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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 "/><link rel="home" href="../spine.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.
Containers
</th></tr><tr><td align="left"><a accesskey="p" href="facets.html">Prev</a> </td><th width="60%" align="center">Part II.
Standard Contents
</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.
Containers
<a id="id478750" class="indexterm"/>
</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><dt><span class="section"><a href="containers.html#containers.sequences.vector">vector</a></span></dt></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></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>
Yes it is, and that's okay. This is a decision that we preserved
when we imported SGI's STL implementation. The following is
quoted from <a class="link" href="http://www.sgi.com/tech/stl/FAQ.html">their FAQ</a>:
</p><div class="blockquote"><blockquote class="blockquote"><p>
The size() member function, for list and slist, takes time
proportional to the number of elements in the list. This was a
deliberate tradeoff. The only way to get a constant-time
size() for linked lists would be to maintain an extra member
variable containing the list's size. This would require taking
extra time to update that variable (it would make splice() a
linear time operation, for example), and it would also make the
list larger. Many list algorithms don't require that extra
word (algorithms that do require it might do better with
vectors than with lists), and, when it is necessary to maintain
an explicit size count, it's something that users can do
themselves.
</p><p>
This choice is permitted by the C++ standard. The standard says
that size() <span class="quote">“<span class="quote">should</span>”</span> be constant time, and
<span class="quote">“<span class="quote">should</span>”</span> does not mean the same thing as
<span class="quote">“<span class="quote">shall</span>”</span>. This is the officially recommended ISO
wording for saying that an implementation is supposed to do
something unless there is a good reason not to.
</p><p>
One implication of linear time size(): you should never write
</p><pre class="programlisting">
if (L.size() == 0)
...
</pre><p>
Instead, you should write
</p><pre class="programlisting">
if (L.empty())
...
</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>
</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>
In <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-04/msg00105.html">this
message to the list</a>, Daniel Kostecky announced work on an
alternate form of <code class="code">std::vector</code> that would support
hints on the number of elements to be over-allocated. The design
was also described, along with possible implementation choices.
</p><p>
The first two alpha releases were announced <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-07/msg00048.html">here</a>
and <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-07/msg00111.html">here</a>.
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