Redmond: punt for the moment. We haven't decided yet whether we
want to fix auto_ptr for C++-0x, or remove it and replace it with
move_ptr and unique_ptr.]</i></p>
<hr>
<a name="466"><h3>466. basic_string ctor should prevent null pointer error</h3></a><p><b>Section:</b> 21.3.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-strings.html#lib.string.cons"> [lib.string.cons]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Daniel Frey <b>Date:</b> 10 Jun 2004</p>
<p>
Today, my colleagues and me wasted a lot of time. After some time, I
found the problem. It could be reduced to the following short example:
</p>
<pre> #include <string>
int main() { std::string( 0 ); }
</pre>
<p>The problem is that the tested compilers (GCC 2.95.2, GCC 3.3.1 and
Comeau online) compile the above without errors or warnings! The
programs (at least for the GCC) resulted in a SEGV.</p>
<p>I know that the standard explicitly states that the ctor of string
requires a char* which is not zero. STLs could easily detect the above
case with a private ctor for basic_string which takes a single 'int'
argument. This would catch the above code at compile time and would not
ambiguate any other legal ctors.</p>
<p><b>Proposed resolution:</b></p>
<p><i>[Redmond: No great enthusiasm for doing this. If we do,
however, we want to do it for all places that take <tt>charT*</tt>
pointers, not just the single-argument constructor. The other
question is whether we want to catch this at compile time (in which
case we catch the error of a literal 0, but not an expression whose
value is a null pointer), at run time, or both.]</i></p>
<hr>
<a name="470"><h3>470. accessing containers from their elements' special functions</h3></a><p><b>Section:</b> 23 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.containers"> [lib.containers]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 28 Jun 2004</p>
<p>
The standard doesn't prohibit the destructors (or any other special
functions) of containers' elements invoked from a member function
of the container from "recursively" calling the same (or any other)
member function on the same container object, potentially while the
container is in an intermediate state, or even changing the state
of the container object while it is being modified. This may result
in some surprising (i.e., undefined) behavior.
</p>
<p>Read email thread starting with c++std-lib-13637 for more.</p>
<p><b>Proposed resolution:</b></p>
<p>Add to Container Requirements the following new paragraph:</p>
<pre> Unless otherwise specified, the behavior of a program that
invokes a container member function f from a member function
g of the container's value_type on a container object c that
called g from its mutating member function h, is undefined.
I.e., if v is an element of c, directly or indirectly calling
c.h() from v.g() called from c.f(), is undefined.
</pre>
<p><i>[Redmond: This is a real issue, but it's probably a clause 17
issue, not clause 23. We get the same issue, for example, if we
try to destroy a stream from one of the stream's callback functions.]</i></p>
<hr>
<a name="471"><h3>471. result of what() implementation-defined</h3></a><p><b>Section:</b> 18.6.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-support.html#lib.exception"> [lib.exception]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 28 Jun 2004</p>
<p>[lib.exception] specifies the following:</p>
<pre> exception (const exception&) throw();
exception& operator= (const exception&) throw();
-4- Effects: Copies an exception object.
-5- Notes: The effects of calling what() after assignment
are implementation-defined.
</pre>
<p>
First, does the Note only apply to the assignment operator? If so,
what are the effects of calling what() on a copy of an object? Is
the returned pointer supposed to point to an identical copy of
the NTBS returned by what() called on the original object or not?
</p>
<p>
Second, is this Note intended to extend to all the derived classes
in section 19? I.e., does the standard provide any guarantee for
the effects of what() called on a copy of any of the derived class
described in section 19?
</p>
<p>
Finally, if the answer to the first question is no, I believe it
constitutes a defect since throwing an exception object typically
implies invoking the copy ctor on the object. If the answer is yes,
then I believe the standard ought to be clarified to spell out
exactly what the effects are on the copy (i.e., after the copy
ctor was called).
</p>
<p><i>[Redmond: Yes, this is fuzzy. The issue of derived classes is
fuzzy too.]</i></p>
<p><b>Proposed resolution:</b></p>
<hr>
<a name="473"><h3>473. underspecified ctype calls</h3></a><p><b>Section:</b> 22.2.1.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-locales.html#lib.locale.ctype"> [lib.locale.ctype]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 1 Jul 2004</p>
<p>
Most ctype member functions come in two forms: one that operates
on a single character at a time and another form that operates
on a range of characters. Both forms are typically described by
a single Effects and/or Returns clause.
</p>
<p>
The Returns clause of each of the single-character non-virtual forms
suggests that the function calls the corresponding single character
virtual function, and that the array form calls the corresponding
virtual array form. Neither of the two forms of each virtual member
function is required to be implemented in terms of the other.
</p>
<p>
There are three problems:
</p>
<p>
1. One is that while the standard does suggest that each non-virtual
member function calls the corresponding form of the virtual function,
it doesn't actually explicitly require it.
</p>
<p>
Implementations that cache results from some of the virtual member
functions for some or all values of their arguments might want to
call the array form from the non-array form the first time to fill
the cache and avoid any or most subsequent virtual calls. Programs
that rely on each form of the virtual function being called from
the corresponding non-virtual function will see unexpected behavior
when using such implementations.
</p>
<p>
2. The second problem is that either form of each of the virtual
functions can be overridden by a user-defined function in a derived
class to return a value that is different from the one produced by
the virtual function of the alternate form that has not been
overriden.
</p>
<p>
Thus, it might be possible for, say, ctype::widen(c) to return one
value, while for ctype::widen(&c, &c + 1, &wc) to set
wc to another value. This is almost certainly not intended. Both
forms of every function should be required to return the same result
for the same character, otherwise the same program using an
implementation that calls one form of the functions will behave
differently than when using another implementation that calls the
other form of the function "under the hood."
</p>
<p>
3. The last problem is that the standard text fails to specify whether
one form of any of the virtual functions is permitted to be implemented
in terms of the other form or not, and if so, whether it is required
or permitted to call the overridden virtual function or not.
</p>
<p>
Thus, a program that overrides one of the virtual functions so that
it calls the other form which then calls the base member might end
up in an infinite loop if the called form of the base implementation
of the function in turn calls the other form.
</p>
<p><b>Proposed resolution:</b></p>
<p>
Lillehammer: Part of this isn't a real problem. We already talk about
caching. 22.1.1/6 But part is a real problem. ctype virtuals may call
each other, so users don't know which ones to override to avoid avoid
infinite loops.</p>
<p>This is a problem for all facet virtuals, not just ctype virtuals,
so we probably want a blanket statement in clause 22 for all
facets. The LWG is leaning toward a blanket prohibition, that a
facet's virtuals may never call each other. We might want to do that
in clause 27 too, for that matter. A review is necessary. Bill will
provide wording.</p>
<hr>
<a name="475"><h3>475. May the function object passed to for_each modify the elements of the iterated sequence?</h3></a><p><b>Section:</b> 25.1.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-algorithms.html#lib.alg.foreach"> [lib.alg.foreach]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Ready">Ready</a> <b>Submitter:</b> Stephan T. Lavavej, Jaakko Jarvi <b>Date:</b> 9 Jul 2004</p>
<p>
It is not clear whether the function object passed to for_each is allowed to
modify the elements of the sequence being iterated over.
</p>
<p>
for_each is classified without explanation in [lib.alg.nonmodifying], "25.1
Non-modifying sequence operations". 'Non-modifying sequence operation' is
never defined.
</p>
<p>
25(5) says: "If an algorithm's Effects section says that a value pointed to
by any iterator passed as an argument is modified, then that algorithm has
an additional type requirement: The type of that argument shall satisfy the
requirements of a mutable iterator (24.1)."
</p>
<p>for_each's Effects section does not mention whether arguments can be
modified:</p>
<blockquote>
"Effects: Applies f to the result of dereferencing every iterator in the
range [first, last), starting from first and proceeding to last - 1."
</blockquote>
<p>
Every other algorithm in [lib.alg.nonmodifying] is "really" non-modifying in
the sense that neither the algorithms themselves nor the function objects
passed to the algorithms may modify the sequences or elements in any way.
This DR affects only for_each.
</p>
<p>
We suspect that for_each's classification in "non-modifying sequence
operations" means that the algorithm itself does not inherently modify the
sequence or the elements in the sequence, but that the function object
passed to it may modify the elements it operates on.
</p>
<p>
The original STL document by Stepanov and Lee explicitly prohibited the
function object from modifying its argument.
The "obvious" implementation of for_each found in several standard library
implementations, however, does not impose this restriction.
As a result, we suspect that the use of for_each with function objects that modify
their arguments is wide-spread.
If the restriction was reinstated, all such code would become non-conforming.
Further, none of the other algorithms in the Standard
could serve the purpose of for_each (transform does not guarantee the order in
which its function object is called).
</p>
<p>
We suggest that the standard be clarified to explicitly allow the function object
passed to for_each modify its argument.</p>
<p><b>Proposed resolution:</b></p>
<p>Add a nonnormative note to the Effects in 25.1.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-algorithms.html#lib.alg.foreach"> [lib.alg.foreach]</a>: If
the type of 'first' satisfies the requirements of a mutable iterator,
'f' may apply nonconstant functions through the dereferenced iterators
passed to it.
</p>
<p><b>Rationale:</b></p>
<p>The LWG believes that nothing in the standard prohibits function
objects that modify the sequence elements. The problem is that
for_each is in a secion entitled "nonmutating algorithms", and the
title may be confusing. A nonnormative note should clarify that.</p>
<hr>
<a name="478"><h3>478. Should forward iterator requirements table have a line for r->m?</h3></a><p><b>Section:</b> 24.1.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iterators.html#lib.forward.iterators"> [lib.forward.iterators]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Ready">Ready</a> <b>Submitter:</b> Dave Abrahams <b>Date:</b> 11 Jul 2004</p>
<p>
The Forward Iterator requirements table contains the following:
</p>
<pre> expression return type operational precondition
semantics
========== ================== =========== ==========================
a->m U& if X is mutable, (*a).m pre: (*a).m is well-defined.
otherwise const U&
r->m U& (*r).m pre: (*r).m is well-defined.
