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<html>
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<head>
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<title>GCC Bugs</title>
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</head>
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<body>
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<h1>GCC Bugs</h1>
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<p>The latest version of this document is always available at
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<a href="http://gcc.gnu.org/bugs.html">http://gcc.gnu.org/bugs.html</a>.</p>
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<hr />
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<h2>Table of Contents</h2>
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<ul>
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<li><a href="#report">Reporting Bugs</a>
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<ul>
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<li><a href="#need">What we need</a></li>
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<li><a href="#dontwant">What we DON'T want</a></li>
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<li><a href="#where">Where to post it</a></li>
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<li><a href="#detailed">Detailed bug reporting instructions</a></li>
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<li><a href="#gnat">Detailed bug reporting instructions for GNAT</a></li>
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<li><a href="#pch">Detailed bug reporting instructions when using a precompiled header</a></li>
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</ul>
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</li>
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<li><a href="#known">Frequently Reported Bugs in GCC</a>
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<ul>
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<li><a href="#cxx">C++</a>
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<ul>
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<li><a href="#missing">Missing features</a></li>
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<li><a href="#fixed34">Bugs fixed in the 3.4 series</a></li>
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</ul>
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</li>
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<li><a href="#fortran">Fortran</a></li>
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</ul>
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</li>
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<li><a href="#nonbugs">Non-bugs</a>
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<ul>
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<li><a href="#nonbugs_general">General</a></li>
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<li><a href="#nonbugs_c">C</a></li>
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<li><a href="#nonbugs_cxx">C++</a>
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<ul>
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<li><a href="#upgrading">Common problems when upgrading the compiler</a></li>
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</ul>
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</li>
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</ul>
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</li>
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</ul>
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<hr />
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<h1><a name="report">Reporting Bugs</a></h1>
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<p>The main purpose of a bug report is to enable us to fix the bug. The
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most important prerequisite for this is that the report must be complete
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and self-contained.</p>
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<p>Before you report a bug, please check the
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<a href="#known">list of well-known bugs</a> and, <strong>if possible,
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try a current development snapshot</strong>.
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If you want to report a bug with versions of GCC before 3.4 we strongly
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recommend upgrading to the current release first.</p>
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<p>Before reporting that GCC compiles your code incorrectly, please
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compile it with <code>gcc -Wall</code> and see whether this shows
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anything wrong with your code that could be the cause instead of a bug
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in GCC.</p>
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<h2>Summarized bug reporting instructions</h2>
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<p>After this summary, you'll find detailed bug reporting
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instructions, that explain how to obtain some of the information
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requested in this summary.</p>
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<h3><a name="need">What we need</a></h3>
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<p>Please include in your bug report all of the following items, the first
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three of which can be obtained from the output of <code>gcc -v</code>:</p>
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<ul>
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<li>the exact version of GCC;</li>
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<li>the system type;</li>
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<li>the options given when GCC was configured/built;</li>
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<li>the complete command line that triggers the bug;</li>
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<li>the compiler output (error messages, warnings, etc.); and</li>
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<li>the <em>preprocessed</em> file (<code>*.i*</code>) that triggers the
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bug, generated by adding <code>-save-temps</code> to the complete
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compilation command, or, in the case of a bug report for the GNAT front end,
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a complete set of source files (see below).</li>
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</ul>
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<h3><a name="dontwant">What we do <strong>not</strong> want</a></h3>
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<ul>
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<li>A source file that <code>#include</code>s header files that are left
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out of the bug report (see above)</li>
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<li>That source file and a collection of header files.</li>
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<li>An attached archive (tar, zip, shar, whatever) containing all
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(or some :-) of the above.</li>
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<li>A code snippet that won't cause the compiler to produce the
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exact output mentioned in the bug report (e.g., a snippet with just
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a few lines around the one that <b>apparently</b> triggers the bug,
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with some pieces replaced with ellipses or comments for extra
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obfuscation :-)</li>
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<li>The location (URL) of the package that failed to build (we won't
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download it, anyway, since you've already given us what we need to
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duplicate the bug, haven't you? :-)</li>
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<li>An error that occurs only some of the times a certain file is
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compiled, such that retrying a sufficient number of times results in
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a successful compilation; this is a symptom of a hardware problem,
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not of a compiler bug (sorry)</li>
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<li>Assembly files (<code>*.s</code>) produced by the compiler, or any
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binary files, such as object files, executables, core files, or
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precompiled header files</li>
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<li>Duplicate bug reports, or reports of bugs already fixed in the
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development tree, especially those that have already been reported
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as fixed last week :-)</li>
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<li>Bugs in the assembler, the linker or the C library. These are
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separate projects, with separate mailing lists and different bug
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reporting procedures</li>
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<li>Bugs in releases or snapshots of GCC not issued by the GNU
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Project. Report them to whoever provided you with the release</li>
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<li>Questions about the correctness or the expected behavior of
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certain constructs that are not GCC extensions. Ask them in forums
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dedicated to the discussion of the programming language</li>
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</ul>
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<h3><a name="where">Where to post it</a></h3>
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<p>Please submit your bug report directly to the
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<a href="http://gcc.gnu.org/bugzilla/">GCC bug database</a>.
