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  24 <a name="compressed_pair"></a>Compressed_Pair</h2></div>
  25 <div><div class="authorgroup">
  26 <div class="author"><h3 class="author">
  27 <span class="firstname">Steve</span> <span class="surname">Cleary</span>
  28 </h3></div>
  29 <div class="author"><h3 class="author">
  30 <span class="firstname">Beman</span> <span class="surname">Dawes</span>
  31 </h3></div>
  32 <div class="author"><h3 class="author">
  33 <span class="firstname">Howard</span> <span class="surname">Hinnant</span>
  34 </h3></div>
  35 <div class="author"><h3 class="author">
  36 <span class="firstname">John</span> <span class="surname">Maddock</span>
  37 </h3></div>
  38 </div></div>
  39 <div><p class="copyright">Copyright &#169; 2000 Steve Cleary, Beman Dawes, Howard Hinnant &amp; John
  40       Maddock</p></div>
  41 <div><div class="legalnotice">
  42 <a name="compressed_pair.legal"></a><p>
  43         Distributed under the Boost Software License, Version 1.0. (See accompanying
  44         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>)
  45       </p>
  46 </div></div>
  47 </div>
  48 <hr>
  49 </div>
  50 <div class="toc">
  51 <p><b>Table of Contents</b></p>
  52 <dl class="toc">
  53 <dt><span class="section"><a href="compressed_pair.html#compressed_pair.overview">Overview</a></span></dt>
  54 <dt><span class="section"><a href="compressed_pair.html#compressed_pair.synopsis">Synopsis</a></span></dt>
  55 <dt><span class="section"><a href="compressed_pair.html#compressed_pair.acknowledgments">Acknowledgments</a></span></dt>
  56 </dl>
  57 </div>
  58 <div class="section">
  59 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  60 <a name="compressed_pair.overview"></a><a class="link" href="compressed_pair.html#compressed_pair.overview" title="Overview">Overview</a>
  61 </h2></div></div></div>
  62 <p>
  63       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>
  64       are defined inside <code class="computeroutput"><span class="keyword">namespace</span> <span class="identifier">boost</span></code>.
  65     </p>
  66 <p>
  67       The class <code class="computeroutput"><span class="identifier">compressed_pair</span></code> is
  68       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
  69       are empty classes, then the <span class="emphasis"><em>empty base-class optimisation</em></span>
  70       is applied to compress the size of the pair.
  71     </p>
  72 </div>
  73 <div class="section">
  74 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  75 <a name="compressed_pair.synopsis"></a><a class="link" href="compressed_pair.html#compressed_pair.synopsis" title="Synopsis">Synopsis</a>
  76 </h2></div></div></div>
  77 <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>
  78 <span class="keyword">class</span> <span class="identifier">compressed_pair</span>
  79 <span class="special">{</span>
  80 <span class="keyword">public</span><span class="special">:</span>
  81     <span class="keyword">typedef</span> <span class="identifier">T1</span>                                                 <span class="identifier">first_type</span><span class="special">;</span>
  82     <span class="keyword">typedef</span> <span class="identifier">T2</span>                                                 <span class="identifier">second_type</span><span class="special">;</span>
  83     <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>
  84     <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>
  85     <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>
  86     <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>
  87     <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>
  88     <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>
  89
  90              <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>
  91              <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>
  92     <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>
  93     <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>
  94
  95     <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>
  96
  97     <span class="identifier">first_reference</span>       <span class="identifier">first</span><span class="special">();</span>
  98     <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>
  99
 100     <span class="identifier">second_reference</span>       <span class="identifier">second</span><span class="special">();</span>
 101     <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>
 102
 103     <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>
 104 <span class="special">};</span>
 105 </pre>
 106 <p>
 107       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
 108       <code class="computeroutput"><span class="identifier">second</span><span class="special">()</span></code>.
 109       Note that not all member functions can be instantiated for all template parameter
 110       types. In particular <code class="computeroutput"><span class="identifier">compressed_pair</span></code>
 111       can be instantiated for reference and array types, however in these cases the
 112       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>
 113       are the same type, then there is only one version of the single-argument constructor,
 114       and this constructor initialises both values in the pair to the passed value.
 115     </p>
 116 <p>
 117       Note that if either member is a POD type, then that member is not zero-initialized
 118       by the <code class="computeroutput"><span class="identifier">compressed_pair</span></code> default
 119       constructor: it's up to you to supply an initial value for these types if you
 120       want them to have a default value.
 121     </p>
 122 <p>
 123       Note that <code class="computeroutput"><span class="identifier">compressed_pair</span></code> can
 124       not be instantiated if either of the template arguments is a union type, unless
 125       there is compiler support for <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_union</span></code>,
 126       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.
 127     </p>
 128 <p>
 129       Finally, a word of caution for Visual C++ 6 users: if either argument is an
 130       empty type, then assigning to that member will produce memory corruption, unless
 131       the empty type has a "do nothing" assignment operator defined. This
 132       is due to a bug in the way VC6 generates implicit assignment operators.
 133     </p>
 134 </div>
 135 <div class="section">
 136 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
 137 <a name="compressed_pair.acknowledgments"></a><a class="link" href="compressed_pair.html#compressed_pair.acknowledgments" title="Acknowledgments">Acknowledgments</a>
 138 </h2></div></div></div>
 139 <p>
 140       Based on contributions by Steve Cleary, Beman Dawes, Howard Hinnant and John
 141       Maddock.
 142     </p>
 143 <p>
 144       Maintained by <a href="mailto:john%40johnmaddock.co.uk" target="_top">John Maddock</a>.
 145     </p>
 146 </div>
 147 </div>
 148 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 149 <td align="left"><p><small>Last revised: December 10, 2019 at 00:21:47 GMT</small></p></td>
 150 <td align="right"><div class="copyright-footer"></div></td>
 151 </tr></table>
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