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17<div class="section" id="placeholder-expression">
18<h1><a class="toc-backref" href="./metafunctions-concepts.html#id441" name="placeholder-expression">Placeholder Expression</a></h1>
19<div class="section" id="placeholder-description">
20<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
21<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
22template specialization with at least one argument that itself is a
23<a class="reference" href="./placeholder-expression.html">Placeholder Expression</a>.</p>
24</div>
25<div class="section" id="placeholder-expression-requirements">
26<h3><a class="subsection-title" href="#expression-requirements" name="expression-requirements">Expression requirements</a></h3>
27<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
28<tt class="literal"><span class="pre">X&lt;a1,...,an&gt;</span></tt> is a <a class="reference" href="./placeholder-expression.html">Placeholder Expression</a> if and only if all of the following
29conditions hold:</p>
30<ul class="simple">
31<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>
32or a <a class="reference" href="./placeholder-expression.html">Placeholder Expression</a>.</li>
33<li>All of <tt class="literal"><span class="pre">X</span></tt>'s template parameters, including the default ones, are types.</li>
34<li>The number of <tt class="literal"><span class="pre">X</span></tt>'s template parameters, including the default ones, is
35less 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> 
36<a class="reference" href="./configuration.html">configuration macro</a>.</li>
37</ul>
38</div>
39<div class="section" id="placeholder-models">
40<h3><a class="subsection-title" href="#models" name="models">Models</a></h3>
41<ul class="simple">
42<li><a class="refentry reference" href="./placeholders.html"><tt class="refentry literal"><span class="pre">_1</span></tt></a></li>
43<li><tt class="literal"><span class="pre"><a href="./plus.html" class="identifier">plus</a>&lt;_,</span> <span class="pre"><a href="./int.html" class="identifier">int_</a>&lt;2&gt;</span> <span class="pre">&gt;</span></tt></li>
44<li><tt class="literal"><span class="pre"><a href="./if.html" class="identifier">if_</a>&lt;</span> <span class="pre"><a href="./less.html" class="identifier">less</a>&lt;_1,</span> <span class="pre"><a href="./int.html" class="identifier">int_</a>&lt;7&gt;</span> <span class="pre">&gt;,</span> <span class="pre"><a href="./plus.html" class="identifier">plus</a>&lt;_1,_2&gt;,</span> <span class="pre">_1</span> <span class="pre">&gt;</span></tt></li>
45</ul>
46</div>
47<div class="section" id="placeholder-see-also">
48<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
49<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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