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| 28 | |
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| 29 | <td><a href="../../more/index.htm"><font face="Arial" color= |
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| 30 | "#FFFFFF"><big>More</big></font></a></td> |
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| 31 | </tr> |
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| 32 | </table> |
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| 33 | |
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| 34 | <h1>Member Function Adapters</h1> |
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| 35 | |
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| 36 | <p>The header <a href="../../boost/functional.hpp">functional.hpp</a> |
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| 37 | includes improved versions of the full range of member function adapters |
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| 38 | from the the C++ Standard Library (§20.3.8):</p> |
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| 39 | |
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| 40 | <ul> |
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| 41 | <li><tt>mem_fun_t</tt></li> |
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| 42 | |
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| 43 | <li><tt>mem_fun1_t</tt></li> |
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| 44 | |
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| 45 | <li><tt>const_mem_fun_t</tt></li> |
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| 46 | |
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| 47 | <li><tt>const_mem_fun1_t</tt></li> |
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| 48 | |
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| 49 | <li><tt>mem_fun_ref_t</tt></li> |
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| 50 | |
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| 51 | <li><tt>mem_fun1_ref_t</tt></li> |
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| 52 | |
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| 53 | <li><tt>const_mem_fun_ref_t</tt></li> |
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| 54 | |
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| 55 | <li><tt>const_mem_fun1_ref_t</tt></li> |
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| 56 | </ul> |
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| 57 | |
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| 58 | <p>as well as the corresponding overloaded helper functions</p> |
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| 59 | |
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| 60 | <ul> |
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| 61 | <li><tt>mem_fun</tt></li> |
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| 62 | |
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| 63 | <li><tt>mem_fun_ref</tt></li> |
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| 64 | </ul> |
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| 65 | |
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| 66 | <p>The following changes have been made to the adapters as specified in the |
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| 67 | Standard:</p> |
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| 68 | |
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| 69 | <ul> |
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| 70 | <li>The <tt>first_argument_type</tt> typedef has been corrected for the |
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| 71 | <tt>const_</tt> family of member function adapters (see <a href= |
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| 72 | "#firstarg">below</a>).</li> |
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| 73 | |
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| 74 | <li>The argument passed to <tt>mem_fun1_t</tt> and its variants is passed |
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| 75 | using the <tt>call_traits::param_type</tt> for the member function's |
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| 76 | argument type.</li> |
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| 77 | </ul> |
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| 78 | |
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| 79 | <h3 id="firstarg">first_argument_type</h3> |
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| 80 | |
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| 81 | <p>The standard specifies <tt>const_mem_fun1_t</tt>, for example, like |
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| 82 | this:</p> |
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| 83 | |
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| 84 | <blockquote> |
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| 85 | <pre> |
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| 86 | template <class S, class T, class A> class const_mem_fun1_t |
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| 87 | : public binary_function<<strong>T*</strong>, A, S> { |
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| 88 | public: |
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| 89 | explicit const_mem_fun1_t(S (T::*p)(A) const); |
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| 90 | S operator()(<strong>const T*</strong> p, A x) const; |
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| 91 | }; |
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| 92 | </pre> |
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| 93 | </blockquote> |
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| 94 | |
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| 95 | <p>Note that the first argument to <tt>binary_function</tt> is <tt>T*</tt> |
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| 96 | despite the fact that the first argument to <tt>operator()</tt> is actually |
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| 97 | of type <tt><em>const</em> T*</tt>.</p> |
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| 98 | |
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| 99 | <p>Does this matter? Well, consider what happens when we write</p> |
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| 100 | |
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| 101 | <blockquote> |
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| 102 | <pre> |
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| 103 | struct Foo { void bar(int) const; }; |
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| 104 | const Foo *cp = new Foo; |
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| 105 | std::bind1st(std::mem_fun(&Foo::bar), cp); |
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| 106 | </pre> |
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| 107 | </blockquote> |
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| 108 | |
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| 109 | <p>We have created a <tt>const_mem_fun1_t</tt> object which will |
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| 110 | effectively contain the following</p> |
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| 111 | |
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| 112 | <blockquote> |
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| 113 | <pre> |
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| 114 | typedef Foo* first_argument_type; |
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| 115 | </pre> |
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| 116 | </blockquote> |
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| 117 | |
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| 118 | <p>The <tt>bind1st</tt> will then create a <tt>binder1st</tt> object that |
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| 119 | will use this <tt>typedef</tt> as the type of a member which will be |
