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<h2><a name="creating-concept-checks" id="creating-concept-checks">Creating
Concept Checking Classes</a></h2>
<p>As an example of how to create a concept checking class template, we
look at how to create the corresponding checks for the <a href=
"http://www.sgi.com/tech/stl/InputIterator.html">InputIterator</a> concept.
The complete definition is here:</p>
<pre>
template <class X>
struct InputIterator
: Assignable<X>, EqualityComparable<X>
{
private:
typedef std::iterator_traits<X> t;
public:
typedef typename t::value_type value_type;
typedef typename t::difference_type difference_type;
typedef typename t::reference reference;
typedef typename t::pointer pointer;
typedef typename t::iterator_category iterator_category;
BOOST_CONCEPT_ASSERT((SignedInteger<difference_type>));
BOOST_CONCEPT_ASSERT((Convertible<iterator_category, std::input_iterator_tag>));
BOOST_CONCEPT_USAGE(InputIterator)
{
X j(i); <font color=
"green">// require copy construction</font>
same_type(*i++,v); <font color=
"green">// require postincrement-dereference returning value_type</font>
X& x = ++j; <font color=
"green">// require preincrement returning X&</font>
}
private:
X i;
value_type v;
<font color=
"green">// Type deduction will fail unless the arguments have the same type.</font>
template <typename T>
void same_type(T const&, T const&);
};
</pre>
<h3>Walkthrough</h3>
<p>First, as a convention we name the concept checking class after the
concept. Next, since InputIterator is a refinement of Assignable and
EqualityComparable, we derive its concept checking class from the checking
classes for those other concepts. The library will automatically check for
conformance to Assignable and EqualityComparable whenever it checks the
InputIterator concept.</p>
<p>Next, we declare the concept's <a href=
"http://www.boost.org/more/generic_programming.html#associated_type">associated types</a>
as member typedefs. The associated difference type is required to be a
signed integer, and the iterator category has to be convertible to
std::input_iterator_tag, so we assert those relationships. The syntax for
accessing associated types through the concept-checking template mirrors
the <a href=
"http://www.generic-programming.org/languages/conceptcpp/">proposed</a>
syntax for associated type access in C++0x Finally, we use the
<code>BOOST_CONCEPT_USAGE</code> macro to declare the function that
exercises all the concept's valid expressions. Note that at this point you
may sometimes need to be a little creative: for example, to check that
<code>*i++</code> returns the iterator's value type, we pass both values to
the <code>same_type</code> member function template, which requires both
arguments to have the same type, modulo references and cv-qualification.
It's an imperfect check, but it's better than nothing.</p>
<h3>Values for Usage Patterns Should Be Data Members</h3>
<p>You may be wondering why we declared <code>i</code> and <code>v</code>
as data members in the example above. Why didn't we simply write the
following?</p>
<pre>
BOOST_CONCEPT_USAGE(InputIterator)
{
X i; <font color=
"green">// create the values we need</font>
value_type v;
X j(i); <font color=
"green">// require copy construction</font>
same_type(*i++,v); <font color=
"green">// require postincrement-dereference returning value_type</font>
X& x = ++j; <font color=
"green">// require preincrement returning X&</font>
}
</pre>
<p>Unfortunately, that code wouldn't have worked out so well, because it
unintentionally imposes the requirement that <code>X</code> and its value
type are both default-constructible. On the other hand, since instances of
the <code>InputIterator</code> template will never be constructed, the
compiler never has to check how its data members will be constructed (C++
Standard Section 14.7.1 9). For that reason you should <strong>always
declare values needed for usage patterns as data members</strong>.</p>
<p>These sorts of errors in concept definitions can be detected by the use
of <a href="concept_covering.htm">Concept Archetypes</a>, but it's always
better to avoid them pre-emptively.</p>
<h3>Similarity to Proposed C++0x Language Support for Concepts</h3>
<p>This library's syntaxes for concept refinement and for access of
associated types mirrors the corresponding <a href=
"http://www.generic-programming.org/languages/conceptcpp/">proposed</a>
syntaxes in C++0x. However, C++0x will use
“signatures” rather than usage patterns to
describe the valid operations on types participating in a concept, so when
converting your concept checking classes into language-supported concepts,
you'll need to translate your usage function into a series of
signatures.</p>
<p><a href="./concept_covering.htm">Next: Concept Covering and
Archetypes</a><br />
<a href="./using_concept_check.htm">Prev: Using Concept
Checks</a><br /></p>
<hr />
<table>
<tr valign="top">
<td nowrap="nowrap">Copyright © 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>.
</tr>
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