ceph/src/boost/libs/type_traits/doc/html/boost_typetraits/reference/is_copy_assignable.html

   1 <html>
   2 <head>
   3 <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
   4 <title>is_copy_assignable</title>
   5 <link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
   6 <meta name="generator" content="DocBook XSL Stylesheets V1.77.1">
   7 <link rel="home" href="../../index.html" title="Chapter&#160;1.&#160;Boost.TypeTraits">
   8 <link rel="up" href="../reference.html" title="Alphabetical Reference">
   9 <link rel="prev" href="is_convertible.html" title="is_convertible">
  10 <link rel="next" href="is_copy_constructible.html" title="is_copy_constructible">
  11 </head>
  12 <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
  13 <table cellpadding="2" width="100%"><tr>
  14 <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
  15 <td align="center"><a href="../../../../../../index.html">Home</a></td>
  16 <td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
  17 <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
  18 <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
  19 <td align="center"><a href="../../../../../../more/index.htm">More</a></td>
  20 </tr></table>
  21 <hr>
  22 <div class="spirit-nav">
  23 <a accesskey="p" href="is_convertible.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="is_copy_constructible.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
  24 </div>
  25 <div class="section">
  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_typetraits.reference.is_copy_assignable"></a><a class="link" href="is_copy_assignable.html" title="is_copy_assignable">is_copy_assignable</a>
  28 </h3></div></div></div>
  29 <pre class="programlisting"><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>
  30 <span class="keyword">struct</span> <span class="identifier">is_copy_assignable</span> <span class="special">:</span> <span class="keyword">public</span> <em class="replaceable"><code><a class="link" href="integral_constant.html" title="integral_constant">true_type</a>-or-<a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code></em> <span class="special">{};</span>
  31 </pre>
  32 <p>
  33         <span class="bold"><strong>Inherits:</strong></span> If <code class="computeroutput"><span class="identifier">T</span></code>
  34         is <code class="computeroutput"><span class="identifier">CopyAssignable</span></code> (i.e. has
  35         an accessible explicit or implicit copy assignment operator), then inherits
  36         from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>,
  37         otherwise inherits from <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>.
  38         Type <code class="computeroutput"><span class="identifier">T</span></code> must be a complete
  39         type.
  40       </p>
  41 <p>
  42         In other words, inherits from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>
  43         only if copy assignment of <code class="computeroutput"><span class="identifier">T</span></code>
  44         from <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">T</span>
  45         <span class="special">&amp;</span></code> is not marked with <code class="computeroutput"><span class="special">=</span> <span class="keyword">delete</span></code>,
  46         <code class="computeroutput"><span class="identifier">T</span></code> does not derives from
  47         <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">noncopyable</span></code> and is not marked with <code class="computeroutput"><span class="identifier">BOOST_MOVABLE_BUT_NOT_COPYABLE</span><span class="special">(</span><span class="identifier">T</span><span class="special">)</span></code>.
  48       </p>
  49 <p>
  50         <span class="bold"><strong>Compiler Compatibility:</strong></span> Requires the C++11
  51         features <code class="computeroutput"><span class="keyword">decltype</span></code> and SFINAE-expressions
  52         for full support.
  53       </p>
  54 <p>
  55         If your compiler does not support C++11 deleted functions (<code class="computeroutput"><span class="special">=</span> <span class="keyword">delete</span></code>)
  56         or does not support SFINAE for the deleted assignments, then derive your
  57         classes from <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">noncopyable</span></code> or mark them with <code class="computeroutput"><span class="identifier">BOOST_MOVABLE_BUT_NOT_COPYABLE</span><span class="special">(</span><span class="identifier">T</span><span class="special">)</span></code> to show
  58         that class is non-assignable.
  59       </p>
  60 <p>
  61         Trait does not care about access modifiers, so if you see errors like this:
  62       </p>
  63 <pre class="programlisting"><span class="char">'T::operator=(const T&amp;)'</span> <span class="identifier">is</span> <span class="keyword">private</span>
  64 <span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">is_copy_assignable</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">:</span><span class="number">68</span><span class="special">:</span><span class="number">5</span><span class="special">:</span> <span class="identifier">error</span><span class="special">:</span> <span class="identifier">within</span> <span class="keyword">this</span> <span class="identifier">context</span>
  65 </pre>
  66 <p>
  67         then you are trying to call that macro for a structure with private assignment:
  68       </p>
  69 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">T</span> <span class="special">{</span>
  70     <span class="comment">// ...</span>
  71 <span class="keyword">private</span><span class="special">:</span>
  72     <span class="identifier">T</span> <span class="special">&amp;</span><span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="identifier">T</span> <span class="special">&amp;);</span>
  73     <span class="comment">// ...</span>
  74 <span class="special">};</span>
  75 </pre>
  76 <p>
  77         To fix that you must modify your structure, explicitly marking it as noncopyable
  78         (<code class="computeroutput"><span class="special">=</span> <span class="keyword">delete</span></code>,
  79         <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">noncopyable</span></code> or <code class="computeroutput"><span class="identifier">BOOST_MOVABLE_BUT_NOT_COPYABLE</span><span class="special">(</span><span class="identifier">T</span><span class="special">)</span></code>)
  80         or explicitly <a class="link" href="../user_defined.html" title="User Defined Specializations">specializing
  81         the trait</a>.
  82       </p>
  83 <p>
  84         <span class="bold"><strong>Header:</strong></span> <code class="computeroutput"> <span class="preprocessor">#include</span>
  85         <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">is_copy_assignable</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
  86         or <code class="computeroutput"> <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
  87       </p>
  88 </div>
  89 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
  90 <td align="left"></td>
  91 <td align="right"><div class="copyright-footer">Copyright &#169; 2000, 2011 Adobe Systems Inc, David Abrahams,
  92       Frederic Bron, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant,
  93       Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten
  94       Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert Stewart and Steven
  95       Watanabe<p>
  96         Distributed under the Boost Software License, Version 1.0. (See accompanying
  97         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>)
  98       </p>
  99 </div></td>
 100 </tr></table>
 101 <hr>
 102 <div class="spirit-nav">
 103 <a accesskey="p" href="is_convertible.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="is_copy_constructible.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
 104 </div>
 105 </body>
 106 </html>