1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2 3<html> 4<head> 5 <meta http-equiv="Content-Language" content="en-us"> 6 <meta http-equiv="Content-Type" content="text/html; charset=us-ascii"> 7 8 <title>Boost Function Object Adapter Library</title> 9</head> 10 11<body bgcolor="#FFFFFF" text="#000000"> 12 <table border="1" bgcolor="#007F7F" cellpadding="2" summary=""> 13 <tr> 14 <td bgcolor="#FFFFFF"><img src="../../boost.png" alt= 15 "boost.png (6897 bytes)" width="277" height="86"></td> 16 17 <td><a href="../../index.htm"><font face="Arial" color= 18 "#FFFFFF"><big>Home</big></font></a></td> 19 20 <td><a href="../libraries.htm"><font face="Arial" color= 21 "#FFFFFF"><big>Libraries</big></font></a></td> 22 23 <td><a href="http://www.boost.org/people/people.htm"><font face="Arial" color= 24 "#FFFFFF"><big>People</big></font></a></td> 25 26 <td><a href="http://www.boost.org/more/faq.htm"><font face="Arial" color= 27 "#FFFFFF"><big>FAQ</big></font></a></td> 28 29 <td><a href="../../more/index.htm"><font face="Arial" color= 30 "#FFFFFF"><big>More</big></font></a></td> 31 </tr> 32 </table> 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> 74bool bad(const Foo &foo) { ... } 75... 76std::vector<Foo> c; 77... 78std::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> 89template <class Predicate> 90 class unary_negate 91 : public unary_function<typename Predicate::argument_type,bool> { 92public: 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> 124bool 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> 157</body> 158</html> 159
