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<h2><a name="concept-covering" id="concept-covering">Concept Covering and
Archetypes</a></h2>
<p>We have discussed how it is important to select the minimal requirements
(concepts) for the inputs to a component, but it is equally important to
verify that the chosen concepts <i>cover</i> the algorithm. That is, any
possible user error should be caught by the concept checks and not let slip
through. Concept coverage can be verified through the use of <i>archetype
classes</i>. An archetype class is an exact implementation of the interface
associated with a particular concept. The run-time behavior of the
archetype class is not important, the functions can be left empty. A simple
test program can then be compiled with the archetype classes as the inputs
to the component. If the program compiles then one can be sure that the
concepts cover the component. The following code shows the archetype class
for the <a href="http://www.sgi.com/tech/stl/InputIterator.html">Input
Iterator</a> concept. Some care must be taken to ensure that the archetype
is an exact match to the concept. For example, the concept states that the
return type of <tt>operator*()</tt> must be convertible to the value type.
It does not state the more stringent requirement that the return type be
<tt>T&</tt> or <tt>const T&</tt>. That means it would be a mistake
to use <tt>T&</tt> or <tt>const T&</tt> for the return type of the
archetype class. The correct approach is to create an artificial return
type that is convertible to <tt>T</tt>, as we have done here with
<tt>reference</tt>. The validity of the archetype class test is completely
dependent on it being an exact match with the concept, which must be
verified by careful (manual) inspection.</p>
<pre>
template <class T>
class input_iterator_archetype
{
private:
typedef input_iterator_archetype self;
public:
typedef std::input_iterator_tag iterator_category;
typedef T value_type;
struct reference {
operator const value_type&() const { return static_object<T>::get(); }
};
typedef const T* pointer;
typedef std::ptrdiff_t difference_type;
self& operator=(const self&) { return *this; }
bool operator==(const self&) const { return true; }
bool operator!=(const self&) const { return true; }
reference operator*() const { return reference(); }
self& operator++() { return *this; }
self operator++(int) { return *this; }
};
</pre>
<p>Generic algorithms are often tested by being instantiated with a number
of common input types. For example, one might apply
<tt>std::stable_sort()</tt> with basic pointer types as the iterators.
Though appropriate for testing the run-time behavior of the algorithm, this
is not helpful for ensuring concept coverage because C++ types never match
particular concepts exactly. Instead, they often provide more than the
minimal functionality required by any one concept. Even though the function
template has concept checks, and compiles with a given type, the checks may
still fall short of covering all the functionality that is actually used.
This is why it is important to compile with archetype classes in addition
to testing with common input types.</p>
<p>The following is an excerpt from <a href=
"./stl_concept_covering.cpp"><tt>stl_concept_covering.cpp</tt></a> that
shows how archetypes can be used to check the requirement documentation for
<a href=
"http://www.sgi.com/tech/stl/stable_sort.html"><tt>std::stable_sort()</tt></a>.
In this case, it looks like the <a href=
"../utility/CopyConstructible.html">CopyConstructible</a> and <a href=
"../utility/Assignable.html">Assignable</a> requirements were forgotten in
the SGI STL documentation (try removing those archetypes). The Boost
archetype classes have been designed so that they can be layered. In this
example the value type of the iterator is composed out of three archetypes.
In the <a href="reference.htm#basic-archetype">archetype class
reference</a>, template parameters named <tt>Base</tt> indicate where the
layered archetype paradigm can be used.</p>
<pre>
{
typedef less_than_comparable_archetype<
sgi_assignable_archetype<> > ValueType;
random_access_iterator_archetype<ValueType> ri;
std::stable_sort(ri, ri);
}
</pre>
<p><a href="./prog_with_concepts.htm">Next: Programming with
Concepts</a><br />
<a href="./creating_concepts.htm">Prev: Creating Concept Checking
Classes</a><br />
<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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