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>Function Pointer Adapters</h1> 35 36 <p>The header <a href="../../boost/functional.hpp">functional.hpp</a> 37 provides enhanced versions of both the function pointer adapters from the 38 C++ Standard Library (§20.3.7):</p> 39 40 <ul> 41 <li><tt>pointer_to_unary_function</tt></li> 42 43 <li><tt>pointer_to_binary_function</tt></li> 44 </ul> 45 46 <p>As well as the corresponding helper function template:</p> 47 48 <ul> 49 <li><tt>ptr_fun</tt></li> 50 </ul> 51 52 <p>However, you should not need to use the adapters in conjunction with the 53 adapters in this library due to our use of <a href= 54 "function_traits.html">function object traits</a>. You will however need to 55 use them if your implementation fails to work properly with our traits 56 classes (due to lack if partial specialisation), or if you wish to use a 57 function object adapter from a third party.</p> 58 59 <h3>Usage</h3> 60 61 <p>If you need to use these adapters, usage is identical to the standard 62 function pointer adapters. For example,</p> 63 64 <blockquote> 65 <pre> 66bool bad(std::string foo) { ... } 67... 68std::vector<std::string> c; 69... 70std::vector<std::string>::iterator it 71 = std::find_if(c.begin(), c.end(), std::not1(boost::ptr_fun(bad))); 72</pre> 73 </blockquote> 74 75 <p>Note however that this library contains enhanced <a href= 76 "negators.html">negators</a> that support function object traits, so the 77 line above could equally be written</p> 78 79 <blockquote> 80 <pre> 81std::vector<std::string>::iterator it 82 = std::find_if(c.begin(), c.end(), boost::not1(bad)); 83</pre> 84 </blockquote> 85 86 <h3>Argument Types</h3> 87 88 <p>The standard defines <tt>pointer_to_unary_function</tt> like this 89 (§20.3.8 ¶2):</p> 90 91 <blockquote> 92 <pre> 93template <class Arg, class Result> 94class pointer_to_unary_function : public unary_function<Arg, Result> { 95public: 96 explicit pointer_to_unary_function(Result (* f)(<strong>Arg</strong>)); 97 Result operator()(<strong>Arg</strong> x) const; 98}; 99</pre> 100 </blockquote> 101 102 <p>Note that the argument to <tt>operator()</tt> is exactly the same type 103 as the argument to the wrapped function. If this is a value type, the 104 argument will be passed by value and copied twice. 105 <tt>pointer_to_binary_function</tt> has a similar problem.</p> 106 107 <p>However, if we were to try and eliminate this inefficiency by instead 108 declaring the argument as <tt>const Arg&</tt>, then if Arg were a 109 reference type, we would have a reference to a reference, which is 110 currently illegal (but see <a href= 111 "http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#106">C++ core 112 language issue number 106)</a></p> 113 114 <p>So the way in which we want to declare the argument for 115 <tt>operator()</tt> depends on whether or not the wrapped function's 116 argument is a reference. If it is a reference, we want to declare it simply 117 as <tt>Arg</tt>; if it is a value we want to declare it as 118 <tt>const Arg&</tt>.</p> 119 120 <p>The Boost <a href="../utility/call_traits.htm">call_traits</a> class 121 template contains a <tt>param_type</tt> typedef, which uses partial 122 specialisation to make precisely this decision. By declaring the 123 <tt>operator()</tt> as</p> 124 125 <blockquote> 126 <pre> 127Result operator()(typename call_traits<Arg>::param_type x) const 128</pre> 129 </blockquote> 130 131 <p>we achieve the desired result - we improve efficiency without generating 132 references to references.</p> 133 134 <h3>Limitations</h3> 135 136 <p>The call traits template used to realise this improvement relies on 137 partial specialisation, so this improvement is only available on compilers 138 that support that feature. With other compilers, the argument passed to the 139 function will always be passed by reference, thus generating the 140 possibility of references to references.</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