</pre>
<p>The second line may be unnecessary. Paragraph 11 of
[lib.iterator.requirements] says:
</p>
<blockquote>
In the following sections, a and b denote values of type const X, n
denotes a value of the difference type Distance, u, tmp, and m
denote identifiers, r denotes a value of X&, t denotes a value of
value type T, o denotes a value of some type that is writable to
the output iterator.
</blockquote>
<p>
Because operators can be overloaded on an iterator's const-ness, the
current requirements allow iterators to make many of the operations
specified using the identifiers a and b invalid for non-const
iterators.</p>
<p>Related issue: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#477">477</a></p>
<p><b>Proposed resolution:</b></p>
<p>Remove the "r->m" line from the Forward Iterator requirements
table. Change</p>
<blockquote>
"const X"
</blockquote>
<p> to </p>
<blockquote>
"X or const X"
</blockquote>
<p>in paragraph 11 of [lib.iterator.requirements].</p>
<p><b>Rationale:</b></p>
<p>
This is a defect because it constrains an lvalue to returning a modifiable lvalue.
</p>
<hr>
<a name="479"><h3>479. Container requirements and placement new</h3></a><p><b>Section:</b> 23.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.container.requirements"> [lib.container.requirements]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Herb Sutter <b>Date:</b> 1 Aug 2004</p>
<p>Nothing in the standard appears to make this program ill-formed:</p>
<pre> struct C {
void* operator new( size_t s ) { return ::operator new( s ); }
// NOTE: this hides in-place and nothrow new
};
int main() {
vector<C> v;
v.push_back( C() );
}
</pre>
<p>Is that intentional? We should clarify whether or not we intended
to require containers to support types that define their own special
versions of <tt>operator new</tt>.</p>
<p><i>[
Lillehammer: A container will definitely never use this overridden
operator new, but whether it will fail to compile is unclear from the
standard. Are containers supposed to use qualified or unqualified
placement new? 20.4.1.1 is somewhat relevant, but the standard
doesn't make it completely clear whether containers have to use
Allocator::construct(). If containers don't use it, the details of how
containers use placement new are unspecified. That is the real bug,
but it needs to be fixed as part of the allocator overhaul. Weak
support that the eventual solution should make this code well formed.
]</i></p>
<p><b>Proposed resolution:</b></p>
<hr>
<a name="482"><h3>482. Swapping pairs</h3></a><p><b>Section:</b> 20.2.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-utilities.html#lib.pairs"> [lib.pairs]</a>, 25.2.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-algorithms.html#lib.alg.swap"> [lib.alg.swap]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Andrew Koenig <b>Date:</b> 14 Sep 2004</p>
<p>(Based on recent comp.std.c++ discussion)</p>
<p>Pair (and tuple) should specialize std::swap to work in terms of
std::swap on their components. For example, there's no obvious reason
why swapping two objects of type pair<vector<int>,
list<double> > should not take O(1).</p>
<p><b>Proposed resolution:</b></p>
<p><i>[Lillehammer: We agree it should be swappable. Howard will
provide wording.]</i></p>
<hr>
<a name="484"><h3>484. Convertible to T</h3></a><p><b>Section:</b> 24.1.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iterators.html#lib.input.iterators"> [lib.input.iterators]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Chris <b>Date:</b> 16 Sep 2004</p>
<p>From comp.std.c++:</p>
<p>
I note that given an input iterator a for type T,
then *a only has to be "convertable to T", not actually of type T.
</p>
<p>Firstly, I can't seem to find an exact definition of "convertable to T".
While I assume it is the obvious definition (an implicit conversion), I
can't find an exact definition. Is there one?</p>
<p>Slightly more worryingly, there doesn't seem to be any restriction on
the this type, other than it is "convertable to T". Consider two input
iterators a and b. I would personally assume that most people would
expect *a==*b would perform T(*a)==T(*b), however it doesn't seem that
the standard requires that, and that whatever type *a is (call it U)
could have == defined on it with totally different symantics and still
be a valid inputer iterator.</p>
<p>Is this a correct reading? When using input iterators should I write
T(*a) all over the place to be sure that the object i'm using is the
class I expect?</p>
<p>This is especially a nuisance for operations that are defined to be
"convertible to bool". (This is probably allowed so that
implementations could return say an int and avoid an unnessary
conversion. However all implementations I have seen simply return a
bool anyway. Typical implemtations of STL algorithms just write
things like <tt>while(a!=b && *a!=0)</tt>. But strictly
speaking, there are lots of types that are convertible to T but
that also overload the appropriate operators so this doesn't behave
as expected.</p>
<p>If we want to make code like this legal (which most people seem to
expect), then we'll need to tighten up what we mean by "convertible
to T".</p>
<p><b>Proposed resolution:</b></p>
<p><i>[Lillehammer: The first part is NAD, since "convertible" is
well-defined in core. The second part is basically about pathological
overloads. It's a minor problem but a real one. So leave open for
now, hope we solve it as part of iterator redesign.]</i></p>
<hr>
<a name="485"><h3>485. output iterator insufficently constrained</h3></a><p><b>Section:</b> 24.1.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iterators.html#lib.output.iterators"> [lib.output.iterators]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Chris <b>Date:</b> 13 Oct 2004</p>
<p>
The note on 24.1.2 Output iterators insufficently limits what can be
performed on output iterators. While it requires that each iterator is
progressed through only once and that each iterator is written to only
once, it does not require the following things:</p>
<p>Note: Here it is assumed that x is an output iterator of type X which
has not yet been assigned to.</p>
<p>a) That each value of the output iterator is written to:
The standard allows:
++x; ++x; ++x;
</p>
<p>
b) That assignments to the output iterator are made in order
X a(x); ++a; *a=1; *x=2; is allowed
</p>
<p>
c) Chains of output iterators cannot be constructed:
X a(x); ++a; X b(a); ++b; X c(b); ++c; is allowed, and under the current
wording (I believe) x,a,b,c could be written to in any order.
</p>
<p>I do not believe this was the intension of the standard?</p>
<p><b>Proposed resolution:</b></p>
<p><i>[Lillehammer: Real issue. There are lots of constraints we
intended but didn't specify. Should be solved as part of iterator
redesign.]</i></p>
<hr>
<a name="488"><h3>488. rotate throws away useful information</h3></a><p><b>Section:</b> 25.2.10 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-algorithms.html#lib.alg.rotate"> [lib.alg.rotate]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Howard Hinnant <b>Date:</b> 22 Nov 2004</p>
<p>
rotate takes 3 iterators: first, middle and last which point into a
sequence, and rearranges the sequence such that the subrange [middle,
last) is now at the beginning of the sequence and the subrange [first,
middle) follows. The return type is void.
</p>
<p>
In many use cases of rotate, the client needs to know where the
subrange [first, middle) starts after the rotate is performed. This
might look like:
</p>
<pre> rotate(first, middle, last);
Iterator i = advance(first, distance(middle, last));
</pre>
<p>
Unless the iterators are random access, the computation to find the
start of the subrange [first, middle) has linear complexity. However,
it is not difficult for rotate to return this information with
negligible additional computation expense. So the client could code:
</p>
<pre> Iterator i = rotate(first, middle, last);
</pre>
<p>
and the resulting program becomes significantly more efficient.
</p>
<p>
While the backwards compatibility hit with this change is not zero, it
is very small (similar to that of lwg <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#130">130</a>), and there is
a significant benefit to the change.
</p>
<p><b>Proposed resolution:</b></p>
<p>In 25p2, change:</p>
<pre> template<class ForwardIterator>
void rotate(ForwardIterator first, ForwardIterator middle,
ForwardIterator last);
</pre>
<p>to:</p>
<pre> template<class ForwardIterator>
ForwardIterator rotate(ForwardIterator first, ForwardIterator middle,
ForwardIterator last);
</pre>
<p>In 25.2.10, change:</p>
<pre> template<class ForwardIterator>
void rotate(ForwardIterator first, ForwardIterator middle,
ForwardIterator last);
</pre>
<p>to:</p>
<pre> template<class ForwardIterator>
ForwardIterator rotate(ForwardIterator first, ForwardIterator middle,
ForwardIterator last);
</pre>
<p>In 25.2.10 insert a new paragraph after p1:</p>
<blockquote>
<p><b>Returns</b>: <tt>first + (last - middle)</tt>.</p>
</blockquote>
<p><i>[
The LWG agrees with this idea, but has one quibble: we want to make
sure not to give the impression that the function "advance" is
actually called, just that the nth iterator is returned. (Calling
advance is observable behavior, since users can specialize it for
their own iterators.) Howard will provide wording.
]</i></p>
<p><i>[Howard provided wording for mid-meeting-mailing Jun. 2005.]</i></p>
<hr>
<a name="492"><h3>492. Invalid iterator arithmetic expressions</h3></a><p><b>Section:</b> 23 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.containers"> [lib.containers]</a>, 24 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-iterators.html#lib.iterators"> [lib.iterators]</a>, 25 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-algorithms.html#lib.algorithms"> [lib.algorithms]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Thomas Mang <b>Date:</b> 12 Dec 2004</p>
<p>Various clauses other than clause 25 make use of iterator arithmetic not
supported by the iterator category in question.
