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| author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 18:24:20 +0000 |
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| committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 18:24:20 +0000 |
| commit | 483eb2f56657e8e7f419ab1a4fab8dce9ade8609 (patch) | |
| tree | e5d88d25d870d5dedacb6bbdbe2a966086a0a5cf /src/boost/libs/functional/negators.html | |
| parent | Initial commit. (diff) | |
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Adding upstream version 14.2.21.upstream/14.2.21upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'src/boost/libs/functional/negators.html')
| -rw-r--r-- | src/boost/libs/functional/negators.html | 158 |
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diff --git a/src/boost/libs/functional/negators.html b/src/boost/libs/functional/negators.html new file mode 100644 index 00000000..5af3c54a --- /dev/null +++ b/src/boost/libs/functional/negators.html @@ -0,0 +1,158 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> + +<html> +<head> + <meta http-equiv="Content-Language" content="en-us"> + <meta http-equiv="Content-Type" content="text/html; charset=us-ascii"> + + <title>Boost Function Object Adapter Library</title> +</head> + +<body bgcolor="#FFFFFF" text="#000000"> + <table border="1" bgcolor="#007F7F" cellpadding="2" summary=""> + <tr> + <td bgcolor="#FFFFFF"><img src="../../boost.png" alt= + "boost.png (6897 bytes)" width="277" height="86"></td> + + <td><a href="../../index.htm"><font face="Arial" color= + "#FFFFFF"><big>Home</big></font></a></td> + + <td><a href="../libraries.htm"><font face="Arial" color= + "#FFFFFF"><big>Libraries</big></font></a></td> + + <td><a href="http://www.boost.org/people/people.htm"><font face="Arial" color= + "#FFFFFF"><big>People</big></font></a></td> + + <td><a href="http://www.boost.org/more/faq.htm"><font face="Arial" color= + "#FFFFFF"><big>FAQ</big></font></a></td> + + <td><a href="../../more/index.htm"><font face="Arial" color= + "#FFFFFF"><big>More</big></font></a></td> + </tr> + </table> + + <h1>Negators</h1> + + <p>The header <a href="../../boost/functional.hpp">functional.hpp</a> + provides enhanced versions of both the negator adapters from the C++ + Standard Library (§20.3.5):</p> + + <ul> + <li><tt>unary_negate</tt></li> + + <li><tt>binary_negate</tt></li> + </ul> + + <p>As well as the corresponding helper functions</p> + + <ul> + <li><tt>not1</tt></li> + + <li><tt>not2</tt></li> + </ul> + + <p>However, the negators in this library improve on the standard versions + in two ways:</p> + + <ul> + <li>They use <a href="function_traits.html">function object traits</a> to + avoid the need for <tt>ptr_fun</tt> when negating a function rather than + an adaptable function object.</li> + + <li>They use Boost <a href= + "../utility/call_traits.htm">call traits</a> to determine the best + way to declare their arguments and pass them through to the adapted + function (see <a href="#arguments">below</a>).</li> + </ul> + + <h3>Usage</h3> + + <p>Usage is identical to the standard negators. For example,</p> + + <blockquote> + <pre> +bool bad(const Foo &foo) { ... } +... +std::vector<Foo> c; +... +std::find_if(c.begin(), c.end(), boost::not1(bad)); +</pre> + </blockquote> + + <h3 id="arguments">Argument Types</h3> + + <p>The C++ Standard (§20.3.5) defines unary negate like this (binary + negate is similar):</p> + + <blockquote> + <pre> +template <class Predicate> + class unary_negate + : public unary_function<typename Predicate::argument_type,bool> { +public: + explicit unary_negate(const Predicate& pred); + bool operator()(<strong>const typename Predicate::argument_type&</strong> x) const; +}; +</pre> + </blockquote> + + <p>Note that if the Predicate's <tt>argument_type</tt> is a reference, the + type of <tt>operator()</tt>'s argument would be a reference to a reference. + Currently this is illegal in C++ (but see the <a href= + "http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#106">C++ + standard core language active issues list</a>).</p> + + <p>However, if we instead defined <tt>operator()</tt> to accept Predicate's + argument_type unmodified, this would be needlessly inefficient if it were a + value type; the argument would be copied twice - once when calling + <tt>unary_negate</tt>'s <tt>operator()</tt>, and again when + <tt>operator()</tt> called the adapted function.</p> + + <p>So how we want to declare the argument for <tt>operator()</tt> depends + on whether or not the Predicate's <tt>argument_type</tt> is a reference. If + it is a reference, we want to declare it simply as <tt>argument_type</tt>; + if it is a value we want to declare it as + <tt>const argument_type&</tt>.</p> + + <p>The Boost <a href="../utility/call_traits.htm">call_traits</a> class + template contains a <tt>param_type</tt> typedef, which uses partial + specialisation to make precisely this decision. If we were to declare + <tt>operator()</tt> as</p> + + <blockquote> + <pre> +bool operator()(typename call_traits<typename Predicate::argument_type>::param_type x) const +</pre> + </blockquote> + + <p>the desired result would be achieved - we would eliminate references to + references without loss of efficiency. In fact, the actual declaration is + slightly more complicated because of the use of function object traits, but + the effect remains the same.</p> + + <h3>Limitations</h3> + + <p>Both the function object traits and call traits used to realise these + improvements rely on partial specialisation, these improvements are only + available on compilers that support that feature. With other compilers, the + negators in this library behave very much like those in the Standard - + <tt>ptr_fun</tt> will be required to adapt functions, and references to + references will not be avoided.</p> + <hr> + + <p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src= + "../../doc/images/valid-html401.png" alt="Valid HTML 4.01 Transitional" + height="31" width="88"></a></p> + + <p>Revised + <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->02 + December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38510" --></p> + + <p><i>Copyright © 2000 Cadenza New Zealand Ltd.</i></p> + + <p><i>Distributed under the Boost Software License, Version 1.0. (See + accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or + copy at <a href= + "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p> +</body> +</html> |