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| 8 | <title>Boost Function Object Adapter Library</title> | |
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| 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> | |
| 66 | bool bad(std::string foo) { ... } | |
| 67 | ... | |
| 68 | std::vector<std::string> c; | |
| 69 | ... | |
| 70 | std::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> | |
| 81 | std::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> | |
| 93 | template <class Arg, class Result> | |
| 94 | class pointer_to_unary_function : public unary_function<Arg, Result> { | |
| 95 | public: | |
| 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> | |
| 127 | Result 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> |