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>Binders</h1> 35 36 <p>The header <a href="../../boost/functional.hpp">functional.hpp</a> 37 provides enhanced versions of both the binder function object adapters from 38 the C++ Standard Library (§20.3.6):</p> 39 40 <ul> 41 <li><tt>binder1st</tt></li> 42 43 <li><tt>binder2nd</tt></li> 44 </ul> 45 46 <p>As well as the corresponding helper functions</p> 47 48 <ul> 49 <li><tt>bind1st</tt></li> 50 51 <li><tt>bind2nd</tt></li> 52 </ul> 53 54 <p>The key benefit of these adapters over those in the Standard Library is 55 they avoid the problem of <a href="#refref">references to 56 references.</a></p> 57 58 <h3>Usage</h3> 59 60 <p>Usage is identical to the standard binders. For example,</p> 61 62 <blockquote> 63 <pre> 64class Foo { 65public: 66 void bar(std::ostream &); 67 // ... 68}; 69// ... 70std::vector<Foo> c; 71// ... 72std::for_each(c.begin(), c.end(), 73 boost::bind2nd(boost::mem_fun_ref(&Foo::bar), std::cout)); 74</pre> 75 </blockquote> 76 77 <h3 id="refref">References to References</h3> 78 79 <p>Consider the usage example above</p> 80 81 <blockquote> 82 <pre> 83class Foo { 84public: 85 void bar(<strong>std::ostream &</strong>); 86 // ... 87}; 88// ... 89std::for_each(c.begin(), c.end(), 90 boost::bind2nd(boost::mem_fun_ref(&Foo::bar), std::cout)); 91</pre> 92 </blockquote> 93 94 <p>If this had been written using <tt>std::bind2nd</tt> and 95 <tt>std::mem_fun_ref</tt>, it would be unlikely to compile.</p> 96 97 <p>The problem arises because <tt>bar</tt> takes a reference argument. The 98 Standard defines <tt>std::mem_fun_ref</tt> such that it creates a function 99 object whose <tt>second_argument_type</tt> will be 100 <tt>std::ostream&</tt>.</p> 101 102 <p>The call to <tt>bind2nd</tt> creates a <tt>binder2nd</tt> which the 103 Standard defines as follows:</p> 104 105 <blockquote> 106 <pre> 107template <class Operation> 108class binder2nd 109 : public unary_function<typename Operation::first_argument_type, 110 typename Operation::result_type> { 111... 112public: 113 binder2nd(const Operation& x, 114 <strong>const typename Operation::second_argument_type& y</strong>); 115 ... 116</pre> 117 </blockquote> 118 119 <p>Since our operation's <tt>second_argument_type</tt> is 120 <tt>std::ostream&</tt>, the type of <tt>y</tt> in the constructor would 121 be <tt>std::ostream&&</tt>. Since you cannot have a reference to a 122 reference, at this point we should get a compilation error because 123 references to references are illegal in C++ (but see <a href= 124 "http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#106">C++ 125 Standard core language active issues list</a>).</p> 126 127 <p>The binders in this library avoid this problem by using the Boost 128 <tt><a href="../utility/call_traits.htm">call_traits</a></tt> 129 templates.</p> 130 131 <p>Our constructor is declared</p> 132 133 <blockquote> 134 <pre> 135binder2nd(const Operation& x, 136 <strong>typename call_traits< 137 typename binary_traits<Operation>::second_argument_type 138 >::param_type y</strong>) 139</pre> 140 </blockquote> 141 142 <p>As a result, <tt>y</tt> has a type of <tt>std::ostream&</tt>, and 143 our example compiles.</p> 144 <hr> 145 146 <p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src= 147 "../../doc/images/valid-html401.png" alt="Valid HTML 4.01 Transitional" 148 height="31" width="88"></a></p> 149 150 <p>Revised 151 <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->02 152 December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38510" --></p> 153 154 <p><i>Copyright © 2000 Cadenza New Zealand Ltd.</i></p> 155 156 <p><i>Distributed under the Boost Software License, Version 1.0. (See 157 accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or 158 copy at <a href= 159 "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p> 160</body> 161</html> 162
