<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0.1 Transitional//EN"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> <title>Boost.Flyweight Documentation - Tutorial - Annex - MPL lambda expressions</title> <link rel="stylesheet" href="../style.css" type="text/css"> <link rel="start" href="../index.html"> <link rel="prev" href="technical.html"> <link rel="up" href="index.html"> <link rel="next" href="../reference/index.html"> </head> <body> <h1><img src="../../../../boost.png" alt="Boost logo" align= "middle" width="277" height="86">Boost.Flyweight Tutorial Annex: MPL lambda expressions</h1> <div class="prev_link"><a href="technical.html"><img src="../prev.gif" alt="technical issues" border="0"><br> Technical issues </a></div> <div class="up_link"><a href="index.html"><img src="../up.gif" alt="Boost.Flyweight tutorial" border="0"><br> Boost.Flyweight tutorial </a></div> <div class="next_link"><a href="../reference/index.html"><img src="../next.gif" alt="Boost.Flyweight reference" border="0"><br> Boost.Flyweight reference </a></div><br clear="all" style="clear: all;"> <hr> <p>This short introduction to lambda expressions is meant for readers unfamiliar with the <a href="../../../mpl/doc/index.html">Boost MPL Library</a> who want to rapidly acquire a working knowledge of the basic concepts for the purposes of using them in Boost.Flyweight. Please refer to the Boost.MPL documentation for further information beyond these introductory notes. </p> <p> The specifiers defined by Boost.Flyweight rely heavily on the <a href="../../../mpl/doc/refmanual/lambda-expression.html"><code>Lambda Expression</code></a> concept defined by the <a href="../../../mpl/doc/index.html">Boost MPL Library</a>. A lambda expression can be thought of as a compile-time "type function", an entity (a concrete type, actually) that can be invoked with a list of types and returns some associated type in its turn. Consider for instance an arbitrary class template: </p> <blockquote><pre> <span class=keyword>template</span><span class=special><</span><span class=keyword>typename</span> <span class=identifier>T</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Q</span><span class=special>></span> <span class=keyword>class</span> <span class=identifier>foo</span> <span class=special>{</span> <span class=special>...</span> <span class=special>};</span> </pre></blockquote> <p> and suppose we want to have a lambda expression that, when invoked with some generic types <code>Arg1</code> and <code>Arg2</code>, returns <code>foo<Arg1,Arg2></code>. Such a lambda expression can be implemented in two ways <ol> <li>As a <a href="../../../mpl/doc/refmanual/metafunction-class.html"><code>MPL Metafunction Class</code></a>, a type with a special nested class template named <code>apply</code>: <blockquote><pre> <span class=keyword>struct</span> <span class=identifier>foo_specifier</span> <span class=special>{</span> <span class=keyword>template</span><span class=special><</span><span class=keyword>typename</span> <span class=identifier>Arg1</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Arg2</span><span class=special>></span> <span class=keyword>struct</span> <span class=identifier>apply</span> <span class=special>{</span> <span class=comment>// this is the "return type" of foo_specifier</span> <span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special><</span><span class=identifier>Arg1</span><span class=special>,</span><span class=identifier>Arg2</span><span class=special>></span> <span class=identifier>type</span><span class=special>;</span> <span class=special>};</span> <span class=special>};</span> </pre></blockquote> </li> <li> As a <a href="../../../mpl/doc/refmanual/placeholder-expression.html"><code>MPL Placeholder Expression</code></a>, a class template instantiated with one or more <i>placeholders</i>: <blockquote><pre> <span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special><</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_1</span><span class=special>,</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_2</span><span class=special>></span> <span class=identifier>foo_specifier</span><span class=special>;</span> </pre></blockquote> Note that, in this case, <code>foo_specifier</code> is a concrete type, much as <code>int</code> or <code>std::set<std::string></code> are; yet, MPL internal mechanisms are able to detect that this type has been gotten from instantiating a class template with placeholders <code>boost::mpl::_1</code> and <code>boost::mpl::_2</code> and take these placeholders as slots to be substituted for actual types (the first and second type supplied, respectively) when <code>foo_specifier</code> is invoked. So, an instantiation of <code>foo</code> can be used to refer back to the <code>foo</code> class template itself! The net effect is the same as with metafunctions, but placeholder expressions spare us the need to write boilerplate metafunction classes --and the kind of metaprogramming magic they depend on has an undeniable beauty to it. </li> </ol> So far the examples shown just forward the arguments <code>Arg1</code> and <code>Arg2</code> directly to a class template without further elaboration, but there is nothing preventing us from doing some argument manipulation, like, for instance, switching their places: </p> <blockquote><pre> <span class=keyword>struct</span> <span class=identifier>foo_specifier</span> <span class=special>{</span> <span class=keyword>template</span><span class=special><</span><span class=keyword>typename</span> <span class=identifier>Arg1</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Arg2</span><span class=special>></span> <span class=keyword>struct</span> <span