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  <h2><a name="using-concept-checks" id="using-concept-checks">Using Concept
  Checks</a></h2>

  <p>For each concept there is a concept checking class template that can be
  used to make sure that a given type (or set of types) models the concept.
  The Boost Concept Checking Library (BCCL) includes concept checking class
  templates for all of the concepts used in the C++ standard library and a
  few more. See the <a href="./reference.htm">Reference</a> section for a
  complete list. In addition, other boost libraries come with concept
  checking classes for the concepts that are particular to those libraries.
  For example, there are <a href="../graph/doc/graph_concepts.html">graph
  concepts</a> and <a href="../property_map/doc/property_map.html">property map
  concepts</a>. Also, whenever <b>anyone</b> writing function templates needs
  to express requirements that are not yet stated by an existing concept, a
  new concept checking class should be created. How to do this is explained
  in <a href="./creating_concepts.htm">Creating Concept Checking
  Classes</a>.</p>

  <p>An example of a concept checking class from the BCCL is the
  <tt>EqualityComparableConcept</tt> class. The class corresponds to the
  EqualityComparable requirements described in 20.1.1 of the C++ Standard,
  and to the <a href=
  "http://www.boost.org/sgi/stl/EqualityComparable.html">EqualityComparable</a>
  concept documented in the SGI STL.</p>
  <pre>
template &lt;class T&gt;
struct EqualityComparable;
</pre>

  <p>The template argument is the type to be checked. That is, the purpose of
  <tt>EqualityComparable&lt;<em>T</em>&gt;</tt> is to make sure that
  <tt><em>T</em></tt> models the EqualityComparable concept.</p>

  <h4><tt>BOOST_CONCEPT_ASSERT()</tt></h4>

  <p>The most versatile way of checking concept requirements is to use the
  <code>BOOST_CONCEPT_ASSERT()</code> macro. You can use this macro at any
  scope, by passing a concept checking template specialization enclosed in
  parentheses. <strong>Note:</strong> that means invocations of
  <code>BOOST_CONCEPT_ASSERT</code> will appear to use <strong>double
  parentheses</strong>.</p>
  <pre>
<font color="green">// In my library:</font>
template &lt;class T&gt;
void generic_library_function(T x)
{
  BOOST_CONCEPT_ASSERT<strong>((</strong>EqualityComparable&lt;T&gt;<strong>))</strong>;
  <font color="green">// ...</font>
};

template &lt;class It&gt;
class generic_library_class
{
  BOOST_CONCEPT_ASSERT<strong>((</strong>RandomAccessIterator&lt;It&gt;<strong>))</strong>;
  <font color="green">// ...</font>
};

<font color="green">// In the user's code:</font>  
class foo {
  <font color="green">//... </font>
};

int main() {
  foo x;
  generic_library_function(x);
  generic_library_class&lt;std::vector&lt;char&gt;::iterator&gt; y;
  <font color="green">//...</font> 
}
</pre>

  <h4><tt>BOOST_CONCEPT_REQUIRES</tt></h4>

  <p>One of the nice things about the proposed C++0x <a href=
  "http://www.generic-programming.org/languages/conceptcpp/tutorial">syntax
  for declaring concept constrained function templates</a> is the way that
  constraints are part of the function <em>declaration</em>, so clients will
  see them. <code>BOOST_CONCEPT_ASSERT</code> can only express constraints
  within the function template definition, which hides the constraint in the
  function body. Aside from the loss of a self-documenting interface,
  asserting conformance only in the function body can undesirably delay
  checking if the function is explicitly instantiated in a different
  translation unit from the one in which it is called, or if the compiler
  does link-time instantiation.</p>

  <p>The <tt>BOOST_CONCEPT_REQUIRES</tt> macro can be used in a function
  template declaration to check whether some type models a concept. It
  accepts two arguments, a <strong>list of constraints</strong>, and the
  function template's return type. The list of constraints takes the form of
  a sequence of adjacent concept checking template specializations,
  <strong>in double parentheses</strong>, and the function's return type must
  also be parenthesized. For example, the standard <code>stable_sort</code>
  algorithm might be declared as follows: </p>
  <pre>
template &lt;class RanIter&gt;
BOOST_CONCEPT_REQUIRES(
    ((Mutable_RandomAccessIterator&lt;RanIter&gt;))
    ((LessThanComparable&lt;typename Mutable_RandomAccessIterator&lt;RanIter&gt;::value_type&gt;)),
    (void)) <font color="green">// return type</font>
    stable_sort(RanIter,RanIter);
</pre>

  <p>Note that the algorithm requires that the value type of the iterator be
  LessThanComparable, and it accesses that value type through the
  <code>Mutable_RandomAccessIterator</code> concept checking template. In
  general, the Boost concept checking classes expose associated types as
  nested member typedefs so that you can use this syntax, which mimics the
  approach used in the concept support proposed for the next version of
  C++.</p>

  <h4>Multi-Type Concepts</h4>

  <p>Some concepts deal with more than one type. In this case the
  corresponding concept checking class will have multiple template
  parameters. The following example shows how <tt>BOOST_CONCEPT_REQUIRES</tt>
  is used with the <a href=
  "../property_map/doc/ReadWritePropertyMap.html">ReadWritePropertyMap</a>
  concept, which takes two type parameters: a property map and the key type
  for the map.</p>
  <pre>
template &lt;class G, class Buffer, class BFSVisitor, 
          class ColorMap&gt;
BOOST_CONCEPT_REQUIRES(
  ((ReadWritePropertyMap&lt;ColorMap, typename IncidenceGraph&lt;G&gt;::vertex_descriptor&gt;)),
  (void)) <font color="green">// return type</font>
breadth_first_search(G&amp; g, 
  typename graph_traits&lt;IncidenceGraph&gt;::vertex_descriptor s, 
  Buffer&amp; Q, BFSVisitor vis, ColorMap color)
{
  typedef typename IncidenceGraph&lt;G&gt;::vertex_descriptor Vertex;
  ...
}
</pre>

  <p>Although concept checks are designed for use by generic library
  implementors, they can also be useful to end users. Sometimes one may not
  be sure whether some type models a particular concept. The syntactic
  requirements, at least, can easily be checked by creating a small program
  and using <tt>BOOST_CONCEPT_ASSERT</tt> with the type and concept in
  question. For example:</p>
  <pre>
<font color=
"green">// Make sure list&lt;int&gt; has bidirectional iterators.</font>
BOOST_CONCEPT_ASSERT((BidirectionalIterator&lt;std::list&lt;int&gt;::iterator&gt;));
</pre>

  <p><a href="./concept_check.htm">Prev: Concept Checking
  Introduction</a><br />
  <a href="./creating_concepts.htm">Next: Creating Concept Checking
  Classes</a><br /></p>
  <hr />

  <table>
    <tr valign="top">
      <td nowrap="nowrap">Copyright &copy; 2000</td>

      <td><a href="http://www.boost.org/people/jeremy_siek.htm">Jeremy Siek</a>(<a href=
      "mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</a>) Andrew
      Lumsdaine(<a href="mailto:lums@osl.iu.edu">lums@osl.iu.edu</a>), 2007
      <a href="mailto:dave@boost-consulting.com">David Abrahams</a>.</td>
    </tr>
  </table>
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