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14 <title>Concept Covering and Archetypes</title>
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23
24 <h2><a name="concept-covering" id="concept-covering">Concept Covering and
25 Archetypes</a></h2>
26
27 <p>We have discussed how it is important to select the minimal requirements
28 (concepts) for the inputs to a component, but it is equally important to
29 verify that the chosen concepts <i>cover</i> the algorithm. That is, any
30 possible user error should be caught by the concept checks and not let slip
31 through. Concept coverage can be verified through the use of <i>archetype
32 classes</i>. An archetype class is an exact implementation of the interface
33 associated with a particular concept. The run-time behavior of the
34 archetype class is not important, the functions can be left empty. A simple
35 test program can then be compiled with the archetype classes as the inputs
36 to the component. If the program compiles then one can be sure that the
37 concepts cover the component. The following code shows the archetype class
38 for the <a href="http://www.sgi.com/tech/stl/InputIterator.html">Input
39 Iterator</a> concept. Some care must be taken to ensure that the archetype
40 is an exact match to the concept. For example, the concept states that the
41 return type of <tt>operator*()</tt> must be convertible to the value type.
42 It does not state the more stringent requirement that the return type be
43 <tt>T&amp;</tt> or <tt>const T&amp;</tt>. That means it would be a mistake
44 to use <tt>T&amp;</tt> or <tt>const T&amp;</tt> for the return type of the
45 archetype class. The correct approach is to create an artificial return
46 type that is convertible to <tt>T</tt>, as we have done here with
47 <tt>reference</tt>. The validity of the archetype class test is completely
48 dependent on it being an exact match with the concept, which must be
49 verified by careful (manual) inspection.</p>
50 <pre>
51template &lt;class T&gt;
52class input_iterator_archetype
53{
54private:
55 typedef input_iterator_archetype self;
56public:
57 typedef std::input_iterator_tag iterator_category;
58 typedef T value_type;
59 struct reference {
60 operator const value_type&amp;() const { return static_object&lt;T&gt;::get(); }
61 };
62 typedef const T* pointer;
63 typedef std::ptrdiff_t difference_type;
64 self&amp; operator=(const self&amp;) { return *this; }
65 bool operator==(const self&amp;) const { return true; }
66 bool operator!=(const self&amp;) const { return true; }
67 reference operator*() const { return reference(); }
68 self&amp; operator++() { return *this; }
69 self operator++(int) { return *this; }
70};
71</pre>
72
73 <p>Generic algorithms are often tested by being instantiated with a number
74 of common input types. For example, one might apply
75 <tt>std::stable_sort()</tt> with basic pointer types as the iterators.
76 Though appropriate for testing the run-time behavior of the algorithm, this
77 is not helpful for ensuring concept coverage because C++ types never match
78 particular concepts exactly. Instead, they often provide more than the
79 minimal functionality required by any one concept. Even though the function
80 template has concept checks, and compiles with a given type, the checks may
81 still fall short of covering all the functionality that is actually used.
82 This is why it is important to compile with archetype classes in addition
83 to testing with common input types.</p>
84
85 <p>The following is an excerpt from <a href=
86 "./stl_concept_covering.cpp"><tt>stl_concept_covering.cpp</tt></a> that
87 shows how archetypes can be used to check the requirement documentation for
88 <a href=
89 "http://www.sgi.com/tech/stl/stable_sort.html"><tt>std::stable_sort()</tt></a>.
90 In this case, it looks like the <a href=
91 "../utility/CopyConstructible.html">CopyConstructible</a> and <a href=
92 "../utility/Assignable.html">Assignable</a> requirements were forgotten in
93 the SGI STL documentation (try removing those archetypes). The Boost
94 archetype classes have been designed so that they can be layered. In this
95 example the value type of the iterator is composed out of three archetypes.
96 In the <a href="reference.htm#basic-archetype">archetype class
97 reference</a>, template parameters named <tt>Base</tt> indicate where the
98 layered archetype paradigm can be used.</p>
99 <pre>
100{
101 typedef less_than_comparable_archetype&lt;
102 sgi_assignable_archetype&lt;&gt; &gt; ValueType;
103 random_access_iterator_archetype&lt;ValueType&gt; ri;
104 std::stable_sort(ri, ri);
105}
106</pre>
107
108 <p><a href="./prog_with_concepts.htm">Next: Programming with
109 Concepts</a><br />
110 <a href="./creating_concepts.htm">Prev: Creating Concept Checking
111 Classes</a><br />
112 <hr />
113
114 <table>
115 <tr valign="top">
116 <td nowrap="nowrap">Copyright &copy; 2000</td>
117
118 <td><a href="http://www.boost.org/people/jeremy_siek.htm">Jeremy Siek</a>(<a href=
119 "mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</a>) Andrew
120 Lumsdaine(<a href="mailto:lums@osl.iu.edu">lums@osl.iu.edu</a>),
121 2007 <a href="mailto:dave@boost-consulting.com">David Abrahams</a>.
122 </tr>
123 </table>
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