Algorithms in clause 25 are exceptional because of 25 [lib.algorithms],
paragraph 9, but this paragraph does not provide semantics to the
expression "iterator - n", where n denotes a value of a distance type
between iterators.</p>
<p>1) Examples of current wording:</p>
<p>Current wording outside clause 25:</p>
<p>
23.2.2.4 [lib.list.ops], paragraphs 19-21: "first + 1", "(i - 1)",
"(last - first)"
23.3.1.1 [lib.map.cons], paragraph 4: "last - first"
23.3.2.1 [lib.multimap.cons], paragraph 4: "last - first"
23.3.3.1 [lib.set.cons], paragraph 4: "last - first"
23.3.4.1 [lib.multiset.cons], paragraph 4: "last - first"
24.4.1 [lib.reverse.iterators], paragraph 1: "(i - 1)"
</p>
<p>
[Important note: The list is not complete, just an illustration. The
same issue might well apply to other paragraphs not listed here.]</p>
<p>None of these expressions is valid for the corresponding iterator
category.</p>
<p>Current wording in clause 25:</p>
<p>
25.1.1 [lib.alg.foreach], paragraph 1: "last - 1"
25.1.3 [lib.alg.find.end], paragraph 2: "[first1, last1 -
(last2-first2))"
25.2.8 [lib.alg.unique], paragraph 1: "(i - 1)"
25.2.8 [lib.alg.unique], paragraph 5: "(i - 1)"
</p>
<p>
However, current wording of 25 [lib.algorithms], paragraph 9 covers
neither of these four cases:</p>
<p>Current wording of 25 [lib.algorithms], paragraph 9:</p>
<p>
"In the description of the algorithms operator + and - are used for some
of the iterator categories for which they do not have to be defined. In
these cases the semantics of a+n is the same as that of</p>
<pre>{X tmp = a;
advance(tmp, n);
return tmp;
}
</pre>
<p>and that of b-a is the same as of return distance(a, b)"</p>
<p>
This paragrpah does not take the expression "iterator - n" into account,
where n denotes a value of a distance type between two iterators [Note:
According to current wording, the expression "iterator - n" would be
resolved as equivalent to "return distance(n, iterator)"]. Even if the
expression "iterator - n" were to be reinterpreted as equivalent to
"iterator + -n" [Note: This would imply that "a" and "b" were
interpreted implicitly as values of iterator types, and "n" as value of
a distance type], then 24.3.4/2 interfers because it says: "Requires: n
may be negative only for random access and bidirectional iterators.",
and none of the paragraphs quoted above requires the iterators on which
the algorithms operate to be of random access or bidirectional category.
</p>
<p>2) Description of intended behavior:</p>
<p>
For the rest of this Defect Report, it is assumed that the expression
"iterator1 + n" and "iterator1 - iterator2" has the semantics as
described in current 25 [lib.algorithms], paragraph 9, but applying to
all clauses. The expression "iterator1 - n" is equivalent to an
result-iterator for which the expression "result-iterator + n" yields an
iterator denoting the same position as iterator1 does. The terms
"iterator1", "iterator2" and "result-iterator" shall denote the value of
an iterator type, and the term "n" shall denote a value of a distance
type between two iterators.</p>
<p>
All implementations known to the author of this Defect Report comply
with these assumptions.
No impact on current code is expected.</p>
<p>3) Proposed fixes:</p>
<p>Change 25 [lib.algorithms], paragraph 9 to:</p>
<p>
"In the description of the algorithms operator + and - are used for some
of the iterator categories for which they do not have to be defined. In
this paragraph, a and b denote values of an iterator type, and n denotes
a value of a distance type between two iterators. In these cases the
semantics of a+n is the same as that of</p>
<pre>{X tmp = a;
advance(tmp, n);
return tmp;
}
</pre>
<p>,the semantics of a-n denotes the value of an iterator i for which the
following condition holds:
advance(i, n) == a,
and that of b-a is the same as of
return distance(a, b)".
</p>
<p>Comments to the new wording:</p>
<p>
a) The wording " In this paragraph, a and b denote values of an iterator
type, and n denotes a value of a distance type between two iterators."
was added so the expressions "b-a" and "a-n" are distinguished regarding
the types of the values on which they operate.
b) The wording ",the semantics of a-n denotes the value of an iterator i
for which the following condition holds: advance(i, n) == a" was added
to cover the expression 'iterator - n'. The wording "advance(i, n) == a"
was used to avoid a dependency on the semantics of a+n, as the wording
"i + n == a" would have implied. However, such a dependency might well
be deserved.
c) DR 225 is not considered in the new wording.
</p>
<p>
Proposed fixes regarding invalid iterator arithmetic expressions outside
clause 25:</p>
<p>
Either
a) Move modified 25 [lib.algorithms], paragraph 9 (as proposed above)
before any current invalid iterator arithmetic expression. In that case,
the first sentence of 25 [lib.algorithms], paragraph 9, need also to be
modified and could read: "For the rest of this International Standard,
...." / "In the description of the following clauses including this
...." / "In the description of the text below ..." etc. - anyways
substituting the wording "algorithms", which is a straight reference to
clause 25.
In that case, 25 [lib.algorithms] paragraph 9 will certainly become
obsolete.
Alternatively,
b) Add an appropiate paragraph similar to resolved 25 [lib.algorithms],
paragraph 9, to the beginning of each clause containing invalid iterator
arithmetic expressions.
Alternatively,
c) Fix each paragraph (both current wording and possible resolutions of
DRs) containing invalid iterator arithmetic expressions separately.
</p>
<p>5) References to other DRs:</p>
<p>
See DR 225.
See DR 237. The resolution could then also read "Linear in last -
first".
</p>
<p><b>Proposed resolution:</b></p>
<p><i>[Lillehammer: Minor issue, but real. We have a blanket statement
about this in 25/11. But (a) it should be in 17, not 25; and (b) it's
not quite broad enough, because there are some arithmetic expressions
it doesn't cover. Bill will provide wording.]</i></p>
<hr>
<a name="495"><h3>495. Clause 22 template parameter requirements</h3></a><p><b>Section:</b> 22 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-locales.html#lib.localization"> [lib.localization]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Ready">Ready</a> <b>Submitter:</b> Beman Dawes <b>Date:</b> 10 Jan 2005</p>
<p>It appears that there are no requirements specified for many of the
template parameters in clause 22. It looks like this issue has never
come up, except perhaps for Facet.</p>
<p>Clause 22 isn't even listed in 17.3.2.1 [lib.type.descriptions],
either, which is the wording that allows requirements on template
parameters to be identified by name.</p>
<p>So one issue is that 17.3.2.1 [lib.type.descriptions] Should be
changed to cover clause 22. A better change, which will cover us in
the future, would be to say that it applies to all the library
clauses. Then if a template gets added to any library clause we are
covered.</p>
<p>charT, InputIterator, and other names with requirements defined
elsewhere are fine, assuming the 17.3.2.1 [lib.type.descriptions] fix.
But there are a few template arguments names which I don't think have
requirements given elsewhere:</p>
<ul>
<li>internT and externT. The fix is to add wording saying that internT
and externT must meet the same requirements as template arguments
named charT.</li>
<li>stateT. I'm not sure about this one. There already is some wording,
but it seems a bit vague.</li>
<li>Intl. [lib.locale.moneypunct.byname] The fix for this one is to
rename "Intl" to "International". The name is important because other
text identifies the requirements for the name International but not
for Intl.</li>
</ul>
<p><b>Proposed resolution:</b></p>
<p>Change 17.3.2.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-intro.html#lib.type.descriptions"> [lib.type.descriptions]</a>, paragraph 1, from:</p>
<blockquote>
The Requirements subclauses may describe names that are used to
specify constraints on template arguments.153) These names are used in
clauses 20, 23, 25, and 26 to describe the types that may be supplied
as arguments by a C++ program when instantiating template components
from the library.
</blockquote>
<p>to:</p>
<blockquote>
The Requirements subclauses may describe names that are used to
specify constraints on template arguments.153) These names are used in
library clauses to describe the types that may be supplied as
arguments by a C++ program when instantiating template components from
the library.
</blockquote>
<p>In the front matter of class 22, locales, add:</p>
<blockquote>
Template parameter types internT and externT shall meet the
requirements of charT (described in 21 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-strings.html#lib.strings"> [lib.strings]</a>).
</blockquote>
<p><b>Rationale:</b></p>
<p>
Again, a blanket clause isn't blanket enough. Also, we've got a
couple of names that we don't have blanket requirement statements
for. The only issue is what to do about stateT. This wording is
thin, but probably adequate.</p>
<hr>
<a name="497"><h3>497. meaning of numeric_limits::traps for floating point types</h3></a><p><b>Section:</b> 18.2.1.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-support.html#lib.numeric.limits.members"> [lib.numeric.limits.members]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Ready">Ready</a> <b>Submitter:</b> Martin Sebor <b>Date:</b> 2 Mar 2005</p>
<p>18.2.1.2, p59 says this much about the traps member of numeric_limits:</p>
<blockquote>
<p>static const bool traps;<br>
-59- true if trapping is implemented for the type.204)
<br>
Footnote 204: Required by LIA-1.
</p>
</blockquote>
<p>It's not clear what is meant by "is implemented" here.</p>
<p>
In the context of floating point numbers it seems reasonable to expect
to be able to use traps to determine whether a program can "safely" use
infinity(), quiet_NaN(), etc., in arithmetic expressions, that is
without causing a trap (i.e., on UNIX without having to worry about
getting a signal). When traps is true, I would expect any of the
operations in section 7 of IEEE 754 to cause a trap (and my program
to get a SIGFPE). So, for example, on Alpha, I would expect traps
to be true by default (unless I compiled my program with the -ieee
option), false by default on most other popular architectures,
including IA64, MIPS, PA-RISC, PPC, SPARC, and x86 which require
traps to be explicitly enabled by the program.
</p>
<p>
Another possible interpretation of p59 is that traps should be true
on any implementation that supports traps regardless of whether they
are enabled by default or not. I don't think such an interpretation
makes the traps member very useful, even though that is how traps is
implemented on several platforms. It is also the only way to implement
traps on platforms that allow programs to enable and disable trapping
at runtime.
