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59 lines
6.7 KiB
HTML
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<title>The MPL Reference Manual: Placeholder Expression</title>
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<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.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.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>
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<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> / <a href="./metafunctions-concepts.html" class="navigation-link">Concepts</a> / <a href="./placeholder-expression.html" class="navigation-link">Placeholder Expression</a></td>
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</tr></table><div class="header-separator"></div>
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<div class="section" id="placeholder-expression">
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<h1><a class="toc-backref" href="./metafunctions-concepts.html#id441" name="placeholder-expression">Placeholder Expression</a></h1>
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<div class="section" id="placeholder-description">
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<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
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<p>A <a class="reference" href="./placeholder-expression.html">Placeholder Expression</a> is a type that is either a <a class="reference" href="./placeholders.html">placeholder</a> or a class
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template specialization with at least one argument that itself is a
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<a class="reference" href="./placeholder-expression.html">Placeholder Expression</a>.</p>
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</div>
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<div class="section" id="placeholder-expression-requirements">
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<h3><a class="subsection-title" href="#expression-requirements" name="expression-requirements">Expression requirements</a></h3>
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<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
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<tt class="literal"><span class="pre">X<a1,...,an></span></tt> is a <a class="reference" href="./placeholder-expression.html">Placeholder Expression</a> if and only if all of the following
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conditions hold:</p>
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<ul class="simple">
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<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" href="./placeholders.html">placeholder</a>
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or a <a class="reference" href="./placeholder-expression.html">Placeholder Expression</a>.</li>
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<li>All of <tt class="literal"><span class="pre">X</span></tt>'s template parameters, including the default ones, are types.</li>
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<li>The number of <tt class="literal"><span class="pre">X</span></tt>'s template parameters, including the default ones, is
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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>
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<a class="reference" href="./configuration.html">configuration macro</a>.</li>
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</ul>
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</div>
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<div class="section" id="placeholder-models">
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<h3><a class="subsection-title" href="#models" name="models">Models</a></h3>
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<ul class="simple">
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<li><a class="refentry reference" href="./placeholders.html"><tt class="refentry literal"><span class="pre">_1</span></tt></a></li>
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<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>
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<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><_1,</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><_1,_2>,</span> <span class="pre">_1</span> <span class="pre">></span></tt></li>
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</ul>
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</div>
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<div class="section" id="placeholder-see-also">
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<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
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<p><a class="reference" href="./lambda-expression.html">Lambda Expression</a>, <a class="reference" href="./placeholders.html">Placeholders</a>, <a class="reference" href="./metafunctions.html">Metafunctions</a>, <a class="refentry reference" href="./apply.html"><tt class="refentry literal"><span class="pre">apply</span></tt></a>, <a class="refentry reference" href="./lambda.html"><tt class="refentry literal"><span class="pre">lambda</span></tt></a></p>
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