162 lines
4.9 KiB
HTML
162 lines
4.9 KiB
HTML
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<title>Boost Function Object Adapter Library</title>
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<body bgcolor="#FFFFFF" text="#000000">
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<table border="1" bgcolor="#007F7F" cellpadding="2" summary="">
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<td bgcolor="#FFFFFF"><img src="../../boost.png" alt=
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"boost.png (6897 bytes)" width="277" height="86"></td>
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<td><a href="../../index.htm"><font face="Arial" color=
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"#FFFFFF"><big>Home</big></font></a></td>
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<td><a href="../libraries.htm"><font face="Arial" color=
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"#FFFFFF"><big>Libraries</big></font></a></td>
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"#FFFFFF"><big>People</big></font></a></td>
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<td><a href="../../more/index.htm"><font face="Arial" color=
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"#FFFFFF"><big>More</big></font></a></td>
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</table>
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<h1>Binders</h1>
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<p>The header <a href="../../boost/functional.hpp">functional.hpp</a>
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provides enhanced versions of both the binder function object adapters from
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the C++ Standard Library (§20.3.6):</p>
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<ul>
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<li><tt>binder1st</tt></li>
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<li><tt>binder2nd</tt></li>
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</ul>
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<p>As well as the corresponding helper functions</p>
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<ul>
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<li><tt>bind1st</tt></li>
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<li><tt>bind2nd</tt></li>
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</ul>
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<p>The key benefit of these adapters over those in the Standard Library is
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they avoid the problem of <a href="#refref">references to
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references.</a></p>
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<h3>Usage</h3>
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<p>Usage is identical to the standard binders. For example,</p>
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<blockquote>
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<pre>
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class Foo {
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public:
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void bar(std::ostream &);
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// ...
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};
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// ...
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std::vector<Foo> c;
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// ...
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std::for_each(c.begin(), c.end(),
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boost::bind2nd(boost::mem_fun_ref(&Foo::bar), std::cout));
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</pre>
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</blockquote>
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<h3 id="refref">References to References</h3>
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<p>Consider the usage example above</p>
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<blockquote>
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<pre>
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class Foo {
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public:
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void bar(<strong>std::ostream &</strong>);
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// ...
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};
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// ...
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std::for_each(c.begin(), c.end(),
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boost::bind2nd(boost::mem_fun_ref(&Foo::bar), std::cout));
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</pre>
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</blockquote>
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<p>If this had been written using <tt>std::bind2nd</tt> and
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<tt>std::mem_fun_ref</tt>, it would be unlikely to compile.</p>
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<p>The problem arises because <tt>bar</tt> takes a reference argument. The
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Standard defines <tt>std::mem_fun_ref</tt> such that it creates a function
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object whose <tt>second_argument_type</tt> will be
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<tt>std::ostream&</tt>.</p>
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<p>The call to <tt>bind2nd</tt> creates a <tt>binder2nd</tt> which the
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Standard defines as follows:</p>
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<blockquote>
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<pre>
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template <class Operation>
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class binder2nd
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: public unary_function<typename Operation::first_argument_type,
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typename Operation::result_type> {
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...
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public:
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binder2nd(const Operation& x,
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<strong>const typename Operation::second_argument_type& y</strong>);
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...
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</pre>
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</blockquote>
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<p>Since our operation's <tt>second_argument_type</tt> is
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<tt>std::ostream&</tt>, the type of <tt>y</tt> in the constructor would
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be <tt>std::ostream&&</tt>. Since you cannot have a reference to a
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reference, at this point we should get a compilation error because
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references to references are illegal in C++ (but see <a href=
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"http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#106">C++
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Standard core language active issues list</a>).</p>
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<p>The binders in this library avoid this problem by using the Boost
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<tt><a href="../utility/call_traits.htm">call_traits</a></tt>
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templates.</p>
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<p>Our constructor is declared</p>
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<blockquote>
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<pre>
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binder2nd(const Operation& x,
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<strong>typename call_traits<
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typename binary_traits<Operation>::second_argument_type
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>::param_type y</strong>)
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</pre>
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</blockquote>
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<p>As a result, <tt>y</tt> has a type of <tt>std::ostream&</tt>, and
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our example compiles.</p>
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<hr>
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<p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src=
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"../../doc/images/valid-html401.png" alt="Valid HTML 4.01 Transitional"
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height="31" width="88"></a></p>
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<p>Revised
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<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->02
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December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38510" --></p>
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<p><i>Copyright © 2000 Cadenza New Zealand Ltd.</i></p>
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<p><i>Distributed under the Boost Software License, Version 1.0. (See
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accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
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copy at <a href=
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"http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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</body>
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