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| 13 | <td><a href="../../index.htm"><font face="Arial" color="#FFFFFF"><big>Home </big></font></a></td> |
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| 19 | </table> |
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| 20 | |
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| 21 | <h1>Function Pointer Adapters</h1> |
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| 22 | |
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| 23 | <p>The header <nobr><a |
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| 24 | href="../../boost/functional.hpp">functional.hpp</a></nobr> provides |
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| 25 | enhanced versions of both the function pointer adapters from the C++ |
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| 26 | Standard Library <nobr>(§ 20.3.7):</nobr></p> |
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| 27 | |
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| 28 | <ul> |
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| 29 | <li><tt>pointer_to_unary_function</tt></li> |
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| 30 | <li><tt>pointer_to_binary_function</tt></li> |
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| 31 | </ul> |
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| 32 | |
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| 33 | <p>As well as the corresponding helper function template:</p> |
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| 34 | |
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| 35 | <ul> |
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| 36 | <li><tt>ptr_fun</tt></li> |
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| 37 | </ul> |
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| 38 | |
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| 39 | <p>However, you should not need to use the adapters in conjunction |
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| 40 | with the adapters in this library due to our use of <a |
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| 41 | href="function_traits.html">function object traits</a>. You will |
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| 42 | however need to use them if your implementation fails to work properly |
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| 43 | with our traits classes (due to lack if partial specialisation), or if |
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| 44 | you wish to use a function object adapter from a third party. |
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| 45 | |
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| 46 | <h3>Usage</h3> |
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| 47 | |
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| 48 | <p>If you need to use these adapters, usage is identical to the |
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| 49 | standard function pointer adapters. For example,</p> |
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| 50 | |
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| 51 | <blockquote><pre> |
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| 52 | bool bad(std::string foo) { ... } |
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| 53 | ... |
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| 54 | std::vector<std::string> c; |
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| 55 | ... |
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| 56 | std::vector<std::string>::iterator it |
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| 57 | = std::find_if(c.begin(), c.end(), std::not1(boost::ptr_fun(bad))); |
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| 58 | </pre></blockquote> |
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| 59 | |
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| 60 | <p>Note however that this library contains enhanced <a |
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| 61 | href="negators.html">negators</a> that support function object traits, |
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| 62 | so the line above could equally be written |
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| 63 | |
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| 64 | <blockquote><pre> |
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| 65 | std::vector<std::string>::iterator it |
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| 66 | = std::find_if(c.begin(), c.end(), boost::not1(bad)); |
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| 67 | </pre></blockquote> |
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| 68 | |
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| 69 | <h3>Argument Types</h3> |
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| 70 | |
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| 71 | <p>The standard defines |
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| 72 | <nobr><tt>pointer_to_unary_function</tt></nobr> like this |
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| 73 | <nobr>(§20.3.8 ¶2):</nobr> |
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| 74 | |
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| 75 | <blockquote><pre> |
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| 76 | template <class Arg, class Result> |
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| 77 | class pointer_to_unary_function : public unary_function<Arg, Result> { |
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| 78 | public: |
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| 79 | explicit pointer_to_unary_function(Result (* f)(<strong>Arg</strong>)); |
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| 80 | Result operator()(<strong>Arg</strong> x) const; |
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| 81 | }; |
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| 82 | </pre></blockquote> |
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| 83 | |
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| 84 | <p>Note that the argument to <nobr><tt>operator()</tt></nobr> is |
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| 85 | exactly the same type as the argument to the wrapped function. If this |
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| 86 | is a value type, the argument will be passed by value and copied twice. |
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| 87 | <nobr><tt>pointer_to_binary_function</tt></nobr> has a similar problem. |
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| 88 | |
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| 89 | <p>However, if we were to try and eliminate this inefficiency by |
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| 90 | instead declaring the argument as <nobr><tt>const Arg&</tt></nobr>, then |
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| 91 | if Arg were a reference type, we would have a reference to a reference, |
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| 92 | which is currently illegal (but see <a |
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| 93 | href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_active.html#106">C++ |
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| 94 | core language issue number 106)</a> |
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| 95 | |
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| 96 | <p>So the way in which we want to declare the argument for |
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| 97 | <nobr><tt>operator()</tt></nobr> depends on whether or not the |
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| 98 | wrapped function's argument is a reference. If it |
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| 99 | is a reference, we want to declare it simply as |
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| 100 | <nobr><tt>Arg</tt></nobr>; if it is a value we want to |
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| 101 | declare it as <nobr><tt>const Arg&</tt></nobr>. |
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| 102 | |
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| 103 | <p>The Boost <nobr><a |
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| 104 | href="../utility/call_traits.htm">call_traits</a></nobr> class |
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| 105 | template contains a <tt><nobr>param_type</nobr></tt> typedef, which |
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| 106 | uses partial specialisation to make precisely this decision. By |
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| 107 | declaring the <nobr><tt>operator()</tt></nobr> as |
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| 108 | |
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| 109 | <blockquote><pre> |
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| 110 | Result operator()(typename call_traits<Arg>::param_type x) const |
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| 111 | </pre></blockquote> |
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| 112 | |
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| 113 | <p>we achieve the desired result - we improve efficiency without |
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| 114 | generating references to references.</p> |
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| 115 | |
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| 116 | <h3>Limitations</h3> |
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| 117 | |
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| 118 | <p>The call traits template used to realise this improvement relies |
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| 119 | on partial specialisation, so this improvement is only available on |
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| 120 | compilers that support that feature. With other compilers, the |
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| 121 | argument passed to the function will always be passed by |
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| 122 | reference, thus generating the possibility of references to references. |
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| 123 | |
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| 124 | <hr> |
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| 125 | |
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| 126 | <p>Copyright © 2000 Cadenza New Zealand Ltd. Permission to copy, |
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| 127 | use, modify, sell and distribute this document is granted provided |
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| 128 | this copyright notice appears in all copies. This document is provided |
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| 129 | "as is" without express or implied warranty, and with no claim as to |
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| 130 | its suitability for any purpose.</p> |
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| 131 | |
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| 132 | <p>Revised 28 June 2000</p> |
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| 133 | |
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| 134 | </body> |
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| 135 | </html> |
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