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| 120 | initialised with <tt>cp</tt>. In other words, we will need to initialise a |
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| 121 | <tt>Foo*</tt> member with a <tt>const Foo*</tt> pointer! Clearly this |
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| 122 | is not possible, so to implement this your Standard Library vendor will |
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| 123 | have had to cast away the constness of <tt>cp</tt>, probably within the |
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| 124 | body of <tt>bind1st</tt>.</p> |
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| 125 | |
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| 126 | <p>This hack will not suffice with the improved <a href= |
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| 127 | "binders.html">binders</a> in this library, so we have had to provide |
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| 128 | corrected versions of the member function adapters as well.</p> |
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| 129 | |
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| 130 | <h3 id="args">Argument Types</h3> |
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| 131 | |
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| 132 | <p>The standard defines <tt>mem_fun1_t</tt>, for example, like this |
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| 133 | (§20.3.8 ¶2):</p> |
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| 134 | |
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| 135 | <blockquote> |
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| 136 | <pre> |
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| 137 | template <class S, class T, class A> class mem_fun1_t |
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| 138 | : public binary_function<T*, A, S> { |
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| 139 | public: |
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| 140 | explicit mem_fun1_t(S (T::*p)(<strong>A</strong>)); |
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| 141 | S operator()(T* p, <strong>A</strong> x) const; |
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| 142 | }; |
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| 143 | </pre> |
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| 144 | </blockquote> |
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| 145 | |
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| 146 | <p>Note that the second argument to <tt>operator()</tt> is exactly the same |
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| 147 | type as the argument to the member function. If this is a value type, the |
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| 148 | argument will be passed by value and copied twice.</p> |
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| 149 | |
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| 150 | <p>However, if we were to try and eliminate this inefficiency by instead |
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| 151 | declaring the argument as <tt>const A&</tt>, then if A were a |
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| 152 | reference type, we would have a reference to a reference, which is |
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| 153 | currently illegal (but see <a href= |
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| 154 | "http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_active.html#106">C++ core |
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| 155 | language issue number 106)</a></p> |
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| 156 | |
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| 157 | <p>So the way in which we want to declare the second argument for |
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| 158 | <tt>operator()</tt> depends on whether or not the member function's |
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| 159 | argument is a reference. If it is a reference, we want to declare it simply |
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| 160 | as <tt>A</tt>; if it is a value we want to declare it as |
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| 161 | <tt>const A&</tt>.</p> |
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| 162 | |
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| 163 | <p>The Boost <a href="../utility/call_traits.htm">call_traits</a> class |
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| 164 | template contains a <tt>param_type</tt> typedef, which uses partial |
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| 165 | specialisation to make precisely this decision. By declaring the |
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| 166 | <tt>operator()</tt> as</p> |
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| 167 | |
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| 168 | <blockquote> |
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| 169 | <pre> |
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| 170 | S operator()(T* p, typename call_traits<A>::param_type x) const |
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| 171 | </pre> |
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| 172 | </blockquote> |
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| 173 | |
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| 174 | <p>we achieve the desired result - we improve efficiency without generating |
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| 175 | references to references.</p> |
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| 176 | |
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| 177 | <h3>Limitations</h3> |
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| 178 | |
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| 179 | <p>The call traits template used to realise some improvements relies on |
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| 180 | partial specialisation, so these improvements are only available on |
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| 181 | compilers that support that feature. With other compilers, the argument |
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| 182 | passed to the member function (in the <tt>mem_fun1_t</tt> family) will |
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| 183 | always be passed by reference, thus generating the possibility of |
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| 184 | references to references.</p> |
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| 185 | <hr> |
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| 186 | |
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| 187 | <p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src= |
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| 189 | height="31" width="88"></a></p> |
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| 190 | |
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| 191 | <p>Revised |
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| 192 | <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->02 December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38510" --></p> |
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| 193 | |
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| 194 | <p><i>Copyright © 2000 Cadenza New Zealand Ltd.</i></p> |
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| 195 | |
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| 196 | <p><i>Distributed under the Boost Software License, Version 1.0. (See |
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| 197 | accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or |
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| 198 | copy at <a href= |
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| 199 | "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p> |
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