</p>
<p><b>Proposed resolution:</b></p>
<p>Change p59 to read:</p>
<blockquote>True if, at program startup, there exists a value of the type that
would cause an arithmetic operation using that value to trap.</blockquote>
<p><b>Rationale:</b></p>
<p>
Real issue, since trapping can be turned on and off. Unclear what a
static query can say about a dynamic issue. The real advice we should
give users is to use cfenv for these sorts of queries. But this new
proposed resolution is at least consistent and slightly better than
nothing.</p>
<hr>
<a name="498"><h3>498. Requirements for partition() and stable_partition() too strong</h3></a><p><b>Section:</b> 25.2.12 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-algorithms.html#lib.alg.partitions"> [lib.alg.partitions]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Sean Parent, Joe Gottman <b>Date:</b> 4 May 2005</p>
<p>
Problem:
The iterator requirements for partition() and stable_partition() [25.2.12]
are listed as BidirectionalIterator, however, there are efficient algorithms
for these functions that only require ForwardIterator that have been known
since before the standard existed. The SGI implementation includes these (see
<a href="http://www.sgi.com/tech/stl/partition.html">http://www.sgi.com/tech/stl/partition.html</a>
and
<a href="http://www.sgi.com/tech/stl/stable_partition.html">http://www.sgi.com/tech/stl/stable_partition.html</a>).
</p>
<p><b>Proposed resolution:</b></p>
<p>
Change 25.2.12 from </p>
<blockquote><pre>template<class BidirectionalIterator, class Predicate>
BidirectionalIterator partition(BidirectionalIterato r first,
BidirectionalIterator last,
Predicate pred);
</pre></blockquote>
<p>to </p>
<blockquote><pre>template<class ForwardIterator, class Predicate>
ForwardIterator partition(ForwardIterator first,
ForwardIterator last,
Predicate pred);
</pre></blockquote>
<p>Change the complexity from </p>
<blockquote><p>
At most (last - first)/2 swaps are done. Exactly (last - first)
applications of the predicate are done.
</p></blockquote>
<p>to </p>
<blockquote><p>
If ForwardIterator is a bidirectional_iterator, at most (last - first)/2
swaps are done; otherwise at most (last - first) swaps are done. Exactly
(last - first) applications of the predicate are done.
</p></blockquote>
<p><b>Rationale:</b></p>
Partition is a "foundation" algorithm useful in many contexts (like sorting
as just one example) - my motivation for extending it to include forward
iterators is slist - without this extension you can't partition an slist
(without writing your own partition). Holes like this in the standard
library weaken the argument for generic programming (ideally I'd be able
to provide a library that would refine std::partition() to other concepts
without fear of conflicting with other libraries doing the same - but
that is a digression). I consider the fact that partition isn't defined
to work for ForwardIterator a minor embarrassment.
<p><i>[Mont Tremblant: Moved to Open, request motivation and use cases
by next meeting. Sean provided further rationale by post-meeting
mailing.]</i></p>
<hr>
<a name="499"><h3>499. Std. doesn't seem to require stable_sort() to be stable!</h3></a><p><b>Section:</b> 25.3.1.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-algorithms.html#lib.stable.sort"> [lib.stable.sort]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a> <b>Submitter:</b> Prateek Karandikar <b>Date:</b> 12 Apr 2005</p>
<blockquote>
<p>
17.3.1.1 Summary</p>
<p>
1 The Summary provides a synopsis of the category, and introduces the
first-level subclauses. Each subclause also provides a summary, listing
the headers specified in the subclause and the library entities
provided in each header.
</p>
<p>
2 Paragraphs labelled "Note(s):" or "Example(s):" are informative,
other paragraphs are normative.
</p>
</blockquote>
<p>So this means that a "Notes" paragraph wouldn't be normative. </p>
<blockquote>
<p>
25.3.1.2 stable_sort
</p>
<pre>template<class RandomAccessIterator>
void stable_sort(RandomAccessIterat or first, RandomAccessIterator last);
template<class RandomAccessIterator, class Compare>
void stable_sort(RandomAccessIterat or first, RandomAccessIterator last, Compare comp);
</pre>
<p>
1 Effects: Sorts the elements in the range [first, last).
</p>
<p>
2 Complexity: It does at most N(log N)^2 (where N == last - first)
comparisons; if enough extra memory is available, it is N log N.
</p>
<p>
3 Notes: Stable: the relative order of the equivalent elements is
preserved.
</p>
</blockquote>
<p>
The Notes para is informative, and nowhere else is stability mentioned above.
</p>
<p>
Also, I just searched for the word "stable" in my copy of the Standard.
and the phrase "Notes: Stable: the relative order of the elements..."
is repeated several times in the Standard library clauses for
describing various functions. How is it that stability is talked about
in the informative paragraph? Or am I missing something obvious?
</p>
<p><b>Proposed resolution:</b></p>
<p>
</p>
<hr>
<a name="501"><h3>501. Proposal: strengthen guarantees of lib.comparisons</h3></a><p><b>Section:</b> 20.3.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-utilities.html#lib.comparisons"> [lib.comparisons]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a> <b>Submitter:</b> Me <anti_spam_email2003@yahoo.com> <b>Date:</b> 7 Jun 2005</p>
<blockquote>
"For templates greater, less, greater_equal, and less_equal,
the specializations for any pointer type yield a total order, even if
the built-in operators <, >, <=, >= do not."
</blockquote>
<p>
The standard should do much better than guarantee that these provide a
total order, it should guarantee that it can be used to test if memory
overlaps, i.e. write a portable memmove. You can imagine a platform
where the built-in operators use a uint32_t comparison (this tests for
overlap on this platform) but the less<T*> functor is allowed to be
defined to use a int32_t comparison. On this platform, if you use
std::less with the intent of making a portable memmove, comparison on
an array that straddles the 0x7FFFFFFF/0x8000000 boundary can give
incorrect results.
</p>
<p><b>Proposed resolution:</b></p>
<p>
Add a footnote to 20.3.3/8 saying:
</p>
<blockquote>
Given a p1 and p2 such that p1 points to N objects of type T and p2
points to M objects of type T. If [p1,p1+N) does not overlap [p2,p2+M),
less returns the same value when comparing all pointers in [p1,p1+N) to
all pointers in [p2,p2+M). Otherwise, there is a value Q and a value R
such that less returns the same value when comparing all pointers in
[p1,p1+Q) to all pointers in [p2,p2+R) and an opposite value when
comparing all pointers in [p1+Q,p1+N) to all pointers in [p2+R,p2+M).
For the sake of completeness, the null pointer value (4.10) for T is
considered to be an array of 1 object that doesn't overlap with any
non-null pointer to T. less_equal, greater, greater_equal, equal_to,
and not_equal_to give the expected results based on the total ordering
semantics of less. For T of void, treat it as having similar semantics
as T of char i.e. less<cv T*>(a, b) gives the same results as less<cv
void*>(a, b) which gives the same results as less<cv char*>((cv
char*)(cv void*)a, (cv char*)(cv void*)b).
</blockquote>
<p>
I'm also thinking there should be a footnote to 20.3.3/1 saying that if
A and B are similar types (4.4/4), comp<A>(a,b) returns the same value
as comp<B>(a,b) (where comp is less, less_equal, etc.). But this might
be problematic if there is some really funky operator overloading going
on that does different things based on cv (that should be undefined
behavior if somebody does that though). This at least should be
guaranteed for all POD types (especially pointers) that use the
built-in comparison operators.
</p>
<hr>
<a name="502"><h3>502. Proposition: Clarification of the interaction between a facet and an iterator</h3></a><p><b>Section:</b> 22.1.1.1.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-locales.html#lib.locale.category"> [lib.locale.category]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a> <b>Submitter:</b> Christopher Conrade Zseleghovski <b>Date:</b> 7 Jun 2005</p>
<p>
Motivation:
</p>
<p>
This requirement seems obvious to me, it is the essence of code modularity.
I have complained to Mr. Plauger that the Dinkumware library does not
observe this principle but he objected that this behaviour is not covered in
the standard.
</p>
<p><b>Proposed resolution:</b></p>
<p>
Append the following point to 22.1.1.1.1:
</p>
<p>
6. The implementation of a facet of Table 52 parametrized with an
InputIterator/OutputIterator should use that iterator only as character
source/sink respectively.
For a *_get facet, it means that the value received depends only on the
sequence of input characters and not on how they are accessed.
For a *_put facet, it means that the sequence of characters output depends
only on the value to be formatted and not of how the characters are stored.
</p>
<hr>
<a name="503"><h3>503. more on locales</h3></a><p><b>Section:</b> 22.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-locales.html#lib.locale.categories"> [lib.locale.categories]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a> <b>Submitter:</b> P.J. Plauger <b>Date:</b> 20 Jun 2005</p>
<p>
a) In 22.2.1.1 para. 2 we refer to "the instantiations required in Table
51" to refer to the facet *objects* associated with a locale. And we
almost certainly mean just those associated with the default or "C"
locale. Otherwise, you can't switch to a locale that enforces a different
mapping between narrow and wide characters, or that defines additional
uppercase characters.
</p>
<p>
b) 22.2.1.5 para. 3 (codecvt) has the same issues.
</p>
<p>
c) 22.2.1.5.2 (do_unshift) is even worse. It *forbids* the generation of
a homing sequence for the basic character set, which might very well need
one.
</p>
<p>
d) 22.2.1.5.2 (do_length) likewise dictates that the default mapping
between wide and narrow characters be taken as one-for-one.
