<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost --> <!-- Software License, Version 1.0. (See accompanying --> <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" /> <title>THE BOOST MPL LIBRARY: The Importance of Being Lazy</title> <link rel="stylesheet" href="../style.css" type="text/css" /> </head> <body class="docframe"> <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> <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> </tr></table><div class="header-separator"></div> <div class="section" id="the-importance-of-being"> <h1><a class="toc-backref" href="./lambda-details.html#id58" name="the-importance-of-being">The Importance of Being Lazy</a></h1> <p>Recall the definition of <tt class="literal"><span class="pre">always_int</span></tt> from the previous chapter:</p> <pre class="literal-block"> struct always_int { typedef int type; }; </pre> <p>Nullary metafunctions might not seem very important at first, since something 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 any lambda expression where it appears. Not all nullary metafunctions are that simple, though:</p> <pre class="literal-block"> typedef mpl::vector<int, char*, double&> seq; typedef <strong>mpl::transform<seq, boost::add_pointer<_> ></strong> calc_ptr_seq; </pre> <!-- @ example.prepend(''' #include <boost/mpl/vector.hpp> #include <boost/mpl/transform.hpp> ''') compile('all') --> <p>Note that <tt class="literal"><span class="pre">calc_ptr_seq</span></tt> is a nullary metafunction, since it has <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 instantiated until we actually "look inside it," though. Just naming <tt class="literal"><span class="pre">calc_ptr_seq</span></tt> does not cause it to be evaluated, since we haven't accessed its <tt class="literal"><span class="pre">::type</span></tt> yet.</p> <p>Metafunctions can be invoked <em>lazily</em>, rather than immediately upon supplying all of their arguments. We can use <strong>lazy evaluation</strong> to improve compilation time when a metafunction result is only going to be used conditionally. We can sometimes also avoid contorting program structure by <em>naming</em> an invalid computation without actually performing it. That's what we've done with <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>. Laziness and all of its virtues will be a recurring theme throughout this book.</p> </div> <div class="footer-separator"></div> <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> </tr></table></body> </html>