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+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
+ "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
+
+<html xmlns="http://www.w3.org/1999/xhtml">
+<!-- Copyright (c) Jeremy Siek and Andrew Lumsdaine 2000 -->
+<!-- Distributed under the Boost -->
+<!-- Software License, Version 1.0. (See accompanying -->
+<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
+
+<head>
+ <meta name="generator" content=
+ "HTML Tidy for Linux/x86 (vers 1 September 2005), see www.w3.org" />
+
+ <title>Concept Covering and Archetypes</title>
+ <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
+ <link rel="stylesheet" href="../../rst.css" type="text/css" />
+</head>
+
+<body bgcolor="#FFFFFF" link="#0000EE" text="#000000" vlink="#551A8B" alink=
+"#FF0000">
+ <img src="../../boost.png" alt="C++ Boost" width="277" height=
+ "86" /><br clear="none" />
+
+ <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>
+ </table>
+</body>
+</html>