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<head> 5<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> 6<meta name="generator" content="Docutils 0.7: http://docutils.sourceforge.net/" /> 7<title>The MPL Reference Manual: Metafunctions</title> 8<link rel="stylesheet" href="../style.css" type="text/css" /> 9</head> 10<body class="docframe refmanual"> 11<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./for-each.html" class="navigation-link">Prev</a> <a href="./metafunctions-concepts.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./algorithms.html" class="navigation-link">Back</a> <a href="./data-types.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="../refmanual.html" class="navigation-link">Up</a> <a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td> 12<td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./metafunctions.html" class="navigation-link">Metafunctions</a></td> 13</tr></table><div class="header-separator"></div> 14<div class="section" id="metafunctions"> 15<h1><a class="toc-backref" href="../refmanual.html#id1510">Metafunctions</a></h1> 16<p id="label-metafunctions">The MPL includes a number of predefined metafunctions that can be roughly 17classified in two categories: <cite>general purpose metafunctions</cite>, dealing with 18conditional <a class="reference internal" href="./type-selection.html">type selection</a> and higher-order metafunction <a class="reference internal" href="./invocation.html">invocation</a>, 19<a class="reference internal" href="./composition-and-argument-binding.html">composition</a>, and <a class="reference internal" href="./composition-and-argument-binding.html">argument binding</a>, and <cite>numeric metafunctions</cite>, 20incapsulating built-in and user-defined <a class="reference internal" href="./arithmetic-operations.html">arithmetic</a>, <a class="reference internal" href="./comparisons.html">comparison</a>, 21<a class="reference internal" href="./logical-operations.html">logical</a>, and <a class="reference internal" href="./bitwise-operations.html">bitwise</a> operations.</p> 22<p>Given that it is possible to perform integer numeric computations at 23compile time using the conventional operators notation, the need for the 24second category might be not obvious, but it in fact plays a cental role in 25making programming with MPL seemingly effortless. In 26particular, there are at least two contexts where built-in language 27facilities fall short <a class="footnote-reference" href="#portability" id="id820">[3]</a>:</p> 28<ol class="arabic simple"> 29<li>Passing a computation to an algorithm.</li> 30<li>Performing a computation on non-integer data.</li> 31</ol> 32<p>The second use case deserves special attention. In contrast to the built-in, 33strictly integer compile-time arithmetics, the MPL numeric metafunctions are 34<em>polymorphic</em>, with support for <em>mixed-type arithmetics</em>. This means that they 35can operate on a variety of numeric types — for instance, rational, 36fixed-point or complex numbers, — and that, in general, you are allowed to 37freely intermix these types within a single expression. See <a class="reference internal" href="./numeric-metafunction.html">Numeric Metafunction</a> concept for more details on the MPL numeric infrastructure.</p> 38<!-- The provided `infrastructure`__ allows easy plugging of user-defined numeric 39types 40Naturally, they also , meaning that you can perform a computation on the 41arguments of different types, and the result will yeild the largest/most general 42of them. For user-defined numeric types, they provide an `infrastructure`__ that 43allows easy plugging and seemless integration with predefined library 44types. details. 45 46__ `Numeric Metafunction`_ --> 47<p>To reduce a negative syntactical impact of the metafunctions notation 48over the infix operator notation, all numeric metafunctions 49allow to pass up to N arguments, where N is defined by the value of 50<a class="reference internal" href="./limit-metafunction-arity.html">BOOST_MPL_LIMIT_METAFUNCTION_ARITY</a> configuration macro.</p> 51<table class="docutils footnote" frame="void" id="portability" rules="none"> 52<colgroup><col class="label" /><col /></colgroup> 53<tbody valign="top"> 54<tr><td class="label"><a class="fn-backref" href="#id820">[3]</a></td><td>All other considerations aside, as of the time of this writing 55(early 2004), using built-in operators on integral constants still often 56present a portability problem — many compilers cannot handle particular 57forms of expressions, forcing us to use conditional compilation. Because MPL 58numeric metafunctions work on types and encapsulate these kind of workarounds 59internally, they elude these problems, so if you aim for portability, it is 60generally adviced to use them in the place of the conventional operators, even 61at the price of slightly decreased readability.</td></tr> 62</tbody> 63</table> 64<ul class="toc simple" id="outline"> 65<li><a class="reference internal" href="./metafunctions-concepts.html" id="id1511">Concepts</a></li> 66<li><a class="reference internal" href="./type-selection.html" id="id1519">Type Selection</a></li> 67<li><a class="reference internal" href="./invocation.html" id="id1524">Invocation</a></li> 68<li><a class="reference internal" href="./composition-and-argument-binding.html" id="id1528">Composition and Argument Binding</a></li> 69<li><a class="reference internal" href="./arithmetic-operations.html" id="id1535">Arithmetic Operations</a></li> 70<li><a class="reference internal" href="./comparisons.html" id="id1542">Comparisons</a></li> 71<li><a class="reference internal" href="./logical-operations.html" id="id1549">Logical Operations</a></li> 72<li><a class="reference internal" href="./bitwise-operations.html" id="id1553">Bitwise Operations</a></li> 73<li><a class="reference internal" href="./trivial.html" id="id1559">Trivial</a></li> 74<li><a class="reference internal" href="./string-operations.html" id="id1561">String Operations</a></li> 75<li><a class="reference internal" href="./miscellaneous.html" id="id1563">Miscellaneous</a></li> 76</ul> 77</div> 78 79<div class="footer-separator"></div> 80<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./for-each.html" class="navigation-link">Prev</a> <a href="./metafunctions-concepts.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./algorithms.html" class="navigation-link">Back</a> <a href="./data-types.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="../refmanual.html" class="navigation-link">Up</a> <a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td> 81<td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div> 82Distributed under the Boost Software License, Version 1.0. (See accompanying 83file LICENSE_1_0.txt or copy at <a class="reference external" href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)</div></td></tr></table></body> 84</html> 85