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Alternatively, you can use the <code>gccbug</code> script that mails your bug
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report to the bug database.
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<br />
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Only if all this is absolutely impossible, mail all information to
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<a href="mailto:gcc-bugs@gcc.gnu.org">gcc-bugs@gcc.gnu.org</a>.</p>
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<h2><a name="detailed">Detailed bug reporting instructions</a></h2>
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<p>Please refer to the <a href="#gnat">next section</a> when reporting
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bugs in GNAT, the Ada compiler, or to the <a href="#pch">one after
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that</a> when reporting bugs that appear when using a precompiled header.</p>
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<p>In general, all the information we need can be obtained by
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collecting the command line below, as well as its output and the
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preprocessed file it generates.</p>
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<blockquote><p><code>gcc -v -save-temps <i>all-your-options
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source-file</i></code></p></blockquote>
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<p>The <b>only</b> excuses to not send us the preprocessed sources are
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(i) if you've found a bug in the preprocessor, (ii) if you've reduced
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the testcase to a small file that doesn't include any other file or
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(iii) if the bug appears only when using precompiled headers. If you
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can't post the preprocessed sources because they're proprietary code,
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then try to create a small file that triggers the same problem.</p>
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<p>Since we're supposed to be able to re-create the assembly output
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(extension <code>.s</code>), you usually should not include
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it in the bug report, although you may want to post parts of it to
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point out assembly code you consider to be wrong.</p>
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<p>Please avoid posting an archive (.tar, .shar or .zip); we generally
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need just a single file to reproduce the bug (the .i/.ii/.f preprocessed
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file), and, by storing it in an archive, you're just making our
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volunteers' jobs harder. Only when your bug report requires multiple
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source files to be reproduced should you use an archive. This is, for example,
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the case if you are using <code>INCLUDE</code> directives in Fortran code,
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which are not processed by the preprocessor, but the compiler. In that case,
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we need the main file and all <code>INCLUDE</code>d files. In any case,
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make sure the compiler version, error message, etc, are included in
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the body of your bug report as plain text, even if needlessly
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duplicated as part of an archive.</p>
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<h2><a name="gnat">Detailed bug reporting instructions for GNAT</a></h2>
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<p>See the <a href="#detailed">previous section</a> for bug reporting
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instructions for GCC language implementations other than Ada.</p>
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<p>Bug reports have to contain at least the following information in
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order to be useful:</p>
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<ul>
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<li>the exact version of GCC, as shown by "<code>gcc -v</code>";</li>
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<li>the system type;</li>
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<li>the options when GCC was configured/built;</li>
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<li>the exact command line passed to the <code>gcc</code> program
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triggering the bug
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(not just the flags passed to <code>gnatmake</code>, but
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<code>gnatmake</code> prints the parameters it passed to <code>gcc</code>)</li>
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<li>a collection of source files for reproducing the bug,
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preferably a minimal set (see below);</li>
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<li>a description of the expected behavior;</li>
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<li>a description of actual behavior.</li>
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</ul>
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<p>If your code depends on additional source files (usually package
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specifications), submit the source code for these compilation units in
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a single file that is acceptable input to <code>gnatchop</code>,
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i.e. contains no non-Ada text. If the compilation terminated
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normally, you can usually obtain a list of dependencies using the
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"<code>gnatls -d <i>main_unit</i></code>" command, where
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<code><i>main_unit</i></code> is the file name of the main compilation
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unit (which is also passed to <code>gcc</code>).</p>
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<p>If you report a bug which causes the compiler to print a bug box,
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include that bug box in your report, and do not forget to send all the
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source files listed after the bug box along with your report.</p>
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<p>If you use <code>gnatprep</code>, be sure to send in preprocessed
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sources (unless you have to report a bug in <code>gnatprep</code>).</p>
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<p>When you have checked that your report meets these criteria, please
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submit it according to our <a href="#where">generic instructions</a>.
