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| 13 | <Head> |
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| 14 | <Title>Creating Concept Checking Classes</Title> |
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| 15 | <BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b" |
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| 16 | ALINK="#ff0000"> |
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| 17 | <IMG SRC="../../boost.png" |
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| 18 | ALT="C++ Boost" width="277" height="86"> |
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| 19 | |
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| 20 | <BR Clear> |
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| 21 | |
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| 22 | |
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| 23 | <h2><a name="creating-concept-checks">Creating Concept Checking Classes</a></h2> |
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| 24 | |
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| 25 | As an example of how to create a concept checking class, we look |
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| 26 | at how to create the corresponding checks for the |
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| 27 | <a href="http://www.sgi.com/tech/stl/RandomAccessIterator.html"> |
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| 28 | RandomAccessIterator</a> concept. First, as a convention we name the |
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| 29 | concept checking class after the concept, and add the suffix |
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| 30 | ``<tt>Concept</tt>''. Next we must define a member function named |
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| 31 | <tt>constraints()</tt> in which we will exercise the valid expressions |
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| 32 | of the concept. <tt>function_requires()</tt> expects this function's |
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| 33 | signature to appear exactly as it is appears below: a <tt>void</tt> |
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| 34 | non-const member function with no parameters. |
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| 35 | |
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| 36 | <p> |
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| 37 | The first part of the <tt>constraints()</tt> function includes |
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| 38 | the requirements that correspond to the <i>refinement</i> relationship |
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| 39 | between <a href="http://www.sgi.com/tech/stl/RandomAccessIterator.html"> |
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| 40 | RandomAccessIterator</a> and the concepts which it builds upon: |
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| 41 | <a href="http://www.sgi.com/tech/stl/BidirectionalIterator.html"> |
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| 42 | BidirectionalIterator</a> and |
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| 43 | <a href="http://www.sgi.com/tech/stl/LessThanComparable.html"> |
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| 44 | LessThanComparable</a>. We could have instead used |
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| 45 | <tt>BOOST_CLASS_REQUIRE</tt> and placed these requirements in the class |
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| 46 | body, however <tt>BOOST_CLASS_REQUIRE</tt> uses C++ language features that |
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| 47 | are less portable. |
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| 48 | |
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| 49 | <p> |
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| 50 | Next we check that the <tt>iterator_category</tt> of the iterator is |
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| 51 | either <tt>std::random_access_iterator_tag</tt> or a derived class. |
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| 52 | After that we write out some code that corresponds to the valid |
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| 53 | expressions of the <a |
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| 54 | href="http://www.sgi.com/tech/stl/RandomAccessIterator.html"> |
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| 55 | RandomAccessIterator</a> concept. Typedefs can also be added to |
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| 56 | enforce the associated types of the concept. |
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| 57 | |
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| 58 | <pre> |
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| 59 | template <class Iter> |
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| 60 | struct RandomAccessIterator_concept |
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| 61 | { |
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| 62 | void constraints() { |
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| 63 | function_requires< BidirectionalIteratorConcept<Iter> >(); |
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| 64 | function_requires< LessThanComparableConcept<Iter> >(); |
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| 65 | function_requires< ConvertibleConcept< |
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| 66 | typename std::iterator_traits<Iter>::iterator_category, |
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| 67 | std::random_access_iterator_tag> >(); |
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| 68 | |
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| 69 | i += n; |
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| 70 | i = i + n; i = n + i; |
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| 71 | i -= n; |
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| 72 | i = i - n; |
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| 73 | n = i - j; |
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| 74 | i[n]; |
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| 75 | } |
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| 76 | Iter i, j; |
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| 77 | typename std::iterator_traits<Iter>::difference_type n; |
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| 78 | }; |
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| 79 | } |
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| 80 | </pre> |
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| 81 | |
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| 82 | One potential pitfall in designing concept checking classes is using |
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| 83 | more expressions in the constraint function than necessary. For |
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| 84 | example, it is easy to accidentally use the default constructor to |
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| 85 | create the objects that will be needed in the expressions (and not all |
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| 86 | concepts require a default constructor). This is the reason we write |
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| 87 | the constraint function as a member function of a class. The objects |
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| 88 | involved in the expressions are declared as data members of the class. |
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| 89 | Since objects of the constraints class template are never |
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| 90 | instantiated, the default constructor for the concept checking class |
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| 91 | is never instantiated. Hence the data member's default constructors |
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| 92 | are never instantiated (C++ Standard Section 14.7.1 9). |
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| 93 | |
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| 94 | <p> |
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| 95 | <a href="./concept_covering.htm">Next: Concept Covering and Archetypes</a><br> |
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| 96 | <a href="./using_concept_check.htm">Prev: Using Concept Checks</a> |
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| 97 | |
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| 98 | |
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| 99 | <br> |
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| 100 | <HR> |
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| 101 | <TABLE> |
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| 102 | <TR valign=top> |
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| 103 | <TD nowrap>Copyright © 2000</TD><TD> |
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| 104 | <A HREF="../../people/jeremy_siek.htm">Jeremy Siek</A>(<A |
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| 105 | HREF="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</A>) |
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| 106 | Andrew Lumsdaine</A>(<A HREF="mailto:lums@osl.iu.edu">lums@osl.iu.edu</A>) |
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| 107 | </TD></TR></TABLE> |
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| 108 | |
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| 109 | </BODY> |
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| 110 | </HTML> |
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