<?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"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <meta name="generator" content="Docutils 0.7: http://docutils.sourceforge.net/" /> <title>The MPL Reference Manual: Placeholder Expression</title> <link rel="stylesheet" href="../style.css" type="text/css" /> </head> <body class="docframe refmanual"> <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> <div class="section" id="id835"> <h3><a class="subsection-title" href="#description" name="description">Description</a></h3> <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 template specialization with at least one argument that itself is a <a class="reference internal" href="./placeholder-expression.html">Placeholder Expression</a>.</p> </div> <div class="section" id="id836"> <h3><a class="subsection-title" href="#expression-requirements" name="expression-requirements">Expression requirements</a></h3> <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 <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 conditions hold:</p> <ul class="simple"> <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> or a <a class="reference internal" href="./placeholder-expression.html">Placeholder Expression</a>.</li> <li>All of <tt class="literal"><span class="pre">X</span></tt>'s template parameters, including the default ones, are types.</li> <li>The number of <tt class="literal"><span class="pre">X</span></tt>'s template parameters, including the default ones, is less 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> <a class="reference internal" href="./configuration.html">configuration macro</a>.</li> </ul> </div> <div class="section" id="id838"> <h3><a class="subsection-title" href="#models" name="models">Models</a></h3> <ul class="simple"> <li><a class="reference internal" href="./placeholders.html">_1</a></li> <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> <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> </ul> </div> <div class="section" id="id839"> <h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> <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> <!-- Metafunctions/Concepts//Tag Dispatched Metafunction |50 --> </div> </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-expression.html" 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(See accompanying file 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> </html>