1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0.1 Transitional//EN"> 2 3<html> 4<head> 5<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> 6<title>Boost.Flyweight Documentation - Tutorial - Annex - MPL lambda expressions</title> 7<link rel="stylesheet" href="../style.css" type="text/css"> 8<link rel="start" href="../index.html"> 9<link rel="prev" href="technical.html"> 10<link rel="up" href="index.html"> 11<link rel="next" href="../reference/index.html"> 12</head> 13 14<body> 15<h1><img src="../../../../boost.png" alt="Boost logo" align= 16"middle" width="277" height="86">Boost.Flyweight Tutorial Annex: MPL lambda expressions</h1> 17 18<div class="prev_link"><a href="technical.html"><img src="../prev.gif" alt="technical issues" border="0"><br> 19Technical issues 20</a></div> 21<div class="up_link"><a href="index.html"><img src="../up.gif" alt="Boost.Flyweight tutorial" border="0"><br> 22Boost.Flyweight tutorial 23</a></div> 24<div class="next_link"><a href="../reference/index.html"><img src="../next.gif" alt="Boost.Flyweight reference" border="0"><br> 25Boost.Flyweight reference 26</a></div><br clear="all" style="clear: all;"> 27 28<hr> 29 30<p>This short introduction to lambda expressions is meant for readers unfamiliar 31with the <a href="../../../mpl/doc/index.html">Boost MPL Library</a> who 32want to rapidly acquire a working knowledge of the basic concepts for the purposes 33of using them in Boost.Flyweight. Please refer to the Boost.MPL documentation 34for further information beyond these introductory notes. 35</p> 36 37<p> 38The specifiers defined by Boost.Flyweight rely heavily on the 39<a href="../../../mpl/doc/refmanual/lambda-expression.html"><code>Lambda 40Expression</code></a> concept defined by the 41<a href="../../../mpl/doc/index.html">Boost MPL Library</a>. A lambda 42expression can be thought of as a compile-time "type function", an entity (a 43concrete type, actually) that can be invoked with a list of types and returns 44some associated type in its turn. Consider for instance an arbitrary class 45template: 46</p> 47 48<blockquote><pre> 49<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> 50<span class=keyword>class</span> <span class=identifier>foo</span> 51<span class=special>{</span> 52 <span class=special>...</span> 53<span class=special>};</span> 54</pre></blockquote> 55 56<p> 57and suppose we want to have a lambda expression that, when invoked 58with some generic types <code>Arg1</code> and <code>Arg2</code>, 59returns <code>foo<Arg1,Arg2></code>. Such a lambda expression 60can be implemented in two ways 61<ol> 62 <li>As a 63 <a href="../../../mpl/doc/refmanual/metafunction-class.html"><code>MPL 64 Metafunction Class</code></a>, a type with a special nested class template 65 named <code>apply</code>: 66<blockquote><pre> 67<span class=keyword>struct</span> <span class=identifier>foo_specifier</span> 68<span class=special>{</span> 69 <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> 70 <span class=keyword>struct</span> <span class=identifier>apply</span> 71 <span class=special>{</span> 72 <span class=comment>// this is the "return type" of foo_specifier</span> 73 <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> 74 <span class=special>};</span> 75<span class=special>};</span> 76</pre></blockquote> 77 </li> 78 <li> 79 As a 80 <a href="../../../mpl/doc/refmanual/placeholder-expression.html"><code>MPL 81 Placeholder Expression</code></a>, a class template instantiated with one or 82 more <i>placeholders</i>: 83<blockquote><pre> 84<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> 85</pre></blockquote> 86 Note that, in this case, <code>foo_specifier</code> is a concrete type, much 87 as <code>int</code> or <code>std::set<std::string></code> are; yet, 88 MPL internal mechanisms are able to detect that this type has been gotten 89 from instantiating a class template with placeholders <code>boost::mpl::_1</code> 90 and <code>boost::mpl::_2</code> and take these placeholders as slots to 91 be substituted for actual types (the first and second type supplied, 92 respectively) when <code>foo_specifier</code> is 93 invoked. So, an instantiation of <code>foo</code> can be used 94 to refer back to the <code>foo</code> class template itself! The net 95 effect is the same as with metafunctions, but placeholder expressions spare 96 us the need to write boilerplate metafunction classes 97 --and the kind of metaprogramming magic they depend on has an undeniable 98 beauty to it. 99 </li> 100</ol> 101So far the examples shown just forward the arguments <code>Arg1</code> and 102<code>Arg2</code> directly to a class template without further elaboration, 103but there is nothing preventing us from doing some argument manipulation, 104like, for instance, switching their places: 105</p> 106 107<blockquote><pre> 108<span class=keyword>struct</span> <span class=identifier>foo_specifier</span> 109<span class=special>{</span> 110 <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> 111 <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> 112<span class=special>};</span> 113 114<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> 115</pre></blockquote> 116 117<p> 118passing placeholder subexpressions as arguments to the overall expression: 119</p> 120 121<blockquote><pre> 122<span class=keyword>struct</span> <span class=identifier>foo_specifier</span> 123<span class=special>{</span> 124 <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> 125 <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> 126<span class=special>};</span> 127 128<span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special><</span> 129 <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> 130 <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> 131<span class=special>></span> <span class=identifier>foo_specifier</span><span class=special>;</span> 132</pre></blockquote> 133 134<p> 135or accepting less or more arguments than the class template itself 136(the number of parameters of a lambda expression is called its <i>arity</i>): 137</p> 138 139<blockquote><pre> 140<span class=keyword>struct</span> <span class=identifier>foo_specifier</span> 141<span class=special>{</span> 142 <span class=keyword>template</span><span class=special><</span><span class=keyword>typename</span> <span class=identifier>Arg1</span><span class=special>></span> 143 <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> 144<span class=special>};</span> 145 146<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> 147 148<span class=keyword>struct</span> <span class=identifier>foo_specifier</span> 149<span class=special>{</span> 150 <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> 151 <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> 152<span class=special>};</span> 153 154<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> 155</pre></blockquote> 156 157<hr> 158 159<div class="prev_link"><a href="technical.html"><img src="../prev.gif" alt="technical issues" border="0"><br> 160Technical issues 161</a></div> 162<div class="up_link"><a href="index.html"><img src="../up.gif" alt="Boost.Flyweight tutorial" border="0"><br> 163Boost.Flyweight tutorial 164</a></div> 165<div class="next_link"><a href="../reference/index.html"><img src="../next.gif" alt="Boost.Flyweight reference" border="0"><br> 166Boost.Flyweight reference 167</a></div><br clear="all" style="clear: all;"> 168 169<br> 170 171<p>Revised August 13th 2008</p> 172 173<p>© Copyright 2006-2008 Joaquín M López Muñoz. 174Distributed under the Boost Software 175License, Version 1.0. (See accompanying file <a href="../../../../LICENSE_1_0.txt"> 176LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt"> 177http://www.boost.org/LICENSE_1_0.txt</a>) 178</p> 179 180</body> 181</html> 182
