boost-react-native-bundle

<!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"><big>Home</big></a></td> <td><a href="../libraries.htm"><big>Libraries</big></a></td> <td><a href="http://www.boost.org/people/people.htm"><big>People</big></a></td> <td><a href="http://www.boost.org/more/faq.htm"><big>FAQ</big></a></td> <td><a href="../../more/index.htm"><big>More</big></a></td> </tr> </table> <h1>Binders</h1> 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): <ul> <li><tt>binder1st</tt></li> <li><tt>binder2nd</tt></li> </ul> As well as the corresponding helper functions <ul> <li><tt>bind1st</tt></li> <li><tt>bind2nd</tt></li> </ul> 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> <h3>Usage</h3> Usage is identical to the standard binders. For example, <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> Consider the usage example above <blockquote> <pre> class Foo { public: void bar(std::ostream &); // ... }; // ... std::for_each(c.begin(), c.end(), boost::bind2nd(boost::mem_fun_ref(&Foo::bar), std::cout)); </pre> </blockquote> If this had been written using <tt>std::bind2nd</tt> and <tt>std::mem_fun_ref</tt>, it would be unlikely to compile. 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>. The call to <tt>bind2nd</tt> creates a <tt>binder2nd</tt> which the Standard defines as follows: <blockquote> <pre> template <class Operation> class binder2nd : public unary_function<typename Operation::first_argument_type, typename Operation::result_type> { ... public: binder2nd(const Operation& x, const typename Operation::second_argument_type& y); ... </pre> </blockquote> 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://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#106">C++ Standard core language active issues list</a>). The binders in this library avoid this problem by using the Boost <tt><a href="../utility/call_traits.htm">call_traits</a></tt> templates. Our constructor is declared <blockquote> <pre> binder2nd(const Operation& x, typename call_traits< typename binary_traits<Operation>::second_argument_type >::param_type y) </pre> </blockquote> As a result, <tt>y</tt> has a type of <tt>std::ostream&</tt>, and our example compiles. <hr> <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> Revised 02 December, 2006 Copyright © 2000 Cadenza New Zealand Ltd. 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>) </body> </html>