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| <h1>Binders</h1> |
| |
| <p>The header <a href="../../boost/functional.hpp">functional.hpp</a> |
| provides enhanced versions of both the binder function object adapters from |
| the C++ Standard Library (§20.3.6):</p> |
| |
| <ul> |
| <li><tt>binder1st</tt></li> |
| |
| <li><tt>binder2nd</tt></li> |
| </ul> |
| |
| <p>As well as the corresponding helper functions</p> |
| |
| <ul> |
| <li><tt>bind1st</tt></li> |
| |
| <li><tt>bind2nd</tt></li> |
| </ul> |
| |
| <p>The key benefit of these adapters over those in the Standard Library is |
| they avoid the problem of <a href="#refref">references to |
| references.</a></p> |
| |
| <h3>Usage</h3> |
| |
| <p>Usage is identical to the standard binders. For example,</p> |
| |
| <blockquote> |
| <pre> |
| class Foo { |
| public: |
| void bar(std::ostream &); |
| // ... |
| }; |
| // ... |
| std::vector<Foo> c; |
| // ... |
| std::for_each(c.begin(), c.end(), |
| boost::bind2nd(boost::mem_fun_ref(&Foo::bar), std::cout)); |
| </pre> |
| </blockquote> |
| |
| <h3 id="refref">References to References</h3> |
| |
| <p>Consider the usage example above</p> |
| |
| <blockquote> |
| <pre> |
| class Foo { |
| public: |
| void bar(<strong>std::ostream &</strong>); |
| // ... |
| }; |
| // ... |
| std::for_each(c.begin(), c.end(), |
| boost::bind2nd(boost::mem_fun_ref(&Foo::bar), std::cout)); |
| </pre> |
| </blockquote> |
| |
| <p>If this had been written using <tt>std::bind2nd</tt> and |
| <tt>std::mem_fun_ref</tt>, it would be unlikely to compile.</p> |
| |
| <p>The problem arises because <tt>bar</tt> takes a reference argument. The |
| Standard defines <tt>std::mem_fun_ref</tt> such that it creates a function |
| object whose <tt>second_argument_type</tt> will be |
| <tt>std::ostream&</tt>.</p> |
| |
| <p>The call to <tt>bind2nd</tt> creates a <tt>binder2nd</tt> which the |
| Standard defines as follows:</p> |
| |
| <blockquote> |
| <pre> |
| template <class Operation> |
| class binder2nd |
| : public unary_function<typename Operation::first_argument_type, |
| typename Operation::result_type> { |
| ... |
| public: |
| binder2nd(const Operation& x, |
| <strong>const typename Operation::second_argument_type& y</strong>); |
| ... |
| </pre> |
| </blockquote> |
| |
| <p>Since our operation's <tt>second_argument_type</tt> is |
| <tt>std::ostream&</tt>, the type of <tt>y</tt> in the constructor would |
| be <tt>std::ostream&&</tt>. Since you cannot have a reference to a |
| reference, at this point we should get a compilation error because |
| references to references are illegal in C++ (but see <a href= |
| "http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_active.html#106">C++ |
| Standard core language active issues list</a>).</p> |
| |
| <p>The binders in this library avoid this problem by using the Boost |
| <tt><a href="../utility/call_traits.htm">call_traits</a></tt> |
| templates.</p> |
| |
| <p>Our constructor is declared</p> |
| |
| <blockquote> |
| <pre> |
| binder2nd(const Operation& x, |
| <strong>typename call_traits< |
| typename binary_traits<Operation>::second_argument_type |
| >::param_type y</strong>) |
| </pre> |
| </blockquote> |
| |
| <p>As a result, <tt>y</tt> has a type of <tt>std::ostream&</tt>, and |
| our example compiles.</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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