boost_1_45_0/libs/concept_check/concept_covering.htm - nest-learning-thermostat/5.0/boost - Git at Google

 <!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&amp;</tt> or <tt>const T&amp;</tt>. That means it would be a mistake
   to use <tt>T&amp;</tt> or <tt>const T&amp;</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 &lt;class T&gt;
 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&amp;() const { return static_object&lt;T&gt;::get(); }
   };
   typedef const T* pointer;
   typedef std::ptrdiff_t difference_type;
   self&amp; operator=(const self&amp;) { return *this;  }
   bool operator==(const self&amp;) const { return true; }
   bool operator!=(const self&amp;) const { return true; }
   reference operator*() const { return reference(); }
   self&amp; 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&lt;
       sgi_assignable_archetype&lt;&gt; &gt; ValueType;
   random_access_iterator_archetype&lt;ValueType&gt; 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 &copy; 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>
	<!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>