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(If you use a mailing list for reporting, please include an
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"<code>[Ada]</code>" tag in the subject.)</p>
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<h2><a name="pch">Detailed bug reporting instructions when using a
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precompiled header</a></h2>
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<p>If you're encountering a bug when using a precompiled header, the
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first thing to do is to delete the precompiled header, and try running
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the same GCC command again. If the bug happens again, the bug doesn't
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really involve precompiled headers, please report it without using
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them by following the instructions <a href="#detailed">above</a>.</p>
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<p>If you've found a bug while <i>building</i> a precompiled header
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(for instance, the compiler crashes), follow the usual instructions
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<a href="#detailed">above</a>.</p>
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<p>If you've found a real precompiled header bug, what we'll need to
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reproduce it is the sources to build the precompiled header (as a
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single <code>.i</code> file), the source file that uses the
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precompiled header, any other headers that source file includes, and
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the command lines that you used to build the precompiled header and to
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use it.</p>
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<p>Please <strong>don't</strong> send us the actual precompiled
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header. It is likely to be very large and we can't use it to
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reproduce the problem.</p>
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<hr />
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<h1><a name="known">Frequently Reported Bugs in GCC</a></h1>
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<p>This is a list of bugs in GCC that are reported very often, but not
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yet fixed. While it is certainly better to fix bugs instead of documenting
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them, this document might save people the effort of writing a bug report
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when the bug is already well-known.</p>
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<p>There are many reasons why a reported bug doesn't get fixed.
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It might be difficult to fix, or fixing it might break compatibility.
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Often, reports get a low priority when there is a simple work-around.
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In particular, bugs caused by invalid code have a simple work-around:
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<em>fix the code</em>.</p>
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<hr />
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<h2><a name="cxx">C++</a></h2>
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<h3><a name="missing">Missing features</a></h3>
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<dl>
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<dt>The <code>export</code> keyword is not implemented.</dt>
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<dd><p>Most C++ compilers (G++ included) do not yet implement
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<code>export</code>, which is necessary for separate compilation of
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template declarations and definitions. Without <code>export</code>, a
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template definition must be in scope to be used. The obvious
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workaround is simply to place all definitions in the header
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itself. Alternatively, the compilation unit containing template
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definitions may be included from the header.</p></dd>
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</dl>
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<h3><a name="fixed34">Bugs fixed in the 3.4 series</a></h3>
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<p>The following bugs are present up to (and including) GCC 3.3.x.
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They have been fixed in 3.4.0.</p>
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<dl>
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<dt>Two-stage name-lookup.</dt>
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<dd><p>GCC did not implement two-stage name-lookup (also see
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<a href="#new34">below</a>).</p></dd>
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<dt>Covariant return types.</dt>
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<dd><p>GCC did not implement non-trivial covariant returns.</p></dd>
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<dt>Parse errors for "simple" code.</dt>
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<dd><p>GCC gave parse errors for seemingly simple code, such as</p>
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<blockquote><pre>
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struct A
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{
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A();
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A(int);
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};
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struct B
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{
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B(A);
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B(A,A);
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void foo();
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};
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A bar()
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{
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B b(A(),A(1)); // Variable b, initialized with two temporaries
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B(A(2)).foo(); // B temporary, initialized with A temporary
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return (A()); // return A temporary
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}
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</pre></blockquote>
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<p>Although being valid code, each of the three lines with a comment was
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rejected by GCC. The work-arounds for older compiler versions proposed
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below do not change the semantics of the programs at all.</p>
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<p>The problem in the first case was that GCC started to parse the
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declaration of <code>b</code> as a function called <code>b</code> returning
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<code>B</code>, taking a function returning <code>A</code> as an argument.
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When it encountered the <code>1</code>, it was too late. To show the
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compiler that this should be really an expression, a comma operator with
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a dummy argument could be used:</p>
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<blockquote><pre>
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B b((0,A()),A(1));
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</pre></blockquote>
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<p>The work-around for simpler cases like the second one was to add
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additional parentheses around the expressions that were mistaken as
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declarations:</p>
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<blockquote><pre>
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(B(A(2))).foo();
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</pre></blockquote>
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<p>In the third case, however, additional parentheses were causing
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the problems: The compiler interpreted <code>A()</code> as a function
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(taking no arguments, returning <code>A</code>), and <code>(A())</code>
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as a cast lacking an expression to be casted, hence the parse error.
