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that use it are valid. Merged revisions 53047-53048 via svnmerge from https://svn.boost.org/svn/boost/trunk ........ r53047 | danieljames | 2009-05-16 15:17:20 +0100 (Sat, 16 May 2009) | 1 line Fix some validation errors. ........ r53048 | danieljames | 2009-05-16 15:23:59 +0100 (Sat, 16 May 2009) | 1 line Use a local copy of the valid HTML 4.01 icon. ........ [SVN r53258]
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6.9 KiB
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234 lines
6.9 KiB
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<meta http-equiv="Content-Language" content="en-us">
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<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
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<title>Boost Function Object Adapter Library</title>
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</head>
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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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<tr>
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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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<td><a href="http://www.boost.org/people/people.htm"><font face="Arial" color=
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"#FFFFFF"><big>People</big></font></a></td>
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<td><a href="http://www.boost.org/more/faq.htm"><font face="Arial" color=
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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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</tr>
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</table>
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<h1>Function Object Traits</h1>
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<p>The header <a href="../../boost/functional.hpp">functional.hpp</a>
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provides two traits class templates for functions and function objects:</p>
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<table border="1" summary="">
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<tr>
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<th>Type</th>
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<th>Contents</th>
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<th>Description</th>
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</tr>
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<tr>
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<td valign="top" rowspan="4">
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<tt>template <typename T><br>
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struct unary_traits</tt></td>
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<td valign="top"><tt>function_type</tt></td>
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<td valign="top">The type of the function or function object itself
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(i.e., <tt>T</tt>).</td>
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</tr>
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<tr>
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<td valign="top"><tt>param_type</tt></td>
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<td valign="top">The type that should be used to pass the function or
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function object as a parameter.</td>
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</tr>
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<tr>
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<td valign="top"><tt>result_type</tt></td>
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<td valign="top">The type returned by the function or function
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object.</td>
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</tr>
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<tr>
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<td valign="top"><tt>argument_type</tt></td>
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<td valign="top">The type of the argument to the function or function
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object.</td>
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</tr>
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<tr>
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<td valign="top" rowspan="5">
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<tt>template <typename T><br>
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struct binary_traits</tt></td>
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<td valign="top"><tt>function_type</tt></td>
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<td valign="top">The type of the function or function object itself
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(i.e., <tt>T</tt>).</td>
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</tr>
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<tr>
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<td valign="top"><tt>param_type</tt></td>
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<td valign="top">The type that should be used to pass the function or
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function object as a parameter.</td>
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</tr>
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<tr>
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<td valign="top"><tt>result_type</tt></td>
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<td valign="top">The type returned by the function or function
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object.</td>
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</tr>
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<tr>
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<td valign="top"><tt>first_argument_type</tt></td>
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<td valign="top">The type of the first argument to the function or
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function object.</td>
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</tr>
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<tr>
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<td valign="top"><tt>second_argument_type</tt></td>
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<td valign="top">The type of the second argument to the function or
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function object.</td>
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</tr>
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</table>
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<h3>Usage</h3>
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<p><tt>unary_traits</tt> should be instantiated with either a function
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taking a single parameter, or an adaptable unary function object (i.e., a
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class derived from <tt>std::unary_function</tt> or one which provides the
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same typedefs). (See §20.3.1 in the C++ Standard.)</p>
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<p><tt>binary_traits</tt> should be instantiated with either a function
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taking two parameters, or an adaptable binary function object (i.e., a
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class derived from <tt>std::binary_function</tt> or one which provides the
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same typedefs). (See §20.3.1 in the C++ Standard.)</p>
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<p>The most common usage of these templates is in function object adapters,
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thus allowing them to adapt plain functions as well as function objects.
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You can do this by wherever you would normally write, for example,</p>
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<blockquote>
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<pre>
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typename Operation::argument_type
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</pre>
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</blockquote>
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<p>simply writing</p>
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<blockquote>
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<pre>
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typename boost::unary_traits<Operation>::argument_type
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</pre>
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</blockquote>
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<p>instead.</p>
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<h3>Additional Types Defined</h3>
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<p>In addition to the standard result and argument typedefs, these traits
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templates define two additional types.</p>
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<h4><tt>function_type</tt></h4>
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<p>This is the type of the function or function object, and can be used in
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declarations such as</p>
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<blockquote>
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<pre>
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template <class Predicate>
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class unary_negate : // ...
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{
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// ...
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private:
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<strong>typename unary_traits<Predicate>::function_type</strong> pred;
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};
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</pre>
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</blockquote>
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<p>If this typedef were not provided, it would not be possible to declare
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<tt>pred</tt> in a way that would allow <tt>unary_negate</tt> to be
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instantiated with a function type (see the C++ Standard §14.3.1
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¶3).</p>
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<h4><tt>param_type</tt></h4>
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<p>This is a type suitable for passing the function or function object as a
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parameter to another function. For example,</p>
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<blockquote>
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<pre>
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template <class Predicate>
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class unary_negate : // ...
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{
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public:
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explicit unary_negate(<strong>typename unary_traits<Predicate>::param_type</strong> x)
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:
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pred(x)
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{}
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// ...
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};
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</pre>
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</blockquote>
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<p>Function objects are passed by reference to const; function pointers are
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passed by value.</p>
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<h3>Limitations</h3>
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<p>This library uses these traits within all function object adapters,
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theoretically rendering <tt>ptr_fun</tt> obsolete. However, third party
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adapters probably won't take advantage of this mechanism, and so
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<tt>ptr_fun</tt> may still be required. Accordingly, this library also
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provides <a href="ptr_fun.html">improved versions of the standard function
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pointer adapters</a>.</p>
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<p>These traits templates will also not work with compilers that fail to
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support partial specialisation of templates. With these compilers, the
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traits templates can only be instantiated with adaptable function objects,
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thus requiring <tt>ptr_fun</tt> to be used, even with the function object
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adapters in this library.</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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</html>
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