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| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_typetraits.reference.is_convertible"></a><a class="link" href="is_convertible.html" title="is_convertible"> is_convertible</a> |
| </h3></div></div></div> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</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">></span> |
| <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> |
| </pre> |
| <p> |
| <span class="bold"><strong>Inherits:</strong></span> If an imaginary lvalue of type |
| <code class="computeroutput"><span class="identifier">From</span></code> is convertible to type |
| <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>, |
| otherwise inherits from <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>. |
| </p> |
| <p> |
| Type From must not be an incomplete type. |
| </p> |
| <p> |
| Type To must not be an incomplete, or function type. |
| </p> |
| <p> |
| No types are considered to be convertible to array types or abstract-class |
| types. |
| </p> |
| <p> |
| 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> |
| has a <code class="computeroutput"><span class="keyword">private</span></code> converting constructor |
| from type <code class="computeroutput"><span class="identifier">From</span></code> then instantiating |
| <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="identifier">From</span><span class="special">,</span> <span class="identifier">To</span><span class="special">></span></code> |
| will produce a compiler error. For this reason <code class="computeroutput"><span class="identifier">is_convertible</span></code> |
| can not be used to determine whether a type has a <code class="computeroutput"><span class="keyword">public</span></code> |
| copy-constructor or not. |
| </p> |
| <p> |
| This template will also produce compiler errors if the conversion is ambiguous, |
| for example: |
| </p> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{};</span> |
| <span class="keyword">struct</span> <span class="identifier">B</span> <span class="special">:</span> <span class="identifier">A</span> <span class="special">{};</span> |
| <span class="keyword">struct</span> <span class="identifier">C</span> <span class="special">:</span> <span class="identifier">A</span> <span class="special">{};</span> |
| <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> |
| <span class="comment">// This produces a compiler error, the conversion is ambiguous: |
| </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"><</span><span class="identifier">D</span><span class="special">*,</span><span class="identifier">A</span><span class="special">*>::</span><span class="identifier">value</span><span class="special">;</span> |
| </pre> |
| <p> |
| <span class="bold"><strong>C++ Standard Reference:</strong></span> 4 and 8.5. |
| </p> |
| <p> |
| <span class="bold"><strong>Compiler Compatibility:</strong></span> This template is |
| currently broken with Borland C++ Builder 5 (and earlier), for constructor-based |
| conversions, and for the Metrowerks 7 (and earlier) compiler in all cases. |
| If the compiler does not support <code class="computeroutput"><a class="link" href="is_abstract.html" title="is_abstract">is_abstract</a></code>, |
| then the template parameter <code class="computeroutput"><span class="identifier">To</span></code> |
| must not be an abstract type. |
| </p> |
| <p> |
| <span class="bold"><strong>Header:</strong></span> <code class="computeroutput"> <span class="preprocessor">#include</span> |
| <span class="special"><</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">></span></code> |
| or <code class="computeroutput"> <span class="preprocessor">#include</span> <span class="special"><</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">></span></code> |
| </p> |
| <p> |
| <span class="bold"><strong>Examples:</strong></span> |
| </p> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">></span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. |
| </p></blockquote></div> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">>::</span><span class="identifier">type</span></code> |
| is the type <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. |
| </p></blockquote></div> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</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">*>::</span><span class="identifier">value</span></code> is an integral constant expression |
| that evaluates to <span class="emphasis"><em>true</em></span>. |
| </p></blockquote></div> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="keyword">int</span> <span class="keyword">const</span><span class="special">*,</span> <span class="keyword">int</span><span class="special">*>::</span><span class="identifier">value</span></code> is an integral constant expression |
| that evaluates to <span class="emphasis"><em>false</em></span>: the conversion would require |
| a <code class="computeroutput"><span class="keyword">const_cast</span></code>. |
| </p></blockquote></div> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="keyword">int</span> <span class="keyword">const</span><span class="special">&,</span> <span class="keyword">long</span><span class="special">>::</span><span class="identifier">value</span></code> |
| is an integral constant expression that evaluates to <span class="emphasis"><em>true</em></span>. |
| </p></blockquote></div> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">>::</span><span class="identifier">value</span></code> |
| is an integral constant expression that evaluates to <span class="emphasis"><em>false</em></span>. |
| </p></blockquote></div> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">T</span><span class="special">>::</span><span class="identifier">value_type</span></code> |
| is the type <code class="computeroutput"><span class="keyword">bool</span></code>. |
| </p></blockquote></div> |
| </div> |
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| <td align="right"><div class="copyright-footer">Copyright © 2000, 2006 Adobe Systems Inc, David Abrahams, |
| Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant, Jesse Jones, Mat |
| Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten Ottosen, Robert |
| Ramey and Jeremy Siek<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| 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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