class=identifier>apply</span><span class=special>{</span><span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special><</span><span class=identifier>Arg2</span><span class=special>,</span><span class=identifier>Arg1</span><span class=special>></span> <span class=identifier>type</span><span class=special>;};</span> <span class=special>};</span> <span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special><</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_2</span><span class=special>,</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_1</span><span class=special>></span> <span class=identifier>foo_specifier</span><span class=special>;</span> </pre></blockquote> <p> passing placeholder subexpressions as arguments to the overall expression: </p> <blockquote><pre> <span class=keyword>struct</span> <span class=identifier>foo_specifier</span> <span class=special>{</span> <span class=keyword>template</span><span class=special><</span><span class=keyword>typename</span> <span class=identifier>Arg1</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Arg2</span><span class=special>></span> <span class=keyword>struct</span> <span class=identifier>apply</span><span class=special>{</span><span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special><</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>shared_ptr</span><span class=special><</span><span class=identifier>Arg1</span><span class=special>>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>less</span><span class=special><</span><span class=identifier>Arg2</span><span class=special>></span> <span class=special>></span> <span class=identifier>type</span><span class=special>;};</span> <span class=special>};</span> <span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special><</span> <span class=identifier>boost</span><span class=special>::</span><span class=identifier>shared_ptr</span><span class=special><</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_1</span><span class=special>>,</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>less</span><span class=special><</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_2</span><span class=special>></span> <span class=special>></span> <span class=identifier>foo_specifier</span><span class=special>;</span> </pre></blockquote> <p> or accepting less or more arguments than the class template itself (the number of parameters of a lambda expression is called its <i>arity</i>): </p> <blockquote><pre> <span class=keyword>struct</span> <span class=identifier>foo_specifier</span> <span class=special>{</span> <span class=keyword>template</span><span class=special><</span><span class=keyword>typename</span> <span class=identifier>Arg1</span><span class=special>></span> <span class=keyword>struct</span> <span class=identifier>apply</span><span class=special>{</span><span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special><</span><span class=identifier>Arg1</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>less</span><span class=special><</span><span class=identifier>Arg1</span><span class=special>></span> <span class=identifier>type</span><span class=special>;};</span> <span class=special>};</span> <span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special><</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_1</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>less</span><span class=special><</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_1</span><span class=special>></span> <span class=special>></span> <span class=identifier>foo_specifier</span><span class=special>;</span> <span class=keyword>struct</span> <span class=identifier>foo_specifier</span> <span class=special>{</span> <span class=keyword>template</span><span class=special><</span><span class=keyword>typename</span> <span class=identifier>Arg1</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Arg2</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Arg3</span><span class=special>></span> <span class=keyword>struct</span> <span class=identifier>apply</span><span class=special>{</span><span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special><</span><span class=identifier>Arg1</span><span class=special>,</span><span class=identifier>foo</span><span class=special><</span><span class=identifier>Arg2</span><span class=special>,</span><span class=identifier>Arg3</span><span class=special>></span> <span class=special>></span> <span class=identifier>type</span><span class=special>;};</span> <span class=special>};</span> <span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special><</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_1</span><span class=special>,</span><span class=identifier>foo</span><span class=special><</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_2</span><span class=special>,</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_3</span><span class=special>></span> <span class=special>></span> <span class=identifier>foo_specifier</span><span class=special>;</span> </pre></blockquote> <hr> <div class="prev_link"><a href="technical.html"><img src="../prev.gif" alt="technical issues" border="0"><br> Technical issues </a></div> <div class="up_link"><a href="index.html"><img src="../up.gif" alt="Boost.Flyweight tutorial" border="0"><br> Boost.Flyweight tutorial </a></div> <div class="next_link"><a href="../reference/index.html"><img src="../next.gif" alt="Boost.Flyweight reference" border="0"><br> Boost.Flyweight reference </a></div><br clear="all" style="clear: all;"> <br> <p>Revised August 13th 2008</p> <p>© Copyright 2006-2008 Joaquín M López Muñoz. 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>) </p> </body> </html>