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33 | ||
34 | <h1>Negators</h1> | |
35 | ||
36 | <p>The header <a href="../../boost/functional.hpp">functional.hpp</a> | |
37 | provides enhanced versions of both the negator adapters from the C++ | |
38 | Standard Library (§20.3.5):</p> | |
39 | ||
40 | <ul> | |
41 | <li><tt>unary_negate</tt></li> | |
42 | ||
43 | <li><tt>binary_negate</tt></li> | |
44 | </ul> | |
45 | ||
46 | <p>As well as the corresponding helper functions</p> | |
47 | ||
48 | <ul> | |
49 | <li><tt>not1</tt></li> | |
50 | ||
51 | <li><tt>not2</tt></li> | |
52 | </ul> | |
53 | ||
54 | <p>However, the negators in this library improve on the standard versions | |
55 | in two ways:</p> | |
56 | ||
57 | <ul> | |
58 | <li>They use <a href="function_traits.html">function object traits</a> to | |
59 | avoid the need for <tt>ptr_fun</tt> when negating a function rather than | |
60 | an adaptable function object.</li> | |
61 | ||
62 | <li>They use Boost <a href= | |
63 | "../utility/call_traits.htm">call traits</a> to determine the best | |
64 | way to declare their arguments and pass them through to the adapted | |
65 | function (see <a href="#arguments">below</a>).</li> | |
66 | </ul> | |
67 | ||
68 | <h3>Usage</h3> | |
69 | ||
70 | <p>Usage is identical to the standard negators. For example,</p> | |
71 | ||
72 | <blockquote> | |
73 | <pre> | |
74 | bool bad(const Foo &foo) { ... } | |
75 | ... | |
76 | std::vector<Foo> c; | |
77 | ... | |
78 | std::find_if(c.begin(), c.end(), boost::not1(bad)); | |
79 | </pre> | |
80 | </blockquote> | |
81 | ||
82 | <h3 id="arguments">Argument Types</h3> | |
83 | ||
84 | <p>The C++ Standard (§20.3.5) defines unary negate like this (binary | |
85 | negate is similar):</p> | |
86 | ||
87 | <blockquote> | |
88 | <pre> | |
89 | template <class Predicate> | |
90 | class unary_negate | |
91 | : public unary_function<typename Predicate::argument_type,bool> { | |
92 | public: | |
93 | explicit unary_negate(const Predicate& pred); | |
94 | bool operator()(<strong>const typename Predicate::argument_type&</strong> x) const; | |
95 | }; | |
96 | </pre> | |
97 | </blockquote> | |
98 | ||
99 | <p>Note that if the Predicate's <tt>argument_type</tt> is a reference, the | |
100 | type of <tt>operator()</tt>'s argument would be a reference to a reference. | |
101 | Currently this is illegal in C++ (but see the <a href= | |
102 | "http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#106">C++ | |
103 | standard core language active issues list</a>).</p> | |
104 | ||
105 | <p>However, if we instead defined <tt>operator()</tt> to accept Predicate's | |
106 | argument_type unmodified, this would be needlessly inefficient if it were a | |
107 | value type; the argument would be copied twice - once when calling | |
108 | <tt>unary_negate</tt>'s <tt>operator()</tt>, and again when | |
109 | <tt>operator()</tt> called the adapted function.</p> | |
110 | ||
111 | <p>So how we want to declare the argument for <tt>operator()</tt> depends | |
112 | on whether or not the Predicate's <tt>argument_type</tt> is a reference. If | |
113 | it is a reference, we want to declare it simply as <tt>argument_type</tt>; | |
114 | if it is a value we want to declare it as | |
115 | <tt>const argument_type&</tt>.</p> | |
116 | ||
117 | <p>The Boost <a href="../utility/call_traits.htm">call_traits</a> class | |
118 | template contains a <tt>param_type</tt> typedef, which uses partial | |
119 | specialisation to make precisely this decision. If we were to declare | |
120 | <tt>operator()</tt> as</p> | |
121 | ||
122 | <blockquote> | |
123 | <pre> | |
124 | bool operator()(typename call_traits<typename Predicate::argument_type>::param_type x) const | |
125 | </pre> | |
126 | </blockquote> | |
127 | ||
128 | <p>the desired result would be achieved - we would eliminate references to | |
129 | references without loss of efficiency. In fact, the actual declaration is | |
130 | slightly more complicated because of the use of function object traits, but | |
131 | the effect remains the same.</p> | |
132 | ||
133 | <h3>Limitations</h3> | |
134 | ||
135 | <p>Both the function object traits and call traits used to realise these | |
136 | improvements rely on partial specialisation, these improvements are only | |
137 | available on compilers that support that feature. With other compilers, the | |
138 | negators in this library behave very much like those in the Standard - | |
139 | <tt>ptr_fun</tt> will be required to adapt functions, and references to | |
140 | references will not be avoided.</p> | |
141 | <hr> | |
142 | ||
143 | <p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src= | |
144 | "../../doc/images/valid-html401.png" alt="Valid HTML 4.01 Transitional" | |
145 | height="31" width="88"></a></p> | |
146 | ||
147 | <p>Revised | |
148 | <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->02 | |
149 | December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38510" --></p> | |
150 | ||
151 | <p><i>Copyright © 2000 Cadenza New Zealand Ltd.</i></p> | |
152 | ||
153 | <p><i>Distributed under the Boost Software License, Version 1.0. (See | |
154 | accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or | |
155 | copy at <a href= | |
156 | "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p> | |
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