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159 lines
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<h1>Negators</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 negator adapters from the C++
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Standard Library (§20.3.5):</p>
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<ul>
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<li><tt>unary_negate</tt></li>
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<li><tt>binary_negate</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>not1</tt></li>
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<li><tt>not2</tt></li>
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</ul>
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<p>However, the negators in this library improve on the standard versions
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in two ways:</p>
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<ul>
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<li>They use <a href="function_traits.html">function object traits</a> to
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avoid the need for <tt>ptr_fun</tt> when negating a function rather than
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an adaptable function object.</li>
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<li>They use Boost <a href=
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"../utility/call_traits.htm">call traits</a> to determine the best
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way to declare their arguments and pass them through to the adapted
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function (see <a href="#arguments">below</a>).</li>
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</ul>
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<h3>Usage</h3>
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<p>Usage is identical to the standard negators. For example,</p>
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<blockquote>
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<pre>
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bool bad(const Foo &foo) { ... }
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...
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std::vector<Foo> c;
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...
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std::find_if(c.begin(), c.end(), boost::not1(bad));
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</pre>
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</blockquote>
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<h3 id="arguments">Argument Types</h3>
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<p>The C++ Standard (§20.3.5) defines unary negate like this (binary
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negate is similar):</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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: public unary_function<typename Predicate::argument_type,bool> {
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public:
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explicit unary_negate(const Predicate& pred);
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bool operator()(<strong>const typename Predicate::argument_type&</strong> x) const;
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};
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</pre>
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</blockquote>
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<p>Note that if the Predicate's <tt>argument_type</tt> is a reference, the
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type of <tt>operator()</tt>'s argument would be a reference to a reference.
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Currently this is illegal in C++ (but see the <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>However, if we instead defined <tt>operator()</tt> to accept Predicate's
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argument_type unmodified, this would be needlessly inefficient if it were a
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value type; the argument would be copied twice - once when calling
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<tt>unary_negate</tt>'s <tt>operator()</tt>, and again when
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<tt>operator()</tt> called the adapted function.</p>
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<p>So how we want to declare the argument for <tt>operator()</tt> depends
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on whether or not the Predicate's <tt>argument_type</tt> is a reference. If
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it is a reference, we want to declare it simply as <tt>argument_type</tt>;
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if it is a value we want to declare it as
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<tt>const argument_type&</tt>.</p>
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<p>The Boost <a href="../utility/call_traits.htm">call_traits</a> class
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template contains a <tt>param_type</tt> typedef, which uses partial
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specialisation to make precisely this decision. If we were to declare
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<tt>operator()</tt> as</p>
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<blockquote>
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<pre>
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bool operator()(typename call_traits<typename Predicate::argument_type>::param_type x) const
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</pre>
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</blockquote>
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<p>the desired result would be achieved - we would eliminate references to
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references without loss of efficiency. In fact, the actual declaration is
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slightly more complicated because of the use of function object traits, but
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the effect remains the same.</p>
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<h3>Limitations</h3>
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<p>Both the function object traits and call traits used to realise these
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improvements rely on partial specialisation, these improvements are only
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available on compilers that support that feature. With other compilers, the
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negators in this library behave very much like those in the Standard -
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<tt>ptr_fun</tt> will be required to adapt functions, and references to
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references will not be avoided.</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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<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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