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The work-around was to omit the parentheses:</p>
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<blockquote><pre>
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return A();
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</pre></blockquote>
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<p>This problem occurred in a number of variants; in <code>throw</code>
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statements, people also frequently put the object in parentheses.</p></dd>
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</dl>
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<hr />
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<h2><a name="fortran">Fortran</a></h2>
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<p>G77 bugs are documented in the G77 manual rather than
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explicitly listed here. Please see
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<a href="http://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/Trouble.html">Known Causes of
|
|
Trouble with GNU Fortran</a> in the G77 manual.</p>
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<hr />
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<h1><a name="nonbugs">Non-bugs</a></h1>
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<p>The following are not actually bugs, but are reported often
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enough to warrant a mention here.</p>
|
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<p>It is not always a bug in the compiler, if code which "worked" in a
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previous version, is now rejected. Earlier versions of GCC sometimes were
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less picky about standard conformance and accepted invalid source code.
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In addition, programming languages themselves change, rendering code
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invalid that used to be conforming (this holds especially for C++).
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In either case, you should update your code to match recent language
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standards.</p>
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<hr />
|
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<h2><a name="nonbugs_general">General</a></h2>
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<dl>
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<dt>Problems with floating point numbers - the
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<a href="http://gcc.gnu.org/PR323">most often reported non-bug</a>.</dt>
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<dd><p>In a number of cases, GCC appears to perform floating point
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computations incorrectly. For example, the C++ program</p>
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<blockquote><pre>
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#include <iostream>
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int main()
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{
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double a = 0.5;
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double b = 0.01;
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std::cout << (int)(a / b) << std::endl;
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return 0;
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}
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</pre></blockquote>
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<p>might print 50 on some systems and optimization levels, and 49 on
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others.</p>
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<p>This is the result of <em>rounding</em>: The computer cannot
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represent all real numbers exactly, so it has to use
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approximations. When computing with approximation, the computer needs
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to round to the nearest representable number.</p>
|
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<p>This is not a bug in the compiler, but an inherent limitation of
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the floating point types. Please study
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<a href="http://www.validlab.com/goldberg/paper.ps">this paper</a>
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for more information.</p></dd>
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</dl>
|
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<hr />
|
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<h2><a name="nonbugs_c">C</a></h2>
|
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<dl>
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<dt>Increment/decrement operator (<code>++</code>/<code>--</code>) not
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working as expected - a <a href="http://gcc.gnu.org/PR11751">problem with
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many variations</a>.</dt>
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<dd><p>The following expressions have unpredictable results:</p>
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<blockquote><pre>
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x[i]=++i
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foo(i,++i)
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i*(++i) /* special case with foo=="operator*" */
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std::cout << i << ++i /* foo(foo(std::cout,i),++i) */
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</pre></blockquote>
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<p>since the <code>i</code> without increment can be evaluated before or
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after <code>++i</code>.</p>
|
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<p>The C and C++ standards have the notion of "sequence points". Everything
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that happens between two sequence points happens in an unspecified order,
|
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but it has to happen after the first and before the second sequence point.
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The end of a statement and a function call are examples for sequence points,
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whereas assignments and the comma between function arguments are not.</p>
|
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<p>Modifying a value twice between two sequence points as shown in the
|
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following examples is even worse:</p>
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<blockquote><pre>
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i=++i
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foo(++i,++i)
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(++i)*(++i) /* special case with foo=="operator*" */
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std::cout << ++i << ++i /* foo(foo(std::cout,++i),++i) */
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</pre></blockquote>
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<p>This leads to undefined behavior (i.e. the compiler can do
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anything).</p></dd>
|
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|
|
|
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<dt>Casting does not work as expected when optimization is turned on.</dt>
|
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|
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<dd><p>This is often caused by a violation of aliasing rules, which are part
|
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of the ISO C standard. These rules say that a program is invalid if you try
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|
to access a variable through a pointer of an incompatible type. This is
|
|
happening in the following example where a short is accessed through a
|
|
pointer to integer (the code assumes 16-bit <code>short</code>s and 32-bit
|
|
<code>int</code>s):</p>
|
|
<blockquote><pre>
|
|
#include <stdio.h>
|
|
|
|
int main()
|
|
{
|
|
short a[2];
|
|
|
|
a[0]=0x1111;
|
|
a[1]=0x1111;
|
|
|
|
*(int *)a = 0x22222222; /* violation of aliasing rules */
|
|
|
|
printf("%x %x\n", a[0], a[1]);
|
|
return 0;
|
|
}
|
|
</pre></blockquote>
|
|
<p>The aliasing rules were designed to allow compilers more aggressive
|
|
optimization. Basically, a compiler can assume that all changes to variables
|
|
happen through pointers or references to variables of a type compatible to
|
|
the accessed variable. Dereferencing a pointer that violates the aliasing
|
|
rules results in undefined behavior.</p>
|
|
|
|
<p>In the case above, the compiler may assume that no access through an
|
|
integer pointer can change the array <code>a</code>, consisting of shorts.
