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diff --git a/src/boost/libs/concept_check/concept_covering.htm b/src/boost/libs/concept_check/concept_covering.htm new file mode 100644 index 000000000..061f0f643 --- /dev/null +++ b/src/boost/libs/concept_check/concept_covering.htm @@ -0,0 +1,125 @@ +<!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.boost.org/sgi/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.boost.org/sgi/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> |