</p>
<p>
e) 22.2.2 para. 2 (num_get/put) is both muddled and vacuous, as far as
I can tell. The muddle is, as before, calling Table 51 a list of
instantiations. But the constraint it applies seems to me to cover
*all* defined uses of num_get/put, so why bother to say so?
</p>
<p>
f) 22.2.3.1.2 para. 1(do_decimal_point) says "The required instantiations
return '.' or L'.'.) Presumably this means "as appropriate for the
character type. But given the vague definition of "required" earlier,
this overrules *any* change of decimal point for non "C" locales.
Surely we don't want to do that.
</p>
<p>
g) 22.2.3.1.2 para. 2 (do_thousands_sep) says "The required instantiations
return ',' or L','.) As above, this probably means "as appropriate for the
character type. But this overrules the "C" locale, which requires *no*
character ('\0') for the thousands separator. Even if we agree that we
don't mean to block changes in decimal point or thousands separator,
we should also eliminate this clear incompatibility with C.
</p>
<p>
h) 22.2.3.1.2 para. 2 (do_grouping) says "The required instantiations
return the empty string, indicating no grouping." Same considerations
as for do_decimal_point.
</p>
<p>
i) 22.2.4.1 para. 1 (collate) refers to "instantiations required in Table
51". Same bad jargon.
</p>
<p>
j) 22.2.4.1.2 para. 1 (do_compare) refers to "instantiations required
in Table 51". Same bad jargon.
</p>
<p>
k) 22.2.5 para. 1 (time_get/put) uses the same muddled and vacuous
as num_get/put.
</p>
<p>
l) 22.2.6 para. 2 (money_get/put) uses the same muddled and vacuous
as num_get/put.
</p>
<p>
m) 22.2.6.3.2 (do_pos/neg_format) says "The instantiations required
in Table 51 ... return an object of type pattern initialized to
{symbol, sign, none, value}." This once again *overrides* the "C"
locale, as well as any other locale."
</p>
<p>
3) We constrain the use_facet calls that can be made by num_get/put,
so why don't we do the same for money_get/put? Or for any of the
other facets, for that matter?
</p>
<p>
4) As an almost aside, we spell out when a facet needs to use the ctype
facet, but several also need to use a codecvt facet and we don't say so.
</p>
<p><b>Proposed resolution:</b></p>
<p>
</p>
<hr>
<a name="504"><h3>504. Integer types in pseudo-random number engine requirements</h3></a><p><b>Section:</b> TR1 5.1.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.rand.req"> [tr.rand.req]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Walter Brown <b>Date:</b> 3 Jul 2005</p>
<p>
In [tr.rand.req], Paragraph 2 states that "... s is a value of integral type,
g is an ... object returning values of unsigned integral type ..."
</p>
<p><b>Proposed resolution:</b></p>
<p>
In 5.1.1 [tr.rand.req], Paragraph 2 replace
</p>
<blockquote>
... s is a value of integral type, g is an lvalue of a type other than X that
defines a zero-argument function object returning values of <del>unsigned integral</del> type
<ins><tt>unsigned long int</tt></ins>,
...
</blockquote>
<p>
In 5.1.1 [tr.rand.seq], Table 16, replace in the line for X(s)
</p>
<blockquote>
creates an engine with the initial internal state
determined by <ins><tt>static_cast<unsigned long>(</tt></ins><tt><i>s</i></tt><ins><tt>)</tt></ins>
</blockquote>
<p><i>[
Mont Tremblant: Both s and g should be unsigned long.
This should refer to the constructor signatures. Jens provided wording post Mont Tremblant.
]</i></p>
<p><b>Rationale:</b></p>
<p>
Jens: Just requiring X(unsigned long) still makes it possible
for an evil library writer to also supply a X(int) that does something
unexpected. The wording above requires that X(s) always performs
as if X(unsigned long) would have been called. I believe that is
sufficient and implements our intentions from Mont Tremblant. I
see no additional use in actually requiring a X(unsigned long)
signature. u.seed(s) is covered by its reference to X(s), same
arguments.
</p>
<hr>
<a name="505"><h3>505. Result_type in random distribution requirements</h3></a><p><b>Section:</b> TR1 5.1.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.rand.req"> [tr.rand.req]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Ready">Ready</a> <b>Submitter:</b> Walter Brown <b>Date:</b> 3 Jul 2005</p>
<p>
Table 17: Random distribution requirements
</p>
<p>
Row 1 requires that each random distribution provide a nested type "input_type";
this type denotes the type of the values that the distribution consumes.
</p>
<p>
Inspection of all distributions in [tr.rand.dist] reveals that each distribution
provides a second typedef ("result_type") that denotes the type of the values the
distribution produces when called.
</p>
<p><b>Proposed resolution:</b></p>
<p>
It seems to me that this is also a requirement
for all distributions and should therefore be indicated as such via a new second
row to this table 17:
</p>
<table border="1" cellpadding="5">
<tbody><tr>
<td>X::result_type</td>
<td>T</td>
<td>---</td>
<td>compile-time</td>
</tr>
</tbody></table>
<hr>
<a name="506"><h3>506. Requirements of Distribution parameter for variate_generator</h3></a><p><b>Section:</b> TR1 5.1.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.rand.var"> [tr.rand.var]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Walter Brown <b>Date:</b> 3 Jul 2005</p>
<p>
Paragraph 3 requires that template argument U (which corresponds to template
parameter Engine) satisfy all uniform random number generator requirements.
However, there is no analogous requirement regarding the template argument
that corresponds to template parameter Distribution. We believe there should
be, and that it should require that this template argument satisfy all random
distribution requirements.
</p>
<p><b>Proposed resolution:</b></p>
<p>
Consequence 1: Remove the precondition clauses [tr.rand.var]/16 and /18.
</p>
<p>
Consequence 2: Add max() and min() functions to those distributions that
do not already have them.
</p>
<p><i>[
Mont Tremblant: Jens reccommends NAD, min/max not needed everywhere.
Marc supports having min and max to satisfy generic programming interface.
]</i></p>
<hr>
<a name="507"><h3>507. Missing requirement for variate_generator::operator()</h3></a><p><b>Section:</b> TR1 5.1.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.rand.var"> [tr.rand.var]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Ready">Ready</a> <b>Submitter:</b> Walter Brown <b>Date:</b> 3 Jul 2005</p>
<p>
Paragraph 11 of [tr.rand.var] equires that the member template
</p>
<blockquote><pre>template<class T> result_type operator() (T value);
</pre></blockquote>
<p>
return
</p>
<blockquote><pre>distribution()(e, value)
</pre></blockquote>
<p>
However, not all distributions have an operator() with a corresponding signature.
</p>
<p><b>Proposed resolution:</b></p>
<p>
We therefore recommend that we insert the following precondition before paragraph 11:
</p>
<blockquote>
Precondition: <tt>distribution().operator()(e,value)</tt> is well-formed.
</blockquote>
<hr>
<a name="508"><h3>508. Bad parameters for ranlux64_base_01</h3></a><p><b>Section:</b> TR1 5.1.5 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.rand.predef"> [tr.rand.predef]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Ready">Ready</a> <b>Submitter:</b> Walter Brown <b>Date:</b> 3 Jul 2005</p>
<p>
The fifth of these engines with predefined parameters, ranlux64_base_01,
appears to have an unintentional error for which there is a simple correction.
The two pre-defined subtract_with_carry_01 engines are given as:
</p>
<blockquote><pre>typedef subtract_with_carry_01<float, 24, 10, 24> ranlux_base_01;
typedef subtract_with_carry_01<double, 48, 10, 24> ranlux64_base_01;
</pre></blockquote>
<p>
We demonstrate below that ranlux64_base_01 fails to meet the intent of the
random number generation proposal, but that the simple correction to
</p>
<blockquote><pre>typedef subtract_with_carry_01<double, 48, 5, 12> ranlux64_base_01;
</pre></blockquote>
<p>
does meet the intent of defining well-known good parameterizations.
</p>
<p>
The ranlux64_base_01 engine as presented fails to meet the intent for
predefined engines, stated in proposal N1398 (section E):
</p>
<blockquote><p>
In order to make good random numbers available to a large number of library
users, this proposal not only defines generic random-number engines, but also
provides a number of predefined well-known good parameterizations for those.
</p></blockquote>
<p>
The predefined ranlux_base_01 engine has been proven [1,2,3] to have a very
long period and so meets this criterion. This property makes it suitable for
use in the excellent discard_block engines defined subsequently. The proof
of long period relies on the fact (proven in [1]) that 2**(w*r) - 2**(w*s)
+ 1 is prime (w, r, and s are template parameters to subtract_with_carry_01,
as defined in [tr.rand.eng.sub1]).
</p>
<p>
The ranlux64_base_01 engine as presented in [tr.rand.predef] uses w=48, r=24, s=10.
For these numbers, the combination 2**(w*r)-2**(w*s)+1 is non-prime (though
explicit factorization would be a challenge). In consequence, while it is
certainly possible for some seeding states that this engine would have a very
long period, it is not at all Òwell-knownÓ that this is the case. The intent
in the N1398 proposal involved the base of the ranlux64 engine, which finds heavy
use in the physics community. This is isomorphic to the predefined ranlux_base_01,
but exploits the ability of double variables to hold (at least) 48 bits of mantissa,
to deliver 48 random bits at a time rather than 24.
</p>
<p><b>Proposed resolution:</b></p>
<p>
To achieve this intended behavior, the correct template parameteriztion would be:
</p>
<blockquote><pre>typedef subtract_with_carry_01<double, 48, 5, 12> ranlux64_base_01;
</pre></blockquote>
<p>
The sequence of mantissa bits delivered by this is isomorphic (treating each
double as having the bits of two floats) to that delivered by ranlux_base_01.