|
|
Thus, <code>printf</code> may be called with the original values of
|
|
<code>a[0]</code> and <code>a[1]</code>. What really happens is up to
|
|
the compiler and may change with architecture and optimization level.</p>
|
|
|
|
<p>Recent versions of GCC turn on the option <code>-fstrict-aliasing</code>
|
|
(which allows alias-based optimizations) by default with <code>-O2</code>.
|
|
And some architectures then really print "1111 1111" as result. Without
|
|
optimization the executable will generate the "expected" output
|
|
"2222 2222".</p>
|
|
|
|
<p>To disable optimizations based on alias-analysis for faulty legacy code,
|
|
the option <code>-fno-strict-aliasing</code> can be used as a work-around.</p>
|
|
|
|
<p>The option <code>-Wstrict-aliasing</code> (which is included in
|
|
<code>-Wall</code>) warns about some - but not all - cases of violation
|
|
of aliasing rules when <code>-fstrict-aliasing</code> is active.</p>
|
|
|
|
<p>To fix the code above, you can use a <code>union</code> instead of a
|
|
cast (note that this is a GCC extension which might not work with other
|
|
compilers):</p>
|
|
<blockquote><pre>
|
|
#include <stdio.h>
|
|
|
|
int main()
|
|
{
|
|
union
|
|
{
|
|
short a[2];
|
|
int i;
|
|
} u;
|
|
|
|
u.a[0]=0x1111;
|
|
u.a[1]=0x1111;
|
|
|
|
u.i = 0x22222222;
|
|
|
|
printf("%x %x\n", u.a[0], u.a[1]);
|
|
return 0;
|
|
}
|
|
</pre></blockquote>
|
|
<p>Now the result will always be "2222 2222".</p>
|
|
|
|
<p>For some more insight into the subject, please have a look at
|
|
<a href="http://mail-index.NetBSD.org/tech-kern/2003/08/11/0001.html">this
|
|
article</a>.</p></dd>
|
|
|
|
|
|
<dt>Cannot use preprocessor directive in macro arguments.</dt>
|
|
<dd><p>Let me guess... you used an older version of GCC to compile code
|
|
that looks something like this:</p>
|
|
<blockquote><pre>
|
|
memcpy(dest, src,
|
|
#ifdef PLATFORM1
|
|
12
|
|
#else
|
|
24
|
|
#endif
|
|
);
|
|
</pre></blockquote>
|
|
<p>and you got a whole pile of error messages:</p>
|
|
<blockquote><pre>
|
|
test.c:11: warning: preprocessing directive not recognized within macro arg
|
|
test.c:11: warning: preprocessing directive not recognized within macro arg
|
|
test.c:11: warning: preprocessing directive not recognized within macro arg
|
|
test.c: In function `foo':
|
|
test.c:6: undefined or invalid # directive
|
|
test.c:8: undefined or invalid # directive
|
|
test.c:9: parse error before `24'
|
|
test.c:10: undefined or invalid # directive
|
|
</pre></blockquote>
|
|
|
|
<p>This is because your C library's <code><string.h></code> happens
|
|
to define <code>memcpy</code> as a macro - which is perfectly legitimate.
|
|
In recent versions of glibc, for example, <code>printf</code> is among those
|
|
functions which are implemented as macros.</p>
|
|
|
|
<p>Versions of GCC prior to 3.3 did not allow you to put <code>#ifdef</code>
|
|
(or any other preprocessor directive) inside the arguments of a macro. The
|
|
code therefore would not compile.</p>
|
|
|
|
<p>As of GCC 3.3 this kind of construct is always accepted and the
|
|
preprocessor will probably do what you expect, but see the manual for
|
|
detailed semantics.</p>
|
|
|
|
<p>However, this kind of code is not portable. It is "undefined behavior"
|
|
according to the C standard; that means different compilers may do
|
|
different things with it. It is always possible to rewrite code which
|
|
uses conditionals inside macros so that it doesn't. You could write
|
|
the above example</p>
|
|
<blockquote><pre>
|
|
#ifdef PLATFORM1
|
|
memcpy(dest, src, 12);
|
|
#else
|
|
memcpy(dest, src, 24);
|
|
#endif
|
|
</pre></blockquote>
|
|
<p>This is a bit more typing, but I personally think it's better style
|
|
in addition to being more portable.</p></dd>
|
|
|
|
|
|
<dt>Cannot initialize a static variable with <code>stdin</code>.</dt>
|
|
<dd><p>This has nothing to do with GCC, but people ask us about it a
|
|
lot. Code like this:</p>
|
|
|
|
<blockquote><pre>
|
|
#include <stdio.h>
|
|
|
|
FILE *yyin = stdin;
|
|
</pre></blockquote>
|
|
|
|
<p>will not compile with GNU libc, because <code>stdin</code> is not a
|
|
constant. This was done deliberately, to make it easier to maintain
|
|
binary compatibility when the type <code>FILE</code> needs to be changed.
