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<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Chapter 9. Containers</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><meta name="keywords" content=" ISO C++ , library "/><meta name="keywords" content=" ISO C++ , runtime , library "/><link rel="home" href="../index.html" title="The GNU C++ Library"/><link rel="up" href="bk01pt02.html" title="Part II. Standard Contents"/><link rel="prev" href="facets.html" title="Facets"/><link rel="next" href="associative.html" title="Associative"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 9.
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Containers
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</th></tr><tr><td align="left"><a accesskey="p" href="facets.html">Prev</a> </td><th width="60%" align="center">Part II.
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Standard Contents
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</th><td align="right"> <a accesskey="n" href="associative.html">Next</a></td></tr></table><hr/></div><div class="chapter" title="Chapter 9. Containers"><div class="titlepage"><div><div><h2 class="title"><a id="std.containers"/>Chapter 9.
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Containers
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<a id="id503211" class="indexterm"/>
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</h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl><dt><span class="section"><a href="containers.html#std.containers.sequences">Sequences</a></span></dt><dd><dl><dt><span class="section"><a href="containers.html#containers.sequences.list">list</a></span></dt><dd><dl><dt><span class="section"><a href="containers.html#sequences.list.size">list::size() is O(n)</a></span></dt></dl></dd><dt><span class="section"><a href="containers.html#containers.sequences.vector">vector</a></span></dt><dd><dl><dt><span class="section"><a href="containers.html#sequences.vector.management">Space Overhead Management</a></span></dt></dl></dd></dl></dd><dt><span class="section"><a href="associative.html">Associative</a></span></dt><dd><dl><dt><span class="section"><a href="associative.html#containers.associative.insert_hints">Insertion Hints</a></span></dt><dt><span class="section"><a href="associative.html#containers.associative.bitset">bitset</a></span></dt><dd><dl><dt><span class="section"><a href="associative.html#associative.bitset.size_variable">Size Variable</a></span></dt><dt><span class="section"><a href="associative.html#associative.bitset.type_string">Type String</a></span></dt></dl></dd></dl></dd><dt><span class="section"><a href="containers_and_c.html">Interacting with C</a></span></dt><dd><dl><dt><span class="section"><a href="containers_and_c.html#containers.c.vs_array">Containers vs. Arrays</a></span></dt></dl></dd></dl></div><div class="section" title="Sequences"><div class="titlepage"><div><div><h2 class="title"><a id="std.containers.sequences"/>Sequences</h2></div></div></div><div class="section" title="list"><div class="titlepage"><div><div><h3 class="title"><a id="containers.sequences.list"/>list</h3></div></div></div><div class="section" title="list::size() is O(n)"><div class="titlepage"><div><div><h4 class="title"><a id="sequences.list.size"/>list::size() is O(n)</h4></div></div></div><p>
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Yes it is, and that's okay. This is a decision that we preserved
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when we imported SGI's STL implementation. The following is
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quoted from <a class="link" href="http://www.sgi.com/tech/stl/FAQ.html">their FAQ</a>:
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</p><div class="blockquote"><blockquote class="blockquote"><p>
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The size() member function, for list and slist, takes time
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proportional to the number of elements in the list. This was a
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deliberate tradeoff. The only way to get a constant-time
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size() for linked lists would be to maintain an extra member
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variable containing the list's size. This would require taking
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extra time to update that variable (it would make splice() a
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linear time operation, for example), and it would also make the
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list larger. Many list algorithms don't require that extra
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word (algorithms that do require it might do better with
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vectors than with lists), and, when it is necessary to maintain
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an explicit size count, it's something that users can do
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themselves.
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</p><p>
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This choice is permitted by the C++ standard. The standard says
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that size() <span class="quote">“<span class="quote">should</span>”</span> be constant time, and
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<span class="quote">“<span class="quote">should</span>”</span> does not mean the same thing as
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<span class="quote">“<span class="quote">shall</span>”</span>. This is the officially recommended ISO
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wording for saying that an implementation is supposed to do
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something unless there is a good reason not to.
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</p><p>
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One implication of linear time size(): you should never write
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</p><pre class="programlisting">
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if (L.size() == 0)
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...
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</pre><p>
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Instead, you should write
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</p><pre class="programlisting">
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if (L.empty())
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...
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</pre></blockquote></div></div></div><div class="section" title="vector"><div class="titlepage"><div><div><h3 class="title"><a id="containers.sequences.vector"/>vector</h3></div></div></div><p>
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</p><div class="section" title="Space Overhead Management"><div class="titlepage"><div><div><h4 class="title"><a id="sequences.vector.management"/>Space Overhead Management</h4></div></div></div><p>
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In <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-04/msg00105.html">this
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message to the list</a>, Daniel Kostecky announced work on an
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alternate form of <code class="code">std::vector</code> that would support
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hints on the number of elements to be over-allocated. The design
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was also described, along with possible implementation choices.
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</p><p>
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The first two alpha releases were announced <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-07/msg00048.html">here</a>
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and <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-07/msg00111.html">here</a>.
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</p></div></div></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="facets.html">Prev</a> </td><td align="center"><a accesskey="u" href="bk01pt02.html">Up</a></td><td align="right"> <a accesskey="n" href="associative.html">Next</a></td></tr><tr><td align="left" valign="top">Facets </td><td align="center"><a accesskey="h" href="../index.html">Home</a></td><td align="right" valign="top"> Associative</td></tr></table></div></body></html>
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