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

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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_typetraits.reference.is_convertible"></a><a class="link" href="is_convertible.html" title="is_convertible">is_convertible</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">From</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">To</span><span class="special">&gt;</span>
  30 <span class="keyword">struct</span> <span class="identifier">is_convertible</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 an imaginary rvalue of type
  34         <code class="computeroutput"><span class="identifier">From</span></code> is convertible to type
  35         <code class="computeroutput"><span class="identifier">To</span></code> then inherits from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>,
  36         otherwise inherits from <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>.
  37       </p>
  38 <p>
  39         Type From must not be an incomplete type.
  40       </p>
  41 <p>
  42         Type To must not be an incomplete, or function type.
  43       </p>
  44 <p>
  45         No types are considered to be convertible to array types or abstract-class
  46         types.
  47       </p>
  48 <p>
  49         This template can not detect whether a converting-constructor is <code class="computeroutput"><span class="keyword">public</span></code> or not: if type <code class="computeroutput"><span class="identifier">To</span></code>
  50         has a <code class="computeroutput"><span class="keyword">private</span></code> converting constructor
  51         from type <code class="computeroutput"><span class="identifier">From</span></code> then instantiating
  52         <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special">&lt;</span><span class="identifier">From</span><span class="special">,</span> <span class="identifier">To</span><span class="special">&gt;</span></code>
  53         will produce a compiler error. For this reason <code class="computeroutput"><span class="identifier">is_convertible</span></code>
  54         can not be used to determine whether a type has a <code class="computeroutput"><span class="keyword">public</span></code>
  55         copy-constructor or not.
  56       </p>
  57 <p>
  58         This template will also produce compiler errors if the conversion is ambiguous,
  59         for example:
  60       </p>
  61 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{};</span>
  62 <span class="keyword">struct</span> <span class="identifier">B</span> <span class="special">:</span> <span class="identifier">A</span> <span class="special">{};</span>
  63 <span class="keyword">struct</span> <span class="identifier">C</span> <span class="special">:</span> <span class="identifier">A</span> <span class="special">{};</span>
  64 <span class="keyword">struct</span> <span class="identifier">D</span> <span class="special">:</span> <span class="identifier">B</span><span class="special">,</span> <span class="identifier">C</span> <span class="special">{};</span>
  65 <span class="comment">// This produces a compiler error, the conversion is ambiguous:</span>
  66 <span class="keyword">bool</span> <span class="keyword">const</span> <span class="identifier">y</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_convertible</span><span class="special">&lt;</span><span class="identifier">D</span><span class="special">*,</span><span class="identifier">A</span><span class="special">*&gt;::</span><span class="identifier">value</span><span class="special">;</span>
  67 </pre>
  68 <p>
  69         <span class="bold"><strong>C++ Standard Reference:</strong></span> 4 and 8.5.
  70       </p>
  71 <p>
  72         <span class="bold"><strong>Compiler Compatibility:</strong></span> All current compilers
  73         are supported by this trait.
  74       </p>
  75 <p>
  76         <span class="bold"><strong>Header:</strong></span> <code class="computeroutput"> <span class="preprocessor">#include</span>
  77         <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_convertible</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
  78         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>
  79       </p>
  80 <p>
  81         <span class="bold"><strong>Examples:</strong></span>
  82       </p>
  83 <div class="blockquote"><blockquote class="blockquote"><p>
  84           <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;</span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
  85         </p></blockquote></div>
  86 <div class="blockquote"><blockquote class="blockquote"><p>
  87           <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special">&lt;</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
  88           is the type <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
  89         </p></blockquote></div>
  90 <div class="blockquote"><blockquote class="blockquote"><p>
  91           <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">*</span> <span class="keyword">const</span><span class="special">,</span> <span class="keyword">int</span><span class="special">*&gt;::</span><span class="identifier">value</span></code> is an integral constant expression
  92           that evaluates to <span class="emphasis"><em>true</em></span>.
  93         </p></blockquote></div>
  94 <div class="blockquote"><blockquote class="blockquote"><p>
  95           <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="keyword">const</span><span class="special">*,</span> <span class="keyword">int</span><span class="special">*&gt;::</span><span class="identifier">value</span></code> is an integral constant expression
  96           that evaluates to <span class="emphasis"><em>false</em></span>: the conversion would require
  97           a <code class="computeroutput"><span class="keyword">const_cast</span></code>.
  98         </p></blockquote></div>
  99 <div class="blockquote"><blockquote class="blockquote"><p>
 100           <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="keyword">const</span><span class="special">&amp;,</span> <span class="keyword">long</span><span class="special">&gt;::</span><span class="identifier">value</span></code>
 101           is an integral constant expression that evaluates to <span class="emphasis"><em>true</em></span>.
 102         </p></blockquote></div>
 103 <div class="blockquote"><blockquote class="blockquote"><p>
 104           <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">value</span></code>
 105           is an integral constant expression that evaluates to <span class="emphasis"><em>true</em></span>.
 106         </p></blockquote></div>
 107 <div class="blockquote"><blockquote class="blockquote"><p>
 108           <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value_type</span></code>
 109           is the type <code class="computeroutput"><span class="keyword">bool</span></code>.
 110         </p></blockquote></div>
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 114 <td align="right"><div class="copyright-footer">Copyright &#169; 2000, 2011 Adobe Systems Inc, David Abrahams,
 115       Frederic Bron, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant,
 116       Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten
 117       Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert Stewart and Steven
 118       Watanabe<p>
 119         Distributed under the Boost Software License, Version 1.0. (See accompanying
 120         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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