</p>
<p>
<b>References:</b>
</p>
<ol>
<li>F. James, Comput. Phys. Commun. 60(1990) 329</li>
<li>G. Marsaglia and A. Zaman, Ann. Appl. Prob 1(1991) 462</li>
<li>M. Luscher, Comput. Phys. Commun. 79(1994) 100-110</li>
</ol>
<hr>
<a name="509"><h3>509. Uniform_int template parameters</h3></a><p><b>Section:</b> TR1 5.1.7.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.rand.dist.iunif"> [tr.rand.dist.iunif]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Walter Brown <b>Date:</b> 3 Jul 2005</p>
<p>
In [tr.rand.dist.iunif] the uniform_int distribution currently has a single
template parameter, IntType, used as the input_type and as the result_type
of the distribution. We believe there is no reason to conflate these types
in this way.
</p>
<p><b>Proposed resolution:</b></p>
<p>
We recommend that there be a second template parameter to
reflect the distributionÕs input_type, and that the existing first template
parameter continue to reflect (solely) the result_type:
</p>
<blockquote><pre>template< class IntType = int, UIntType = unsigned int >
class uniform_int
{
public:
// types
typedef UIntType input_type;
typedef IntType result_type;
</pre></blockquote>
<hr>
<a name="510"><h3>510. Input_type for bernoulli_distribution</h3></a><p><b>Section:</b> TR1 5.1.7.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.rand.dist.bern"> [tr.rand.dist.bern]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Walter Brown <b>Date:</b> 3 Jul 2005</p>
<p>
In [tr.rand.dist.bern] the distribution currently requires;
</p>
<blockquote><pre>typedef int input_type;
</pre></blockquote>
<p><b>Proposed resolution:</b></p>
<p>
We believe this is an unfortunate choice, and recommend instead:
</p>
<blockquote><pre>typedef unsigned int input_type;
</pre></blockquote>
<hr>
<a name="511"><h3>511. Input_type for binomial_distribution</h3></a><p><b>Section:</b> TR1 5.1.7.5 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.rand.dist.bin"> [tr.rand.dist.bin]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Walter Brown <b>Date:</b> 3 Jul 2005</p>
<p>
Unlike all other distributions in TR1, this binomial_distribution has an
implementation-defined input_type. We believe this is an unfortunate choice,
because it hinders users from writing portable code. It also hinders the
writing of compliance tests. We recommend instead:
</p>
<blockquote><pre>typedef RealType input_type;
</pre></blockquote>
<p>
While this choice is somewhat arbitrary (as it was for some of the other
distributions), we make this particular choice because (unlike all other
distributions) otherwise this template would not publish its RealType
argument and so users could not write generic code that accessed this
second template parameter. In this respect, the choice is consistent with
the other distributions in TR1.
</p>
<p>
We have two reasons for recommending that a real type be specified instead.
One reason is based specifically on characteristics of binomial distribution
implementations, while the other is based on mathematical characteristics of
probability distribution functions in general.
</p>
<p>
Implementations of binomial distributions commonly use Stirling approximations
for values in certain ranges. It is far more natural to use real values to
represent these approximations than it would be to use integral values to do
so. In other ranges, implementations reply on the Bernoulli distribution to
obtain values. While TR1Õs bernoulli_distribution::input_type is specified as
int, we believe this would be better specified as double.
</p>
<p>
This brings us to our main point: The notion of a random distribution rests
on the notion of a cumulative distribution function, which in turn mathematically
depends on a continuous dependent variable. Indeed, such a distribution function
would be meaningless if it depended on discrete values such as integersÑand this
remains true even if the distribution function were to take discrete steps.
</p>
<p>
Although this note is specifically about binomial_distribution::input_type,
we intend to recommend that all of the random distributionsÕ input_types be
specified as a real type (either a RealType template parameter, or double,
as appropriate).
</p>
<p>
Of the nine distributions in TR1, four already have this characteristic
(uniform_real, exponential_distribution, normal_distribution, and
gamma_distribution). We have already argued the case for the binomial the
remaining four distributions.
</p>
<p>
In the case of uniform_int, we believe that the calculations to produce an
integer result in a specified range from an integer in a different specified
range is best done using real arithmetic. This is because it involves a
product, one of whose terms is the ratio of the extents of the two ranges.
Without real arithmetic, the results become less uniform: some numbers become
more (or less) probable that they should be. This is, of course, undesireable
behavior in a uniform distribution.
</p>
<p>
Finally, we believe that in the case of the bernoulli_distribution (briefly
mentioned earlier), as well as the cases of the geometric_distribution and the
poisson_distribution, it would be far more natural to have a real input_type.
This is because the most natural computation involves the random number
delivered and the distributionÕs parameter p (in the case of bernoulli_distribution,
for example, the computation is a comparison against p), and p is already specified
in each case as having some real type.
</p>
<p><b>Proposed resolution:</b></p>
<blockquote><pre>typedef RealType input_type;
</pre></blockquote>
<hr>
<a name="512"></a><h3><a name="512">512. Seeding subtract_with_carry_01 from a single unsigned long</a></h3><p><b>Section:</b> TR1 5.1.4.4 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.rand.eng.sub1"> [tr.rand.eng.sub1]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a> <b>Submitter:</b> Walter Brown <b>Date:</b> 3 Jul 2005</p>
<p>
Paragraph 8 specifies the algorithm by which a subtract_with_carry_01 engine
is to be seeded given a single unsigned long. This algorithm is seriously
flawed in the case where the engine parameter w (also known as word_size)
exceeds 31 [bits]. The key part of the paragraph reads:
</p>
<blockquote>
sets x(-r) ... x(-1) to (lcg(1)*2**(-w)) mod 1
</blockquote>
<p>
and so forth.
</p>
<p>
Since the specified linear congruential engine, lcg, delivers numbers with
a maximum of 2147483563 (just a shade under 31 bits), then when w is, for
example, 48, each of the x(i) will be less than 2**-17. The consequence
is that roughly the first 400 numbers delivered will be conspicuously
close to either zero or one.
</p>
<p>
Unfortunately, this is not an innocuous flaw: One of the predefined engines
in [tr.rand.predef], namely ranlux64_base_01, has w = 48 and would exhibit
this poor behavior, while the original N1378 proposal states that these
pre-defined engines are intended to be of "known good properties."
</p>
<p><b>Proposed resolution:</b></p>
<p>
In 5.1.4.4 [tr.rand.eng.sub1], replace the "effects" clause for
void seed(unsigned long value = 19780503) by
</p>
<blockquote>
<i>Effects:</i> If <tt>value == 0</tt>, sets value to <tt>19780503</tt>. In any
case, <del>with a linear congruential generator <tt>lcg</tt>(i) having parameters
<tt><i>m<sub>lcg</sub></i> = 2147483563</tt>, <tt><i>a<sub>lcg</sub></i> = 40014</tt>,
<tt><i>c<sub>lcg</sub></i> = 0</tt>, and <tt><i>lcg</i>(0) = value</tt>,</del>
sets <ins>carry<tt>(-1)</tt> and</ins> <tt>x(-r) … x(-1)</tt>
<ins>as if executing</ins>
<blockquote><pre><ins>
linear_congruential<unsigned long, 40014, 0, 2147483563> lcg(value);
seed(lcg);
</ins></pre></blockquote>
<del>to <tt>(<i>lcg</i>(1) · 2<sup>-<i>w</i></sup>) mod 1
… (<i>lcg</i>(<i>r</i>) · 2<sup>-<i>w</i></sup>) mod 1</tt>,
respectively. If <tt><i>x</i>(-1) == 0</tt>, sets carry<tt>(-1) = 2<sup>-<i>w</i></sup></tt>,
else sets carry<tt>(-1) = 0</tt>.</del>
</blockquote>
<p><i>[
Jens provided revised wording post Mont Tremblant.
]</i></p>
<p><b>Rationale:</b></p>
<p>
Jens: I'm using an explicit type here, because fixing the
prose would probably not qualify for the (with issue <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#504">504</a> even
stricter) requirements we have for seed(Gen&).
</p>
<hr>
<a name="513"><h3>513. Size of state for subtract_with_carry_01</h3></a><p><b>Section:</b> TR1 5.1.4.4 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.rand.eng.sub1"> [tr.rand.eng.sub1]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Walter Brown <b>Date:</b> 3 Jul 2005</p>
<p>
Paragraph 3 begins:
</p>
<blockquote>
The size of the state is r.
</blockquote>
<p>
However, this is not quite consistent with the remainder of the paragraph
which specifies a total of nr+1 items in the textual representation of
the state. We recommend the sentence be corrected to match:
</p>
<blockquote>
The size of the state is nr+1.
</blockquote>
<p>
To give meaning to the coefficient n, it may be also desirable to move
nÕs definition from later in the paragraph. Either of the following
seem reasonable formulations:
</p>
<blockquote>
With n=..., the size of the state is nr+1.
</blockquote>
<blockquote>
The size of the state is nr+1, where n=... .
</blockquote>
<p>
</p>
<p><b>Proposed resolution:</b></p>
<p><i>[
Jens: I plead for "NAD" on the grounds that "size of state" is only
used as an argument for big-O complexity notation, thus
constant factors and additions don't count.
]</i></p>
<hr>
<a name="514"><h3>514. Size of state for subtract_with_carry</h3></a><p><b>Section:</b> TR1 5.1.4.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.rand.eng.sub"> [tr.rand.eng.sub]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Open">Open</a> <b>Submitter:</b> Walter Brown <b>Date:</b> 3 Jul 2005</p>
<p>
Paragraph 2 begins:
</p>
<blockquote>
The size of the state is r.
</blockquote>
<p>
However, the next sentence specifies a total of r+1 items in the textual
representation of the state, r specific xÕs as well as a specific carry.
This makes a total of r+1 items that constitute the size of the state,
rather than r.
</p>
<p><b>Proposed resolution:</b></p>
<p>
We recommend the sentence be corrected to match:
</p>
<blockquote>
The size of the state is r+1.