|
|
It is surprising for people used to traditional Unix C libraries, but it
|
|
is permitted by the C standard.</p>
|
|
|
|
<p>This construct commonly occurs in code generated by old versions of
|
|
lex or yacc. We suggest you try regenerating the parser with a
|
|
current version of flex or bison, respectively. In your own code, the
|
|
appropriate fix is to move the initialization to the beginning of
|
|
main.</p>
|
|
|
|
<p>There is a common misconception that the GCC developers are
|
|
responsible for GNU libc. These are in fact two entirely separate
|
|
projects; please check the
|
|
<a href="http://www.gnu.org/software/libc/">GNU libc web pages</a>
|
|
for details.
|
|
</p></dd>
|
|
</dl>
|
|
|
|
<hr />
|
|
|
|
<h2><a name="nonbugs_cxx">C++</a></h2>
|
|
|
|
<dl>
|
|
<dt>Nested classes can access private members and types of the containing
|
|
class.</dt>
|
|
|
|
<dd><p>Defect report 45 clarifies that nested classes are members of the
|
|
class they are nested in, and so are granted access to private members of
|
|
that class.</p></dd>
|
|
|
|
<dt>G++ emits two copies of constructors and destructors.</dt>
|
|
|
|
<dd><p>In general there are <em>three</em> types of constructors (and
|
|
destructors).</p>
|
|
<ol>
|
|
<li>The complete object constructor/destructor.</li>
|
|
<li>The base object constructor/destructor.</li>
|
|
<li>The allocating constructor/deallocating destructor.</li>
|
|
</ol>
|
|
<p>The first two are different, when virtual base classes are involved.
|
|
</p></dd>
|
|
|
|
<dt>Global destructors are not run in the correct order.</dt>
|
|
|
|
<dd><p>Global destructors should be run in the reverse order of their
|
|
constructors <em>completing</em>. In most cases this is the same as
|
|
the reverse order of constructors <em>starting</em>, but sometimes it
|
|
is different, and that is important. You need to compile and link your
|
|
programs with <code>--use-cxa-atexit</code>. We have not turned this
|
|
switch on by default, as it requires a <code>cxa</code> aware runtime
|
|
library (<code>libc</code>, <code>glibc</code>, or equivalent).</p></dd>
|
|
|
|
<dt>Classes in exception specifiers must be complete types.</dt>
|
|
|
|
<dd><p>[15.4]/1 tells you that you cannot have an incomplete type, or
|
|
pointer to incomplete (other than <code><i>cv</i> void *</code>) in
|
|
an exception specification.</p></dd>
|
|
|
|
<dt>Exceptions don't work in multithreaded applications.</dt>
|
|
|
|
<dd><p>You need to rebuild g++ and libstdc++ with
|
|
<code>--enable-threads</code>. Remember, C++ exceptions are not like
|
|
hardware interrupts. You cannot throw an exception in one thread and
|
|
catch it in another. You cannot throw an exception from a signal
|
|
handler and catch it in the main thread.</p></dd>
|
|
|
|
<dt>Templates, scoping, and digraphs.</dt>
|
|
|
|
<dd><p>If you have a class in the global namespace, say named <code>X</code>,
|
|
and want to give it as a template argument to some other class, say
|
|
<code>std::vector</code>, then <code>std::vector<::X></code>
|
|
fails with a parser error.</p>
|
|
|
|
<p>The reason is that the standard mandates that the sequence
|
|
<code><:</code> is treated as if it were the token <code>[</code>.
