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  25 <div class="section">
  26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  27 <a name="core.swap"></a><a class="link" href="swap.html" title="swap">swap</a>
  28 </h2></div></div></div>
  29 <div class="toc"><dl class="toc">
  30 <dt><span class="section"><a href="swap.html#core.swap.header_boost_core_swap_hpp">Header &lt;boost/core/swap.hpp&gt;</a></span></dt>
  31 <dt><span class="section"><a href="swap.html#core.swap.introduction">Introduction</a></span></dt>
  32 <dt><span class="section"><a href="swap.html#core.swap.rationale">Rationale</a></span></dt>
  33 <dt><span class="section"><a href="swap.html#core.swap.exception_safety">Exception Safety</a></span></dt>
  34 <dt><span class="section"><a href="swap.html#core.swap.requirements">Requirements</a></span></dt>
  35 <dt><span class="section"><a href="swap.html#core.swap.portability">Portability</a></span></dt>
  36 <dt><span class="section"><a href="swap.html#core.swap.credits">Credits</a></span></dt>
  37 </dl></div>
  38 <div class="simplesect">
  39 <div class="titlepage"><div><div><h3 class="title">
  40 <a name="idm46687073211952"></a>Authors</h3></div></div></div>
  41 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  42 <li class="listitem">
  43           Niels Dekker
  44         </li>
  45 <li class="listitem">
  46           Joseph Gauterin
  47         </li>
  48 <li class="listitem">
  49           Steven Watanabe
  50         </li>
  51 <li class="listitem">
  52           Eric Niebler
  53         </li>
  54 </ul></div>
  55 </div>
  56 <div class="section">
  57 <div class="titlepage"><div><div><h3 class="title">
  58 <a name="core.swap.header_boost_core_swap_hpp"></a><a class="link" href="swap.html#core.swap.header_boost_core_swap_hpp" title="Header &lt;boost/core/swap.hpp&gt;">Header &lt;boost/core/swap.hpp&gt;</a>
  59 </h3></div></div></div>
  60 <p>
  61         <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span><span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">left</span><span class="special">,</span> <span class="identifier">T</span><span class="special">&amp;</span>
  62         <span class="identifier">right</span><span class="special">);</span></code>
  63       </p>
  64 </div>
  65 <div class="section">
  66 <div class="titlepage"><div><div><h3 class="title">
  67 <a name="core.swap.introduction"></a><a class="link" href="swap.html#core.swap.introduction" title="Introduction">Introduction</a>
  68 </h3></div></div></div>
  69 <p>
  70         The template function <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span></code>
  71         allows the values of two variables to be swapped, using argument dependent
  72         lookup to select a specialized swap function if available. If no specialized
  73         swap function is available, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code>
  74         is used.
  75       </p>
  76 </div>
  77 <div class="section">
  78 <div class="titlepage"><div><div><h3 class="title">
  79 <a name="core.swap.rationale"></a><a class="link" href="swap.html#core.swap.rationale" title="Rationale">Rationale</a>
  80 </h3></div></div></div>
  81 <p>
  82         The generic <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code> function requires that the elements
  83         to be swapped are assignable and copy constructible. It is usually implemented
  84         using one copy construction and two assignments - this is often both unnecessarily
  85         restrictive and unnecessarily slow. In addition, where the generic swap implementation
  86         provides only the basic guarantee, specialized swap functions are often able
  87         to provide the no-throw exception guarantee (and it is considered best practice
  88         to do so where possible <a href="#ftn.core.swap.rationale.f0" class="footnote" name="core.swap.rationale.f0"><sup class="footnote">[1]</sup></a>.
  89       </p>
  90 <p>
  91         The alternative to using argument dependent lookup in this situation is to
  92         provide a template specialization of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code>
  93         for every type that requires a specialized swap. Although this is legal C++,
  94         no Boost libraries use this method, whereas many Boost libraries provide
  95         specialized swap functions in their own namespaces.
  96       </p>
  97 <p>
  98         <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span></code> also supports swapping built-in arrays.
  99         Note that <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code> originally did not do so, but a request
 100         to add an overload of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code>
 101         for built-in arrays has been accepted by the C++ Standards Committee<a href="#ftn.core.swap.rationale.f1" class="footnote" name="core.swap.rationale.f1"><sup class="footnote">[2]</sup></a>.
