doc/html/boost_typeerasure/conceptdef.html


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<title>Concept Definitions</title>
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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"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></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_idm46563170497328.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"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>
      from a raw value or by constructing a <code class="computeroutput"><a class="link" href="../boost/type_erasure/binding.html" title="Class template binding">binding</a></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">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span> <span class="special">=</span> <span class="identifier">_self</span><span class="special">&gt;</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">&amp;</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">&lt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">foo1</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</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">&amp;);</span>
<span class="special">};</span>

<span class="comment">// Wrong. apply must be static</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span> <span class="special">=</span> <span class="identifier">_self</span><span class="special">&gt;</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">&amp;);</span>
<span class="special">};</span>

<span class="comment">// Wrong.  apply cannot be overloaded</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span> <span class="special">=</span> <span class="identifier">_self</span><span class="special">&gt;</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">&amp;);</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">&amp;);</span>
<span class="special">};</span>

<span class="comment">// Wrong.  Only top level placeholders are detected</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">foo4</span><span class="special">;</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">foo4</span><span class="special">&lt;</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">&gt;</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">&amp;);</span>
<span class="special">};</span>

<span class="comment">// Wrong.  Template template parameters are not allowed.</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">&gt;</span> <span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</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">&lt;</span><span class="keyword">int</span><span class="special">&gt;&amp;);</span>
<span class="special">};</span>
</pre>
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