|
|
(There are several such combinations of characters - they are called
|
|
<em>digraphs</em>.) Depending on the version, the compiler then reports
|
|
a parse error before the character <code>:</code> (the colon before
|
|
<code>X</code>) or a missing closing bracket <code>]</code>.</p>
|
|
|
|
<p>The simplest way to avoid this is to write <code>std::vector<
|
|
::X></code>, i.e. place a space between the opening angle bracket
|
|
and the scope operator.</p></dd>
|
|
|
|
|
|
<dt><a name="cxx_rvalbind">Copy constructor access check while
|
|
initializing a reference.</a></dt>
|
|
|
|
<dd><p>Consider this code:</p>
|
|
|
|
<blockquote><pre>
|
|
class A
|
|
{
|
|
public:
|
|
A();
|
|
|
|
private:
|
|
A(const A&); // private copy ctor
|
|
};
|
|
|
|
A makeA(void);
|
|
void foo(const A&);
|
|
|
|
void bar(void)
|
|
{
|
|
foo(A()); // error, copy ctor is not accessible
|
|
foo(makeA()); // error, copy ctor is not accessible
|
|
|
|
A a1;
|
|
foo(a1); // OK, a1 is a lvalue
|
|
}</pre></blockquote>
|
|
|
|
<p>Starting with GCC 3.4.0, binding an rvalue to a const reference requires
|
|
an accessible copy constructor. This might be surprising at first sight,
|
|
especially since most popular compilers do not correctly implement this
|
|
rule.</p>
|
|
|
|
<p>The C++ Standard says that a temporary object should be created in
|
|
this context and its contents filled with a copy of the object we are
|
|
trying to bind to the reference; it also says that the temporary copy
|
|
can be elided, but the semantic constraints (eg. accessibility) of the
|
|
copy constructor still have to be checked.</p>
|
|
|
|
<p>For further information, you can consult the following paragraphs of
|
|
the C++ standard: [dcl.init.ref]/5, bullet 2, sub-bullet 1, and
|
|
[class.temporary]/2.</p></dd>
|
|
</dl>
|
|
|
|
<h3><a name="upgrading">Common problems when upgrading the compiler</a></h3>
|
|
|
|
<h4>ABI changes</h4>
|
|
|
|
<p>The C++ application binary interface (ABI) consists of two
|
|
components: the first defines how the elements of classes are laid
|
|
out, how functions are called, how function names are mangled, etc;
|
|
the second part deals with the internals of the objects in libstdc++.
|
|
Although we strive for a non-changing ABI, so far we have had to
|
|
modify it with each major release. If you change your compiler to a
|
|
different major release <em>you must recompile all libraries that
|
|
contain C++ code</em>. If you fail to do so you risk getting linker
|
|
errors or malfunctioning programs. Some of our Java support libraries
|
|
also contain C++ code, so you might want to recompile all libraries to
|
|
be safe. It should not be necessary to recompile if you have changed
|
|
to a bug-fix release of the same version of the compiler; bug-fix
|
|
releases are careful to avoid ABI changes. See also the
|
|
<a href="http://gcc.gnu.org/onlinedocs/gcc/Compatibility.html">compatibility
|
|
section</a> of the GCC manual.</p>
|
|
|
|
<p>Remark: A major release is designated by a change to the first or second
|
|
component of the two- or three-part version number. A minor (bug-fix)
|
|
release is designated by a change to the third component only. Thus GCC
|
|
3.2 and 3.3 are major releases, while 3.3.1 and 3.3.2 are bug-fix releases
|
|
for GCC 3.3. With the 3.4 series we are introducing a new naming scheme;
|
|
the first release of this series is 3.4.0 instead of just 3.4.</p>
|
|
|
|
<h4>Standard conformance</h4>
|
|
|
|
<p>With each release, we try to make G++ conform closer to the ISO C++ standard
|
|
(available at
|
|
<a href="http://www.ncits.org/cplusplus.htm">http://www.ncits.org/cplusplus.htm</a>).
|
|
We have also implemented some of the core and library defect reports
|
|
(available at
|
|
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html">http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html</a>
|
|
&
|
|
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html">http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html</a>
|
|
respectively).</p>
|
|
|
|
<p>Non-conforming legacy code that worked with older versions of GCC may be
|
|
rejected by more recent compilers. There is no command-line switch to ensure
|
|
compatibility in general, because trying to parse standard-conforming and
|
|
old-style code at the same time would render the C++ frontend unmaintainable.
|
|
However, some non-conforming constructs are allowed when the command-line
|
|
option <code>-fpermissive</code> is used.</p>
|
|
|
|
<p>Two milestones in standard conformance are GCC 3.0 (including a major
|
|
overhaul of the standard library) and the 3.4.0 version (with its new C++
|
|
parser).</p>
|
|
|
|
<h4>New in GCC 3.0</h4>
|
|
|
|
<ul>
|
|
|
|
<li>The standard library is much more conformant, and uses the
|
|
<code>std::</code> namespace (which is now a real namespace, not an
|
|
alias for <code>::</code>).</li>
|
|
|
|
<li>The standard header files for the c library don't end with
|
|
<code>.h</code>, but begin with <code>c</code> (i.e.