</blockquote>
<p><i>[
Jens: I plead for "NAD" on the grounds that "size of state" is only
used as an argument for big-O complexity notation, thus
constant factors and additions don't count.
]</i></p>
<hr>
<a name="515"><h3>515. Random number engine traits</h3></a><p><b>Section:</b> TR1 5.1.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.rand.synopsis"> [tr.rand.synopsis]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a> <b>Submitter:</b> Walter Brown <b>Date:</b> 3 Jul 2005</p>
<p>
To accompany the concept of a pseudo-random number engine as defined in Table 17,
we propose and recommend an adjunct template, engine_traits, to be declared in
[tr.rand.synopsis] as:
</p>
<blockquote><pre>template< class PSRE >
class engine_traits;
</pre></blockquote>
<p>
This templateÕs primary purpose would be as an aid to generic programming involving
pseudo-random number engines. Given only the facilities described in tr1, it would
be very difficult to produce any algorithms involving the notion of a generic engine.
The intent of this proposal is to provide, via engine_traits<>, sufficient
descriptive information to allow an algorithm to employ a pseudo-random number engine
without regard to its exact type, i.e., as a template parameter.
</p>
<p>
For example, today it is not possible to write an efficient generic function that
requires any specific number of random bits. More specifically, consider a
cryptographic application that internally needs 256 bits of randomness per call:
</p>
<blockquote><pre>template< class Eng, class InIter, class OutIter >
void crypto( Eng& e, InIter in, OutIter out );
</pre></blockquote>
<p>
Without knowning the number of bits of randomness produced per call to a provided
engine, the algorithm has no means of determining how many times to call the engine.
</p>
<p>
In a new section [tr.rand.eng.traits], we proposed to define the engine_traits
template as:
</p>
<blockquote><pre>template< class PSRE >
class engine_traits
{
static std::size_t bits_of_randomness = 0u;
static std::string name() { return "unknown_engine"; }
// TODO: other traits here
};
</pre></blockquote>
<p>
Further, each engine described in [tr.rand.engine] would be accompanied by a
complete specialization of this new engine_traits template.
</p>
<p><b>Proposed resolution:</b></p>
<p>
</p>
<hr>
<a name="516"><h3>516. Seeding subtract_with_carry_01 using a generator</h3></a><p><b>Section:</b> TR1 5.1.4.4 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.rand.eng.sub1"> [tr.rand.eng.sub1]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a> <b>Submitter:</b> Walter Brown <b>Date:</b> 3 Jul 2005</p>
<p>
Paragraph 6 says:
</p>
<blockquote>
... obtained by successive invocations of g, ...
</blockquote>
<p>
We recommend instead:
</p>
<blockquote>
... obtained by taking successive invocations of g mod 2**32, ...
</blockquote>
<p>
as the context seems to require only 32-bit quantities be used here.
</p>
<p><b>Proposed resolution:</b></p>
<p>
</p>
<hr>
<a name="517"><h3>517. Should include name in external representation</h3></a><p><b>Section:</b> TR1 5.1.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.rand.req"> [tr.rand.req]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a> <b>Submitter:</b> Walter Brown <b>Date:</b> 3 Jul 2005</p>
<p>
The last two rows of Table 16 deal with the i/o requirements of an engine,
specifying that the textual representation of an engineÕs state,
appropriately formatted, constitute the engineÕs external representation.
</p>
<p>
This seems adequate when an engineÕs type is known. However, it seems
inadequate in the context of generic code, where it becomes useful and
perhaps even necessary to determine an engineÕs type via input.
</p>
<p>
</p>
<p><b>Proposed resolution:</b></p>
<p>
We therefore recommend that, in each of these two rows of Table 16, the
text "textual representation" be expanded so as to read "engine name
followed by the textual representation."
</p>
<hr>
<a name="518"><h3>518. Are insert and erase stable for unordered_multiset and unordered_multimap?</h3></a><p><b>Section:</b> TR1 6.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.hash"> [tr.hash]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Review">Review</a> <b>Submitter:</b> Matt Austern <b>Date:</b> 3 Jul 2005</p>
<p>
Issue 371 deals with stability of multiset/multimap under insert and erase
(i.e. do they preserve the relative order in ranges of equal elements).
The same issue applies to unordered_multiset and unordered_multimap.
</p>
<p><b>Proposed resolution:</b></p>
<p>
</p>
<hr>
<a name="519"><h3>519. Data() undocumented</h3></a><p><b>Section:</b> TR1 6.2.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.array.array"> [tr.array.array]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#Ready">Ready</a> <b>Submitter:</b> Pete Becker <b>Date:</b> 3 Jul 2005</p>
<p>
<tt>array<>::data()</tt> is present in the class synopsis, but not documented.
</p>
<p><b>Proposed resolution:</b></p>
<p>
Add a new section, after 6.2.2.3:
</p>
<blockquote><pre>T* data()
const T* data() const;
</pre></blockquote>
<p>
<b>Returns:</b> <tt>elems</tt>.
</p>
<p>
Change 6.2.2.4/2 to:
</p>
<blockquote>
In the case where <tt>N == 0</tt>, <tt>begin() == end()</tt>. The return value
of <tt>data()</tt> is unspecified.
</blockquote>
<hr>
<a name="520"><h3>520. Result_of and pointers to data members</h3></a><p><b>Section:</b> TR1 3.6 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.func.bind"> [tr.func.bind]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a> <b>Submitter:</b> Pete Becker <b>Date:</b> 3 Jul 2005</p>
<p>
In the original proposal for binders, the return type of bind() when
called with a pointer to member data as it's callable object was
defined to be mem_fn(ptr); when Peter Dimov and I unified the
descriptions of the TR1 function objects we hoisted the descriptions
of return types into the INVOKE pseudo-function and into result_of.
Unfortunately, we left pointer to member data out of result_of, so
bind doesn't have any specified behavior when called with a pointer
to member data.
</p>
<p><b>Proposed resolution:</b></p>
<p><i>[
Pete and Peter will provide wording.
]</i></p>
<hr>
<a name="521"><h3>521. Garbled requirements for argument_type in reference_wrapper</h3></a><p><b>Section:</b> TR1 2.1.2 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.util.refwrp.refwrp"> [tr.util.refwrp.refwrp]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a> <b>Submitter:</b> Pete Becker <b>Date:</b> 3 Jul 2005</p>
<p>
2.1.2/3, second bullet item currently says that reference_wrapper<T> is
derived from unary_function<T, R> if T is:
</p>
<blockquote>
a pointer to member function type with cv-qualifier cv and no arguments;
the type T1 is cv T* and R is the return type of the pointer to member function;
</blockquote>
<p>
The type of T1 can't be cv T*, 'cause that's a pointer to a pointer to member
function. It should be a pointer to the class that T is a pointer to member of.
Like this:
</p>
<blockquote>
a pointer to a member function R T0::f() cv (where cv represents the member
function's cv-qualifiers); the type T1 is cv T0*
</blockquote>
<p>
Similarly, bullet item 2 in 2.1.2/4 should be:
</p>
<blockquote>
a pointer to a member function R T0::f(T2) cv (where cv represents the member
function's cv-qualifiers); the type T1 is cv T0*
</blockquote>
<p><b>Proposed resolution:</b></p>
<hr>
<a name="522"></a><h3><a name="522">522. Tuple doesn't define swap</a></h3><p><b>Section:</b> TR1 6.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.tuple"> [tr.tuple]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a> <b>Submitter:</b> Andy Koenig <b>Date:</b> 3 Jul 2005</p>
<p>
Tuple doesn't define swap(). It should.
</p>
<p><b>Proposed resolution:</b></p>
<hr>
<a name="523"><h3>523. regex case-insensitive character ranges are unimplementable as specified</h3></a><p><b>Section:</b> TR1 7 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.re"> [tr.re]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a> <b>Submitter:</b> Eric Niebler <b>Date:</b> 1 Jul 2005</p>
<p>
A problem with TR1 regex is currently being discussed on the Boost
developers list. It involves the handling of case-insensitive matching
of character ranges such as [Z-a]. The proper behavior (according to the
ECMAScript standard) is unimplementable given the current specification
of the TR1 regex_traits<> class template. John Maddock, the author of
the TR1 regex proposal, agrees there is a problem. The full discussion
can be found at http://lists.boost.org/boost/2005/06/28850.php (first
message copied below). We don't have any recommendations as yet.
</p>
<p>
-- Begin original message --
</p>
<p>
The situation of interest is described in the ECMAScript specification
(ECMA-262), section 15.10.2.15:
</p>
<p>
"Even if the pattern ignores case, the case of the two ends of a range
is significant in determining which characters belong to the range.
Thus, for example, the pattern /[E-F]/i matches only the letters E, F,
e, and f, while the pattern /[E-f]/i matches all upper and lower-case
ASCII letters as well as the symbols [, \, ], ^, _, and `."
</p>
<p>
A more interesting case is what should happen when doing a
case-insentitive match on a range such as [Z-a]. It should match z, Z,
a, A and the symbols [, \, ], ^, _, and `. This is not what happens with
Boost.Regex (it throws an exception from the regex constructor).
</p>
<p>
The tough pill to swallow is that, given the specification in TR1, I
don't think there is any effective way to handle this situation.
According to the spec, case-insensitivity is handled with
regex_traits<>::translate_nocase(CharT) -- two characters are equivalent
if they compare equal after both are sent through the translate_nocase
function. But I don't see any way of using this translation function to
make character ranges case-insensitive. Consider the difficulty of
detecting whether "z" is in the range [Z-a]. Applying the transformation
to "z" has no effect (it is essentially std::tolower). And we're not
allowed to apply the transformation to the ends of the range, because as
ECMA-262 says, "the case of the two ends of a range is significant."
