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  25 <div class="section" lang="en">
  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 lvalue 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:
  66 </span><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> This template is
  73         currently broken with Borland C++ Builder 5 (and earlier), for constructor-based
  74         conversions, and for the Metrowerks 7 (and earlier) compiler in all cases.
  75         If the compiler does not support <code class="computeroutput"><a class="link" href="is_abstract.html" title="is_abstract">is_abstract</a></code>,
  76         then the template parameter <code class="computeroutput"><span class="identifier">To</span></code>
  77         must not be an abstract type.
  78       </p>
  79 <p>
  80         <span class="bold"><strong>Header:</strong></span> <code class="computeroutput"> <span class="preprocessor">#include</span>
  81         <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>
  82         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>
  83       </p>
  84 <p>
  85         <span class="bold"><strong>Examples:</strong></span>
  86       </p>
  87 <div class="blockquote"><blockquote class="blockquote">
  88 <p>
  89           </p>
  90 <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">double</span><span class="special">&gt;</span></code>
  92             inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
  93           </p>
  94 <p>
  95         </p>
  96 </blockquote></div>
  97 <div class="blockquote"><blockquote class="blockquote">
  98 <p>
  99           </p>
 100 <p>
 101             <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>
 102             is the type <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
 103           </p>
 104 <p>
 105         </p>
 106 </blockquote></div>
 107 <div class="blockquote"><blockquote class="blockquote">
 108 <p>
 109           </p>
 110 <p>
 111             <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
 112             that evaluates to <span class="emphasis"><em>true</em></span>.
 113           </p>
 114 <p>
 115         </p>
 116 </blockquote></div>
 117 <div class="blockquote"><blockquote class="blockquote">
 118 <p>
 119           </p>
 120 <p>
 121             <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>
 122             is an integral constant expression that evaluates to <span class="emphasis"><em>false</em></span>:
 123             the conversion would require a <code class="computeroutput"><span class="keyword">const_cast</span></code>.
 124           </p>
 125 <p>
 126         </p>
 127 </blockquote></div>
 128 <div class="blockquote"><blockquote class="blockquote">
 129 <p>
 130           </p>
 131 <p>
 132             <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>
 133             is an integral constant expression that evaluates to <span class="emphasis"><em>true</em></span>.
 134           </p>
 135 <p>
 136         </p>
 137 </blockquote></div>
 138 <div class="blockquote"><blockquote class="blockquote">
 139 <p>
 140           </p>
 141 <p>
 142             <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">value</span></code> is an integral constant expression
 143             that evaluates to <span class="emphasis"><em>false</em></span>.
 144           </p>
 145 <p>
 146         </p>
 147 </blockquote></div>
 148 <div class="blockquote"><blockquote class="blockquote">
 149 <p>
 150           </p>
 151 <p>
 152             <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value_type</span></code> is the type <code class="computeroutput"><span class="keyword">bool</span></code>.
 153           </p>
 154 <p>
 155         </p>
 156 </blockquote></div>
 157 </div>
 158 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 159 <td align="left"></td>
 160 <td align="right"><div class="copyright-footer">Copyright © 2000, 2006 Adobe Systems Inc, David Abrahams,
 161       Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant, Jesse Jones, Mat
 162       Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten Ottosen, Robert
 163       Ramey and Jeremy Siek<p>
 164         Distributed under the Boost Software License, Version 1.0. (See accompanying
 165         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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