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8  <title>Boost Function Object Adapter Library</title>
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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 (&sect;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 &amp;);
67  // ...
68};
69// ...
70std::vector&lt;Foo&gt; c;
71// ...
72std::for_each(c.begin(), c.end(),
73              boost::bind2nd(boost::mem_fun_ref(&amp;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 &amp;</strong>);
86  // ...
87};
88// ...
89std::for_each(c.begin(), c.end(),
90              boost::bind2nd(boost::mem_fun_ref(&amp;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&amp;</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 &lt;class Operation&gt;
108class binder2nd
109    : public unary_function&lt;typename Operation::first_argument_type,
110                            typename Operation::result_type&gt; {
111...
112public:
113  binder2nd(const Operation&amp; x,
114            <strong>const typename Operation::second_argument_type&amp; y</strong>);
115  ...
116</pre>
117  </blockquote>
118
119  <p>Since our operation's <tt>second_argument_type</tt> is
120  <tt>std::ostream&amp;</tt>, the type of <tt>y</tt> in the constructor would
121  be <tt>std::ostream&amp;&amp;</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://anubis.dkuug.dk/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&amp; x,
136          <strong>typename call_traits&lt;
137             typename binary_traits&lt;Operation&gt;::second_argument_type
138          &gt;::param_type y</strong>)
139</pre>
140  </blockquote>
141
142  <p>As a result, <tt>y</tt> has a type of <tt>std::ostream&amp;</tt>, and
143  our example compiles.</p>
144  <hr>
145
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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 &copy; 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>
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