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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://anubis.dkuug.dk/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= |
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| 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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