1<?xml version="1.0" encoding="utf-8" ?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> 4<!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost --> 5<!-- Software License, Version 1.0. (See accompanying --> 6<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> 7<head> 8<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> 9<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" /> 10<title>THE BOOST MPL LIBRARY: Lambda and Non-Metafunction Templates</title> 11<link rel="stylesheet" href="../style.css" type="text/css" /> 12</head> 13<body class="docframe"> 14<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./placeholder-expression.html" class="navigation-link">Prev</a> <a href="./the-importance-of-being.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./placeholder-expression.html" class="navigation-link">Back</a> <a href="./the-importance-of-being.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./lambda-details.html" class="navigation-link">Up</a> <a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td> 15<td class="header-group page-location"><a href="../index.html" class="navigation-link">Front Page</a> / <a href="./tutorial-metafunctions.html" class="navigation-link">Tutorial: Metafunctions and Higher-Order Metaprogramming</a> / <a href="./lambda-details.html" class="navigation-link">Lambda Details</a> / <a href="./lambda-and-non.html" class="navigation-link">Lambda and Non-Metafunction Templates</a></td> 16</tr></table><div class="header-separator"></div> 17<div class="section" id="lambda-and-non"> 18<h1><a class="toc-backref" href="./lambda-details.html#id57" name="lambda-and-non">Lambda and Non-Metafunction Templates</a></h1> 19<p>There is just one detail of placeholder expressions that we haven't 20discussed yet. MPL uses a special rule to make it easier to 21integrate ordinary templates into metaprograms: After all of the 22placeholders have been replaced with actual 23arguments, if the resulting template specialization <em>X</em> doesn't 24have a nested <tt class="literal"><span class="pre">::type</span></tt>, the result is just 25<em>X</em> itself.</p> 26<p>For example, <tt class="literal"><span class="pre">mpl::apply<std::vector<_>,</span> <span class="pre">T></span></tt> is always just 27<tt class="literal"><span class="pre">std::vector<T></span></tt>. If it weren't for this behavior, we would 28have to build trivial metafunctions to create ordinary template 29specializations in lambda expressions:</p> 30<pre class="literal-block"> 31// trivial std::vector generator 32template<class U> 33struct make_vector { typedef std::vector<U> type; }; 34 35typedef mpl::apply<<strong>make_vector<_></strong>, T>::type vector_of_t; 36</pre> 37<p>Instead, we can simply write:</p> 38<pre class="literal-block"> 39typedef mpl::apply<<strong>std::vector<_></strong>, T>::type vector_of_t; 40</pre> 41<!-- @ # ensure indentation works. The ReST parser will push the 2nd line left 42while stack: 43 stack[-1].prepend('#include <vector>\ntypedef int T;') 44 compile() --> 45</div> 46 47<div class="footer-separator"></div> 48<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./placeholder-expression.html" class="navigation-link">Prev</a> <a href="./the-importance-of-being.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./placeholder-expression.html" class="navigation-link">Back</a> <a href="./the-importance-of-being.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./lambda-details.html" class="navigation-link">Up</a> <a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td> 49</tr></table></body> 50</html> 51