 102       </p>
 103 </div>
 104 <div class="section">
 105 <div class="titlepage"><div><div><h3 class="title">
 106 <a name="core.swap.exception_safety"></a><a class="link" href="swap.html#core.swap.exception_safety" title="Exception Safety">Exception Safety</a>
 107 </h3></div></div></div>
 108 <p>
 109         <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span></code> provides the same exception guarantee
 110         as the underlying swap function used, with one exception; for an array of
 111         type <code class="computeroutput"><span class="identifier">T</span><span class="special">[</span><span class="identifier">n</span><span class="special">]</span></code>, where
 112         <code class="computeroutput"><span class="identifier">n</span> <span class="special">&gt;</span>
 113         <span class="number">1</span></code> and the underlying swap function
 114         for <code class="computeroutput"><span class="identifier">T</span></code> provides the strong
 115         exception guarantee, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span></code>
 116         provides only the basic exception guarantee.
 117       </p>
 118 </div>
 119 <div class="section">
 120 <div class="titlepage"><div><div><h3 class="title">
 121 <a name="core.swap.requirements"></a><a class="link" href="swap.html#core.swap.requirements" title="Requirements">Requirements</a>
 122 </h3></div></div></div>
 123 <p>
 124         Either:
 125       </p>
 126 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 127 <li class="listitem">
 128             T must be assignable
 129           </li>
 130 <li class="listitem">
 131             T must be copy constructible
 132           </li>
 133 </ul></div>
 134 <p>
 135         Or:
 136       </p>
 137 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 138             A function with the signature <code class="computeroutput"><span class="identifier">swap</span><span class="special">(</span><span class="identifier">T</span><span class="special">&amp;,</span><span class="identifier">T</span><span class="special">&amp;)</span></code> is available via argument dependent
 139             lookup
 140           </li></ul></div>
 141 <p>
 142         Or:
 143       </p>
 144 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 145             A template specialization of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code>
 146             exists for T
 147           </li></ul></div>
 148 <p>
 149         Or:
 150       </p>
 151 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 152             T is a built-in array of swappable elements
 153           </li></ul></div>
 154 </div>
 155 <div class="section">
 156 <div class="titlepage"><div><div><h3 class="title">
 157 <a name="core.swap.portability"></a><a class="link" href="swap.html#core.swap.portability" title="Portability">Portability</a>
 158 </h3></div></div></div>
 159 <p>
 160         Several older compilers do not support argument dependent lookup. On these
 161         compilers <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span></code> will call <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code>,
 162         ignoring any specialized swap functions that could be found as a result of
 163         argument dependent lookup.
 164       </p>
 165 </div>
 166 <div class="section">
 167 <div class="titlepage"><div><div><h3 class="title">
 168 <a name="core.swap.credits"></a><a class="link" href="swap.html#core.swap.credits" title="Credits">Credits</a>
 169 </h3></div></div></div>
 170 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 171 <li class="listitem">
 172             <span class="bold"><strong>Niels Dekker</strong></span> - for implementing and
 173             documenting support for built-in arrays
 174           </li>
 175 <li class="listitem">
 176             <span class="bold"><strong>Joseph Gauterin</strong></span> - for the initial idea,
 177             implementation, tests, and documentation
 178           </li>
 179 <li class="listitem">
 180             <span class="bold"><strong>Steven Watanabe</strong></span> - for the idea to make
 181             <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span></code> less specialized than <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code>, thereby allowing the function
 182             to have the name 'swap' without introducing ambiguity
 183           </li>
 184 </ul></div>
 185 </div>
 186 <div class="footnotes">
 187 <br><hr style="width:100; text-align:left;margin-left: 0">
 188 <div id="ftn.core.swap.rationale.f0" class="footnote"><p><a href="#core.swap.rationale.f0" class="para"><sup class="para">[1] </sup></a>
 189           Scott Meyers, Effective C++ Third Edition, Item 25: "Consider support
 190           for a non-throwing swap"
 191         </p></div>
 192 <div id="ftn.core.swap.rationale.f1" class="footnote"><p><a href="#core.swap.rationale.f1" class="para"><sup class="para">[2] </sup></a>
 193           <a href="http://open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#809" target="_top">LWG
 194           Defect Report 809: std::swap should be overloaded for array types</a>
 195         </p></div>
 196 </div>
 197 </div>
 198 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 199 <td align="left"></td>
 200 <td align="right"><div class="copyright-footer">Copyright &#169; 2014 Peter Dimov<br>Copyright &#169; 2014 Glen Fernandes<br>Copyright &#169; 2014 Andrey Semashev<p>
 201         Distributed under the <a href="http://boost.org/LICENSE_1_0.txt" target="_top">Boost
 202         Software License, Version 1.0</a>.
 203       </p>
 204 </div></td>
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