|
|
<code><cstdlib></code> rather than <code><stdlib.h></code>).
|
|
The <code>.h</code> names are still available, but are deprecated.</li>
|
|
|
|
<li><code><strstream></code> is deprecated, use
|
|
<code><sstream></code> instead.</li>
|
|
|
|
<li><code>streambuf::seekoff</code> &
|
|
<code>streambuf::seekpos</code> are private, instead use
|
|
<code>streambuf::pubseekoff</code> &
|
|
<code>streambuf::pubseekpos</code> respectively.</li>
|
|
|
|
<li>If <code>std::operator << (std::ostream &, long long)</code>
|
|
doesn't exist, you need to recompile libstdc++ with
|
|
<code>--enable-long-long</code>.</li>
|
|
|
|
</ul>
|
|
|
|
<p>If you get lots of errors about things like <code>cout</code> not being
|
|
found, you've most likely forgotten to tell the compiler to look in the
|
|
<code>std::</code> namespace. There are several ways to do this:</p>
|
|
|
|
<ul>
|
|
|
|
<li>Say <code>std::cout</code> at the call. This is the most explicit
|
|
way of saying what you mean.</li>
|
|
|
|
<li>Say <code>using std::cout;</code> somewhere before the call. You
|
|
will need to do this for each function or type you wish to use from the
|
|
standard library.</li>
|
|
|
|
<li>Say <code>using namespace std;</code> somewhere before the call.
|
|
This is the quick-but-dirty fix. This brings the <em>whole</em> of the
|
|
<code>std::</code> namespace into scope. <em>Never</em> do this in a
|
|
header file, as every user of your header file will be affected by this
|
|
decision.</li>
|
|
|
|
</ul>
|
|
|
|
<h4><a name="new34">New in GCC 3.4.0</a></h4>
|
|
|
|
<p>The new parser brings a lot of improvements, especially concerning
|
|
name-lookup.</p>
|
|
|
|
<ul>
|
|
|
|
<li>The "implicit typename" extension got removed (it was already deprecated
|
|
since GCC 3.1), so that the following code is now rejected, see [14.6]:
|
|
<blockquote><pre>
|
|
template <typename> struct A
|
|
{
|
|
typedef int X;
|
|
};
|
|
|
|
template <typename T> struct B
|
|
{
|
|
A<T>::X x; // error
|
|
typename A<T>::X y; // OK
|
|
};
|
|
|
|
B<void> b;
|
|
</pre></blockquote></li>
|
|
|
|
<li>For similar reasons, the following code now requires the
|
|
<code>template</code> keyword, see [14.2]:
|
|
<blockquote><pre>
|
|
template <typename> struct A
|
|
{
|
|
template <int> struct X {};
|
|
};
|
|
|
|
template <typename T> struct B
|
|
{
|
|
typename A<T>::X<0> x; // error
|
|
typename A<T>::template X<0> y; // OK
|
|
};
|
|
|
|
B<void> b;
|
|
</pre></blockquote></li>
|
|
|
|
<li>We now have two-stage name-lookup, so that the following code is
|
|
rejected, see [14.6]/9:
|
|
<blockquote><pre>
|
|
template <typename T> int foo()
|
|
{
|
|
return i; // error
|
|
}
|
|
</pre></blockquote></li>
|
|
|
|
<li>This also affects members of base classes, see [14.6.2]:
|
|
<blockquote><pre>
|
|
template <typename> struct A
|
|
{
|
|
int i, j;
|
|
};
|
|
|
|
template <typename T> struct B : A<T>
|
|
{
|
|
int foo1() { return i; } // error
|
|
int foo2() { return this->i; } // OK
|
|
int foo3() { return B<T>::i; } // OK
|
|
int foo4() { return A<T>::i; } // OK
|
|
|
|
using A<T>::j;
|
|
int foo5() { return j; } // OK
|
|
};
|
|
</pre></blockquote></li>
|
|
|
|
</ul>
|
|
|
|
<p>In addition to the problems listed above, the manual contains a section on
|
|
<a href="http://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Misunderstandings.html">
|
|
Common Misunderstandings with GNU C++</a>.</p>
|
|
|
|
</body>
|
|
</html>
|
|
|
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|
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|
