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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://anubis.dkuug.dk/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> |
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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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