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: The Importance of Being Lazy</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="./lambda-and-non.html" class="navigation-link">Prev</a> <a href="./details.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./lambda-and-non.html" class="navigation-link">Back</a> Along</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="./the-importance-of-being.html" class="navigation-link">The Importance of Being Lazy</a></td> 16</tr></table><div class="header-separator"></div> 17<div class="section" id="the-importance-of-being"> 18<h1><a class="toc-backref" href="./lambda-details.html#id58" name="the-importance-of-being">The Importance of Being Lazy</a></h1> 19<p>Recall the definition of <tt class="literal"><span class="pre">always_int</span></tt> from the previous chapter:</p> 20<pre class="literal-block"> 21struct always_int 22{ 23 typedef int type; 24}; 25</pre> 26<p>Nullary metafunctions might not seem very important at first, since 27something like <tt class="literal"><span class="pre">add_pointer<int></span></tt> could be replaced by <tt class="literal"><span class="pre">int*</span></tt> in 28any lambda expression where it appears. Not all nullary 29metafunctions are that simple, though:</p> 30<pre class="literal-block"> 31typedef mpl::vector<int, char*, double&> seq; 32typedef <strong>mpl::transform<seq, boost::add_pointer<_> ></strong> calc_ptr_seq; 33</pre> 34<!-- @ example.prepend(''' 35 #include <boost/mpl/vector.hpp> 36 #include <boost/mpl/transform.hpp> 37''') 38compile('all') --> 39<p>Note that <tt class="literal"><span class="pre">calc_ptr_seq</span></tt> is a nullary metafunction, since it has 40<tt class="literal"><span class="pre">transform</span></tt>'s nested <tt class="literal"><span class="pre">::type</span></tt>. A C++ template is not 41instantiated until we actually "look inside it," though. Just 42naming <tt class="literal"><span class="pre">calc_ptr_seq</span></tt> does not cause it to be evaluated, since we 43haven't accessed its <tt class="literal"><span class="pre">::type</span></tt> yet.</p> 44<p>Metafunctions can be invoked <em>lazily</em>, rather than immediately upon 45supplying all of their arguments. We can use <strong>lazy evaluation</strong> to 46improve compilation time when a metafunction result is only going 47to be used conditionally. We can sometimes also avoid contorting 48program structure by <em>naming</em> an invalid computation without 49actually performing it. That's what we've done with 50<tt class="literal"><span class="pre">calc_ptr_seq</span></tt> above, since you can't legally form <tt class="literal"><span class="pre">double&*</span></tt>. 51Laziness and all of its virtues will be a recurring theme 52throughout this book.</p> 53</div> 54 55<div class="footer-separator"></div> 56<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./lambda-and-non.html" class="navigation-link">Prev</a> <a href="./details.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./lambda-and-non.html" class="navigation-link">Back</a> Along</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> 57</tr></table></body> 58</html> 59