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<div><h2 class="title">
<a name="compressed_pair"></a>Compressed_Pair</h2></div>
<div><div class="authorgroup">
<div class="author"><h3 class="author">
<span class="firstname">Steve</span> <span class="surname">Cleary</span>
</h3></div>
<div class="author"><h3 class="author">
<span class="firstname">Beman</span> <span class="surname">Dawes</span>
</h3></div>
<div class="author"><h3 class="author">
<span class="firstname">Howard</span> <span class="surname">Hinnant</span>
</h3></div>
<div class="author"><h3 class="author">
<span class="firstname">John</span> <span class="surname">Maddock</span>
</h3></div>
</div></div>
<div><p class="copyright">Copyright © 2000 Steve Cleary, Beman Dawes, Howard Hinnant &amp; John
Maddock</p></div>
<div><div class="legalnotice">
<a name="compressed_pair.legal"></a><p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="compressed_pair.html#compressed_pair.overview">Overview</a></span></dt>
<dt><span class="section"><a href="compressed_pair.html#compressed_pair.synopsis">Synopsis</a></span></dt>
<dt><span class="section"><a href="compressed_pair.html#compressed_pair.acknowledgments">Acknowledgments</a></span></dt>
</dl>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="compressed_pair.overview"></a><a class="link" href="compressed_pair.html#compressed_pair.overview" title="Overview">Overview</a>
</h2></div></div></div>
<p>
All of the contents of <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">compressed_pair</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
are defined inside <code class="computeroutput"><span class="keyword">namespace</span> <span class="identifier">boost</span></code>.
</p>
<p>
The class <code class="computeroutput"><span class="identifier">compressed_pair</span></code> is
very similar to <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code>, but if either of the template arguments
are empty classes, then the <span class="emphasis"><em>empty base-class optimisation</em></span>
is applied to compress the size of the pair.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="compressed_pair.synopsis"></a><a class="link" href="compressed_pair.html#compressed_pair.synopsis" title="Synopsis">Synopsis</a>
</h2></div></div></div>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T1</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">T2</span><span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">compressed_pair</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="keyword">typedef</span> <span class="identifier">T1</span> <span class="identifier">first_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">T2</span> <span class="identifier">second_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">call_traits</span><span class="special">&lt;</span><span class="identifier">first_type</span><span class="special">&gt;::</span><span class="identifier">param_type</span> <span class="identifier">first_param_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">call_traits</span><span class="special">&lt;</span><span class="identifier">second_type</span><span class="special">&gt;::</span><span class="identifier">param_type</span> <span class="identifier">second_param_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">call_traits</span><span class="special">&lt;</span><span class="identifier">first_type</span><span class="special">&gt;::</span><span class="identifier">reference</span> <span class="identifier">first_reference</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">call_traits</span><span class="special">&lt;</span><span class="identifier">second_type</span><span class="special">&gt;::</span><span class="identifier">reference</span> <span class="identifier">second_reference</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">call_traits</span><span class="special">&lt;</span><span class="identifier">first_type</span><span class="special">&gt;::</span><span class="identifier">const_reference</span> <span class="identifier">first_const_reference</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">call_traits</span><span class="special">&lt;</span><span class="identifier">second_type</span><span class="special">&gt;::</span><span class="identifier">const_reference</span> <span class="identifier">second_const_reference</span><span class="special">;</span>
<span class="identifier">compressed_pair</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">base</span><span class="special">()</span> <span class="special">{}</span>
<span class="identifier">compressed_pair</span><span class="special">(</span><span class="identifier">first_param_type</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">second_param_type</span> <span class="identifier">y</span><span class="special">);</span>
<span class="keyword">explicit</span> <span class="identifier">compressed_pair</span><span class="special">(</span><span class="identifier">first_param_type</span> <span class="identifier">x</span><span class="special">);</span>
<span class="keyword">explicit</span> <span class="identifier">compressed_pair</span><span class="special">(</span><span class="identifier">second_param_type</span> <span class="identifier">y</span><span class="special">);</span>
<span class="identifier">compressed_pair</span><span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="identifier">compressed_pair</span><span class="special">&amp;);</span>
<span class="identifier">first_reference</span> <span class="identifier">first</span><span class="special">();</span>
<span class="identifier">first_const_reference</span> <span class="identifier">first</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span>
<span class="identifier">second_reference</span> <span class="identifier">second</span><span class="special">();</span>
<span class="identifier">second_const_reference</span> <span class="identifier">second</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span>
<span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span><span class="identifier">compressed_pair</span><span class="special">&amp;</span> <span class="identifier">y</span><span class="special">);</span>
<span class="special">};</span>
</pre>
<p>
The two members of the pair can be accessed using the member functions <code class="computeroutput"><span class="identifier">first</span><span class="special">()</span></code> and
<code class="computeroutput"><span class="identifier">second</span><span class="special">()</span></code>.
Note that not all member functions can be instantiated for all template parameter
types. In particular <code class="computeroutput"><span class="identifier">compressed_pair</span></code>
can be instantiated for reference and array types, however in these cases the
range of constructors that can be used are limited. If types <code class="computeroutput"><span class="identifier">T1</span></code> and <code class="computeroutput"><span class="identifier">T2</span></code>
are the same type, then there is only one version of the single-argument constructor,
and this constructor initialises both values in the pair to the passed value.
</p>
<p>
Note that if either member is a POD type, then that member is not zero-initialized
by the <code class="computeroutput"><span class="identifier">compressed_pair</span></code> default
constructor: it's up to you to supply an initial value for these types if you
want them to have a default value.
</p>
<p>
Note that <code class="computeroutput"><span class="identifier">compressed_pair</span></code> can
not be instantiated if either of the template arguments is a union type, unless
there is compiler support for <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_union</span></code>,
or if <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_union</span></code> is specialised for the union type.
</p>
<p>
Finally, a word of caution for Visual C++ 6 users: if either argument is an
empty type, then assigning to that member will produce memory corruption, unless
the empty type has a "do nothing" assignment operator defined. This
is due to a bug in the way VC6 generates implicit assignment operators.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="compressed_pair.acknowledgments"></a><a class="link" href="compressed_pair.html#compressed_pair.acknowledgments" title="Acknowledgments">Acknowledgments</a>
</h2></div></div></div>
<p>
Based on contributions by Steve Cleary, Beman Dawes, Howard Hinnant and John
Maddock.
</p>
<p>
Maintained by <a href="mailto:john%40johnmaddock.co.uk" target="_top">John Maddock</a>.
</p>
</div>
</div>
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<td align="left"><p><small>Last revised: April 13, 2021 at 16:25:08 GMT</small></p></td>
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