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| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_typetraits.reference.common_type_hpp"></a><a class="link" href="common_type_hpp.html" title="common_type"> common_type</a> |
| </h3></div></div></div> |
| <p> |
| <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">common_type</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> |
| </p> |
| <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="special">...</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">struct</span> <a class="link" href="../../">common_type</a><span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| <a class="link" href="../../">common_type</a> |
| is a traits class used to deduce a type common to a several types, useful |
| as the return type of functions operating on multiple input types such as |
| in mixed-mode arithmetic.. |
| </p> |
| <p> |
| The nested typedef <code class="computeroutput"><span class="special">::</span><span class="identifier">type</span></code> |
| could be defined as follows: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="special">...</span><span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">,</span> <span class="keyword">class</span> <span class="special">...</span><span class="identifier">V</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">U</span><span class="special">,...</span><span class="identifier">V</span><span class="special">></span> <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">__common_type__</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">__common_type__</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">>::</span><span class="identifier">type</span><span class="special">,</span> <span class="identifier">V</span><span class="special">...>::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span> |
| <span class="special">};</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="identifier">T</span> <span class="identifier">type</span><span class="special">;</span> |
| <span class="special">};</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">></span> <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="identifier">decltype</span><span class="special">(</span><span class="identifier">__declval__</span><span class="special"><</span><span class="keyword">bool</span><span class="special">>()</span> <span class="special">?</span> <span class="identifier">__declval__</span><span class="special"><</span><span class="identifier">T</span><span class="special">>()</span> <span class="special">:</span> <span class="identifier">__declval__</span><span class="special"><</span><span class="identifier">U</span><span class="special">>())</span> <span class="identifier">type</span><span class="special">;</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| All parameter types must be complete. This trait is permitted to be specialized |
| by a user if at least one template parameter is a user-defined type. <span class="bold"><strong>Note:</strong></span> Such specializations are required when only |
| explicit conversions are desired among the <a class="link" href="../../">common_type</a> |
| arguments. |
| </p> |
| <a name="boost_typetraits.reference.common_type_hpp.configuration_macros"></a><h5> |
| <a name="id895242"></a> |
| <a class="link" href="common_type_hpp.html#boost_typetraits.reference.common_type_hpp.configuration_macros">Configuration |
| macros</a> |
| </h5> |
| <p> |
| When the compiler does not support static assertions then the user can select |
| the way static assertions are reported. Define |
| </p> |
| <div class="itemizedlist"><ul type="disc"> |
| <li> |
| BOOST_COMMON_TYPE_USES_STATIC_ASSERT: define it if you want to use Boost.StaticAssert |
| </li> |
| <li> |
| BOOST_COMMON_TYPE_USES_MPL_ASSERT: define it if you want to use Boost.MPL |
| static asertions |
| </li> |
| </ul></div> |
| <p> |
| The default behavior is to use mpl assertions in this case, but setting BOOST_COMMON_TYPE_USES_STATIC_ASSERT |
| may reduce compile times and header dependencies somewhat. |
| </p> |
| <p> |
| Depending on the static assertion used you will have an hint of the failing |
| assertion either through the symbol or through the text. |
| </p> |
| <p> |
| When possible common_type is implemented using <code class="computeroutput"><span class="identifier">decltype</span></code>. |
| Otherwise when BOOST_COMMON_TYPE_DONT_USE_TYPEOF is not defined it uses Boost.TypeOf. |
| </p> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <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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