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- <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>
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- <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>
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