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: Incomplete Support for Lambda Expressions</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="./broken-integral-constant.html" class="navigation-link">Prev</a> <a href="./eti.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./broken-integral-constant.html" class="navigation-link">Back</a> <a href="./eti.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./portability.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="./technical-details.html" class="navigation-link">Technical Details</a> / <a href="./portability.html" class="navigation-link">Portability</a> / <a href="./incomplete-support-for.html" class="navigation-link">Incomplete Support for Lambda Expressions</a></td> 16</tr></table><div class="header-separator"></div> 17<div class="section" id="incomplete-support-for"> 18<h1><a class="toc-backref" href="./portability.html#id75" name="incomplete-support-for">Incomplete Support for Lambda Expressions</a></h1> 19<p>Seasoned MPL users will agree with us that if there is 20anything in the MPL that is seemingly magical both in power and 21its nature, then it's MPL lambda expressions. In reality, the 22mechanism that bring this to life is very straightforward and 23probably can be explained to anyone generally familiar with C++ 24templates in less than 10 minutes.</p> 25<!-- People are going to hate you for saying so and not actually --> 26<!-- explaining it. --> 27<p>Unfortunately, this mechanism also happens to rely on support for 28partial template specialization and template template parameters. 29Among the so-called deficient compilers — basically, most of the 30compilers released before the year 2000 — the chances are poor 31that you'll find <em>complete</em> support for <em>both</em> of these features. 32Please see our <a class="reference" href="./portability.html#compatibility-table">compatibility table</a> for the list of the products 33which fall into this category.</p> 34<p>Although it's not possible to implement <em>fully transparent</em> lambda 35expressions without these two features, a slightly more limited 36implementation that requires some manual assistance from the 37metafunction author is possible. This section describes the manual 38work required and the limitations of the result.</p> 39<div class="section" id="incomplete-the-problem"> 40<h2><a name="incomplete-the-problem">The Problem</a></h2> 41<p>If your compiler falls into the "deficient" category, the following 42valid MPL metaprogram will fail to compile for you:</p> 43<pre class="literal-block"> 44#include <boost/mpl/apply.hpp> 45 46using namespace boost::mpl; 47 48template< typename T > struct add_const 49{ 50 typedef T const type; 51}; 52 53typedef apply1< add_const<_1>,int >::type t; // t == int const 54</pre> 55<p>Worse yet, chances are it wil fail with a diagnostic backtrace 56leading you into the inside of the library and possibly creating an 57impression that there's something's wrong there. The fact is, both 58the program and the library are defect free (for the 59purpose of this particular demonstraction), and it's your compiler 60that is to blame.</p> 61</div> 62<div class="section" id="incomplete-the-solution"> 63<h2><a name="incomplete-the-solution">The Solution</a></h2> 64<p>As previously mentioned, the solution requires some work from 65metafunction authors, but for the users of those metafunctions, the 66result is relatively transparent. Here's what we have to do to our 67earlier example:</p> 68<pre class="literal-block"> 69#include <boost/mpl/apply.hpp> 70<strong>#include <boost/mpl/aux_/lambda_support.hpp></strong> 71 72using namespace boost::mpl; 73 74template< typename T > struct add_const 75{ 76 typedef T const type; 77 <strong>BOOST_MPL_AUX_LAMBDA_SUPPORT(1, add_const, (T))</strong> 78}; 79 80typedef apply1< add_const<_1>,int >::type t; // t == int const 81</pre> 82<p>With these two modifications, now the compiler that has been barking at us now 83happily accepts it. "Hey, that's not that bad at all!" you might say. Just put a 84little macro inside and be happy again.</p> 85</div> 86<div class="section" id="limitations"> 87<h2><a name="limitations">Limitations</a></h2> 88<p>Unfortunately, that's not quite the end of the story. There are 89still cases where the above approach will fail and we will have to 90resort to writing out-of-line metafunction class. Here are the 91details:</p> 92<blockquote> 93<p>To make the lambda expression work without partial template 94specialization and 95template template parameters, the MPL has to implement some other way of 96pulling apart the template instantiations' expression tree, and the only way 97to do it is through an intrusive metafunction introspection 98mechanism. That's what hidden behind the <tt class="literal"><span class="pre">BOOST_MPL_AUX_LAMBDA_SUPPORT</span></tt> macro 99we've seen above.</p> 100<p>But then, after we've got the information we need (the metafunction's arity 101and its exact template arguments) stored inside the metafunction itself, 102the only way for the library to access it is to look inside the metafunction. 103The latter, in its turn, means instantiating the metafunction, prematurely, 104before the actuall call, <em>with one or more placeholder arguments</em>. This last 105part is a potential problem.</p> 106</blockquote> 107<p>In other words, the mechanism works as long as your metafunction is 108"placeholder-safe" (can be safely instantiated on placeholder 109arguments), which comes down to the follwing two criteria:</p> 110<ol class="arabic simple"> 111<li>The metafunction doesn't access its arguments' nested members, or</li> 112<li>The only accessed members are types named <tt class="literal"><span class="pre">::tag</span></tt> or <tt class="literal"><span class="pre">::type</span></tt> (the 113placeholders do contain these).</li> 114</ol> 115<p>If these two hold, you can safely put <tt class="literal"><span class="pre">BOOST_MPL_AUX_LAMBDA_SUPPORT</span></tt> inside 116your metafunction and forget about the issue. If not, you are out of luck and 117probably have to write a metafunction class instead.</p> 118<p>The good news are that most of the MPL's own metafunctions and <a class="reference" href="../../../type_traits/index.html" target="_top">Boost.Type Traits</a> 119templates are "placeholder-safe" and have the workaround applied to them, so 120even on broken compilers things "just work" in about 90% of use cases.</p> 121<p>Please refer to the MPL <a class="reference" href="./reference-manual.html">reference manual</a> for the details on the 122<tt class="literal"><span class="pre">BOOST_MPL_AUX_LAMBDA_SUPPORT</span></tt> macro.</p> 123</div> 124</div> 125 126<div class="footer-separator"></div> 127<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./broken-integral-constant.html" class="navigation-link">Prev</a> <a href="./eti.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./broken-integral-constant.html" class="navigation-link">Back</a> <a href="./eti.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./portability.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> 128</tr></table></body> 129</html> 130