</p>
<p>
So AFAICT, TR1 regex is just broken, as is Boost.Regex. One possible fix
is to redefine translate_nocase to return a string_type containing all
the characters that should compare equal to the specified character. But
this function is hard to implement for Unicode, and it doesn't play nice
with the existing ctype facet. What a mess!
</p>
<p>
-- End original message --
</p>
<p><i>[
John Maddock adds:
]</i></p>
<p>
One small correction, I have since found that ICU's regex package does
implement this correctly, using a similar mechanism to the current
TR1.Regex.
</p>
<p>
Given an expression [c1-c2] that is compiled as case insensitive it:
</p>
<p>
Enumerates every character in the range c1 to c2 and converts it to it's
case folded equivalent. That case folded character is then used a key to a
table of equivalence classes, and each member of the class is added to the
list of possible matches supported by the character-class. This second step
isn't possible with our current traits class design, but isn't necessary if
the input text is also converted to a case-folded equivalent on the fly.
</p>
<p>
ICU applies similar brute force mechanisms to character classes such as
[[:lower:]] and [[:word:]], however these are at least cached, so the impact
is less noticeable in this case.
</p>
<p>
Quick and dirty performance comparisons show that expressions such as
"[X-\\x{fff0}]+" are indeed very slow to compile with ICU (about 200 times
slower than a "normal" expression). For an application that uses a lot of
regexes this could have a noticeable performance impact. ICU also has an
advantage in that it knows the range of valid characters codes: code points
outside that range are assumed not to require enumeration, as they can not
be part of any equivalence class. I presume that if we want the TR1.Regex
to work with arbitrarily large character sets enumeration really does become
impractical.
</p>
<p>
Finally note that Unicode has:
</p>
<p>
Three cases (upper, lower and title).
One to many, and many to one case transformations.
Character that have context sensitive case translations - for example an
uppercase sigma has two different lowercase forms - the form chosen depends
on context(is it end of a word or not), a caseless match for an upper case
sigma should match either of the lower case forms, which is why case folding
is often approximated by tolower(toupper(c)).
</p>
<p>
Probably we need some way to enumerate character equivalence classes,
including digraphs (either as a result or an input), and some way to tell
whether the next character pair is a valid digraph in the current locale.
</p>
<p>
Hoping this doesn't make this even more complex that it was already,
</p>
<p><b>Proposed resolution:</b></p>
<hr>
<a name="524"><h3>524. regex named character classes and case-insensitivity don't mix</h3></a><p><b>Section:</b> TR1 7 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.re"> [tr.re]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a> <b>Submitter:</b> Eric Niebler <b>Date:</b> 1 Jul 2005</p>
<p>
This defect is also being discussed on the Boost developers list. The
full discussion can be found here:
http://lists.boost.org/boost/2005/07/29546.php
</p>
<p>
-- Begin original message --
</p>
<p>
Also, I may have found another issue, closely related to the one under
discussion. It regards case-insensitive matching of named character
classes. The regex_traits<> provides two functions for working with
named char classes: lookup_classname and isctype. To match a char class
such as [[:alpha:]], you pass "alpha" to lookup_classname and get a
bitmask. Later, you pass a char and the bitmask to isctype and get a
bool yes/no answer.
</p>
<p>
But how does case-insensitivity work in this scenario? Suppose we're
doing a case-insensitive match on [[:lower:]]. It should behave as if it
were [[:lower:][:upper:]], right? But there doesn't seem to be enough
smarts in the regex_traits interface to do this.
</p>
<p>
Imagine I write a traits class which recognizes [[:fubar:]], and the
"fubar" char class happens to be case-sensitive. How is the regex engine
to know that? And how should it do a case-insensitive match of a
character against the [[:fubar:]] char class? John, can you confirm this
is a legitimate problem?
</p>
<p>
I see two options:
</p>
<p>
1) Add a bool icase parameter to lookup_classname. Then,
lookup_classname( "upper", true ) will know to return lower|upper
instead of just upper.
</p>
<p>
2) Add a isctype_nocase function
</p>
<p>
I prefer (1) because the extra computation happens at the time the
pattern is compiled rather than when it is executed.
</p>
<p>
-- End original message --
</p>
<p>
For what it's worth, John has also expressed his preference for option
(1) above.
</p>
<p><b>Proposed resolution:</b></p>
<hr>
<a name="525"><h3>525. type traits definitions not clear</h3></a><p><b>Section:</b> TR1 4.5 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.meta.unary"> [tr.meta.unary]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a> <b>Submitter:</b> Robert Klarer <b>Date:</b> 11 Jul 2005</p>
<p>
It is not completely clear how the primary type traits deal with
cv-qualified types. And several of the secondary type traits
seem to be lacking a definition.
</p>
<p><b>Proposed resolution:</b></p>
<hr>
<a name="526"></a><h3><a name="526">526. Is it undefined if a function in the standard changes in parameters?</a></h3><p><b>Section:</b> 23.1.1 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lib-containers.html#lib.sequence.reqmts"> [lib.sequence.reqmts]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a> <b>Submitter:</b> Chris Jefferson <b>Date:</b> 14 Sep 2005</p>
<p>
Problem: There are a number of places in the C++ standard library where
it is possible to write what appear to be sensible ways of calling
functions, but which can cause problems in some (or all)
implementations, as they cause the values given to the function to be
changed in a way not specified in standard (and therefore not coded to
correctly work). These fall into two similar categories.
</p>
<p>
1) Parameters taken by const reference can be changed during execution
of the function
</p>
<p>
Examples:
</p>
<p>
Given std::vector<int> v:
</p>
<p>
v.insert(v.begin(), v[2]);
</p>
<p>
v[2] can be changed by moving elements of vector
</p>
<p>
Given std::list<int> l:
</p>
<p>
l.remove(*l.begin());
</p>
<p>
Will delete the first element, and then continue trying to access it.
This is particularily vicious, as it will appear to work in almost all
cases.
</p>
<p>
2) A range is given which changes during the execution of the function:
Similarly,
</p>
<p>
v.insert(v.begin(), v.begin()+4, v.begin()+6);
</p>
<p>
This kind of problem has been partly covered in some cases. For example
std::copy(first, last, result) states that result cannot be in the range
[first, last). However, does this cover the case where result is a
reverse_iterator built from some iterator in the range [first, last)?
Also, std::copy would still break if result was reverse_iterator(last +
1), yet this is not forbidden by the standard
</p>
<p>
Solution:
</p>
<p>
One option would be to try to more carefully limit the requirements of
each function. There are many functions which would have to be checked.
However as has been shown in the std::copy case, this may be difficult.
A simpler, more global option would be to somewhere insert text similar to:
</p>
<p>
If the execution of any function would change either any values passed
by reference or any value in any range passed to a function in a way not
defined in the definition of that function, the result is undefined.
</p>
<p>
Such code would have to at least cover chapters 23 and 25 (the sections
I read through carefully). I can see no harm on applying it to much of
the rest of the standard.
</p>
<p>
Some existing parts of the standard could be improved to fit with this,
for example the requires for 25.2.1 (Copy) could be adjusted to:
</p>
<p>
Requires: For each non-negative integer n < (last - first), assigning to
*(result + n) must not alter any value in the range [first + n, last).
</p>
<p>
However, this may add excessive complication.
</p>
<p>
One other benefit of clearly introducing this text is that it would
allow a number of small optimisations, such as caching values passed
by const reference.
</p>
<p><b>Proposed resolution:</b></p>
<p>
</p>
<hr>
<a name="527"><h3>527. tr1::bind has lost its Throws clause</h3></a><p><b>Section:</b> TR1 3.6.3 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.func.bind.bind"> [tr.func.bind.bind]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a> <b>Submitter:</b> Peter Dimov <b>Date:</b> 01 Oct 2005</p>
<p>
The original bind proposal gives the guarantee that tr1::bind(f, t1,
..., tN) does not throw when the copy constructors of f, t1, ..., tN
don't.
</p>
<p>
This guarantee is not present in the final version of TR1.
</p>
<p>
I'm pretty certain that we never removed it on purpose. Editorial omission? :-)
</p>
<p><b>Proposed resolution:</b></p>
<p>
</p>
<hr>
<a name="528"><h3>528. TR1: issue 6.19 vs 6.3.4.3/2 (and 6.3.4.5/2)</h3></a><p><b>Section:</b> TR1 6.3.4 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/tr1.html#tr.unord.unord"> [tr.unord.unord]</a> <b>Status:</b> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#New">New</a> <b>Submitter:</b> Paolo Carlini <b>Date:</b> 12 Oct 2005</p>
<p>
while implementing the resolution of issue 6.19 I'm noticing the
following: according to 6.3.4.3/2 (and 6.3.4.5/2), for unordered_set and
unordered_multiset:
</p>
<blockquote>
"The iterator and const_iterator types are both const types. It is
unspecified whether they are the same type"
</blockquote>
<p>
Now, according to the resolution of 6.19, we have overloads of insert
with hint and erase (single and range) both for iterator and
const_iterator, which, AFAICS, can be meaningful at the same time *only*
if iterator and const_iterator *are* in fact different types.
</p>
<p>
Then, iterator and const_iterator are *required* to be different types?
Or that is an unintended consequence? Maybe the overloads for plain
iterators should be added only to unordered_map and unordered_multimap?
Or, of course, I'm missing something?
</p>
<p><b>Proposed resolution:</b></p>
<p>
Add to 6.3.4.3p2 (and 6.3.4.5p2):
</p>
<p>
2 ... The iterator and const_iterator types are both <del>const</del>
<ins>constant</ins> iterator types.
It is unspecified whether they are the same type. <ins>If they are the
same type, those signatures that become otherwise indistinguishable
collapse into a single signature.</ins>
</p>
<p>----- End of document -----</p>
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