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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_typeerasure.conceptdef"></a><a class="link" href="conceptdef.html" title="Concept Definitions">Concept Definitions</a> |
| </h2></div></div></div> |
| <p> |
| A Concept defines a set of constraints on the types that are stored in an |
| <code class="computeroutput">any</code>. |
| </p> |
| <p> |
| There are three kinds of concepts. |
| </p> |
| <div class="orderedlist"><ol class="orderedlist" type="1"> |
| <li class="listitem"> |
| The library defines a number of <a class="link" href="predef.html" title="Predefined Concepts">predefined |
| concepts</a>. Most of these are equivalent to user-defined concepts, |
| but a few require special handling. |
| </li> |
| <li class="listitem"> |
| Users can define their own primitive concepts as described below. The macros |
| <code class="computeroutput"><a class="link" href="../BOOST_TYPE_ERASURE_MEMBER.html" title="Macro BOOST_TYPE_ERASURE_MEMBER">BOOST_TYPE_ERASURE_MEMBER</a></code> |
| and <code class="computeroutput"><a class="link" href="../BOOST_TYPE_ERASURE_FREE.html" title="Macro BOOST_TYPE_ERASURE_FREE">BOOST_TYPE_ERASURE_FREE</a></code> |
| define concepts of this form. |
| </li> |
| <li class="listitem"> |
| Any MPL Forward Sequence whose elements are concepts is also a concept. |
| This allows concepts to be composed easily. |
| </li> |
| </ol></div> |
| <p> |
| Each primitive concept defines a single function. A primitive concept must |
| be a specialization of a class template, with a static member function called |
| <code class="computeroutput"><span class="identifier">apply</span></code>, which will be executed |
| when the function is dispatched by <code class="computeroutput"><a class="link" href="../boost/type_erasure/call_idp304107712.html" title="Function call">call</a></code>. |
| The template can only take template type parameters. non-type template parameters |
| and template template parameters are not allowed. |
| </p> |
| <p> |
| The template parameters of the concept may involve placeholders. The following |
| are considered. |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"> |
| Each template argument may be a cv and/or reference qualified placeholder |
| type. |
| </li> |
| <li class="listitem"> |
| If a template argument is a function type, its arguments and return type |
| may be cv/reference qualified placeholders. |
| </li> |
| </ul></div> |
| <p> |
| Any other placeholders are ignored. |
| </p> |
| <p> |
| A concept is instantiated by constructing an <code class="computeroutput">any</code> |
| from a raw value or by constructing a <code class="computeroutput">binding</code>. |
| When a concept is instantiated with a specific set of type bindings, each placeholder |
| is bound to a cv-unqualified non-reference type. After replacing each placeholder |
| in the template argument list with the type that it binds to, the following |
| must hold. |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"> |
| The number of arguments of apply in the bound concept must be the same |
| as the number of arguments in the unbound concept. |
| </li> |
| <li class="listitem"> |
| The arguments and return type of apply in the bound concept can be derived |
| from the corresponding arguments and the return type in the unbound concept |
| as follows: If the argument in the unbound concept is a placeholder with |
| optional cv and reference qualifiers, then the argument in the bound concept |
| can be found by replacing the placeholder. Otherwise, the argument in the |
| unbound concept must be the same as the argument in the bound concept. |
| </li> |
| </ul></div> |
| <pre class="programlisting"><span class="comment">// Correct.</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="identifier">_self</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">foo1</span> <span class="special">{</span> |
| <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">apply</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">t</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">t</span><span class="special">.</span><span class="identifier">foo</span><span class="special">();</span> <span class="special">}</span> |
| <span class="special">};</span> |
| |
| <span class="comment">// Wrong. The signature of apply is different from the</span> |
| <span class="comment">// primary template</span> |
| <span class="keyword">template</span><span class="special"><></span> |
| <span class="keyword">struct</span> <span class="identifier">foo1</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="special">{</span> |
| <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">apply</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">);</span> |
| <span class="special">};</span> |
| |
| <span class="comment">// Wrong. A concept must be a template</span> |
| <span class="keyword">struct</span> <span class="identifier">foo2</span> <span class="special">{</span> |
| <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">apply</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">_self</span><span class="special">&);</span> |
| <span class="special">};</span> |
| |
| <span class="comment">// Wrong. apply must be static</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="identifier">_self</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">foo3</span> <span class="special">{</span> |
| <span class="keyword">void</span> <span class="identifier">apply</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&);</span> |
| <span class="special">};</span> |
| |
| <span class="comment">// Wrong. apply cannot be overloaded</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="identifier">_self</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">foo3</span> <span class="special">{</span> |
| <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">apply</span><span class="special">(</span><span class="identifier">T</span><span class="special">&);</span> |
| <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">apply</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&);</span> |
| <span class="special">};</span> |
| |
| <span class="comment">// Wrong. Only top level placeholders are detected</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">foo4</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">foo4</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">></span> <span class="special">{</span> |
| <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">apply</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&);</span> |
| <span class="special">};</span> |
| |
| <span class="comment">// Wrong. Template template parameters are not allowed.</span> |
| <span class="keyword">template</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="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">foo5</span> |
| <span class="special">{</span> |
| <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">apply</span><span class="special">(</span><span class="identifier">T</span><span class="special"><</span><span class="keyword">int</span><span class="special">>&);</span> |
| <span class="special">};</span> |
| </pre> |
| </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 © 2011-2013 Steven Watanabe<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>) |
| </p> |
| </div></td> |
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