<!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>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> <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>