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: Placeholder Expression</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="./lambda-expression.html" class="navigation-link">Prev</a> <a href="./tag-dispatched-metafunction.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./lambda-expression.html" class="navigation-link">Back</a> <a 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href="./metafunctions-concepts.html#id1515">Placeholder Expression</a></h1> 16<div class="section" id="id835"> 17<h3><a class="subsection-title" href="#description" name="description">Description</a></h3> 18<p>A <a class="reference internal" href="./placeholder-expression.html">Placeholder Expression</a> is a type that is either a <a class="reference internal" href="./placeholders.html#placeholder">placeholder</a> or a class 19template specialization with at least one argument that itself is a 20<a class="reference internal" href="./placeholder-expression.html">Placeholder Expression</a>.</p> 21</div> 22<div class="section" id="id836"> 23<h3><a class="subsection-title" href="#expression-requirements" name="expression-requirements">Expression requirements</a></h3> 24<p>If <tt class="literal"><span class="pre">X</span></tt> is a class template, and <tt class="literal"><span class="pre">a1</span></tt>,... <tt class="literal"><span class="pre">an</span></tt> are arbitrary types, then 25<tt class="literal"><span class="pre">X<a1,...,an></span></tt> is a <a class="reference internal" href="./placeholder-expression.html">Placeholder Expression</a> if and only if all of the following 26conditions hold:</p> 27<ul class="simple"> 28<li>At least one of the template arguments <tt class="literal"><span class="pre">a1</span></tt>,... <tt class="literal"><span class="pre">an</span></tt> is a <a class="reference internal" href="./placeholders.html#placeholder">placeholder</a> 29or a <a class="reference internal" href="./placeholder-expression.html">Placeholder Expression</a>.</li> 30<li>All of <tt class="literal"><span class="pre">X</span></tt>'s template parameters, including the default ones, are types.</li> 31<li>The number of <tt class="literal"><span class="pre">X</span></tt>'s template parameters, including the default ones, is 32less or equal to the value of <tt class="literal"><span class="pre"><a href="./limit-metafunction-arity.html" class="identifier">BOOST_MPL_LIMIT_METAFUNCTION_ARITY</a></span></tt> 33<a class="reference internal" href="./configuration.html">configuration macro</a>.</li> 34</ul> 35</div> 36<div class="section" id="id838"> 37<h3><a class="subsection-title" href="#models" name="models">Models</a></h3> 38<ul class="simple"> 39<li><a class="reference internal" href="./placeholders.html">_1</a></li> 40<li><tt class="literal"><span class="pre"><a href="./plus.html" class="identifier">plus</a><_,</span> <span class="pre"><a href="./int.html" class="identifier">int_</a><2></span> <span class="pre">></span></tt></li> 41<li><tt class="literal"><span class="pre"><a href="./if.html" class="identifier">if_</a><</span> <span class="pre"><a href="./less.html" class="identifier">less</a><<a href="./placeholders.html" class="identifier">_1</a>,</span> <span class="pre"><a href="./int.html" class="identifier">int_</a><7></span> <span class="pre">>,</span> <span class="pre"><a href="./plus.html" class="identifier">plus</a><<a href="./placeholders.html" class="identifier">_1</a>,<a href="./placeholders.html" class="identifier">_2</a>>,</span> <span class="pre"><a href="./placeholders.html" class="identifier">_1</a></span> <span class="pre">></span></tt></li> 42</ul> 43</div> 44<div class="section" id="id839"> 45<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> 46<p><a class="reference internal" href="./lambda-expression.html">Lambda Expression</a>, <a class="reference internal" href="./placeholders.html">Placeholders</a>, <a class="reference internal" href="./metafunctions.html">Metafunctions</a>, <a class="reference internal" href="./apply.html">apply</a>, <a class="reference internal" href="./lambda.html">lambda</a></p> 47<!-- Metafunctions/Concepts//Tag Dispatched Metafunction |50 --> 48</div> 49</div> 50 51<div class="footer-separator"></div> 52<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./lambda-expression.html" class="navigation-link">Prev</a> <a href="./tag-dispatched-metafunction.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./lambda-expression.html" class="navigation-link">Back</a> <a href="./tag-dispatched-metafunction.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./metafunctions-concepts.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> 53<td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div> 54Distributed under the Boost Software License, Version 1.0. (See accompanying 55file 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> 56</html> 57