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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="./composition-and-argument-binding.html" class="navigation-link">Composition and Argument Binding</a> / <a href="./lambda.html" class="navigation-link">lambda</a></td>
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<div class="section" id="lambda">
<h1><a class="toc-backref" href="./composition-and-argument-binding.html#id1530">lambda</a></h1>
<div class="section" id="id922">
<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
<pre class="literal-block">
template&lt;
      typename X
    , typename Tag = <em>unspecified</em>
    &gt;
struct <a href="./lambda.html" class="identifier">lambda</a>
{
    typedef <em>unspecified</em> type;
};
</pre>
</div>
<div class="section" id="id923">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<p>If <tt class="literal"><span class="pre">X</span></tt> is a <a class="reference internal" href="./placeholder-expression.html">Placeholder Expression</a>, transforms <tt class="literal"><span class="pre">X</span></tt> into a corresponding
<a class="reference internal" href="./metafunction-class.html">Metafunction Class</a>, otherwise <tt class="literal"><span class="pre">X</span></tt> is returned unchanged.</p>
</div>
<div class="section" id="id924">
<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
<pre class="literal-block">
#include &lt;<a href="../../../../boost/mpl/lambda.hpp" class="header">boost/mpl/lambda.hpp</a>&gt;
</pre>
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<div class="section" id="id925">
<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
<table border="1" class="docutils table">
<colgroup>
<col width="18%" />
<col width="27%" />
<col width="55%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Parameter</th>
<th class="head">Requirement</th>
<th class="head">Description</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">X</span></tt></td>
<td>Any type</td>
<td>An expression to transform.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">Tag</span></tt></td>
<td>Any type</td>
<td>A tag determining transform semantics.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="id926">
<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
<p>For arbitrary types <tt class="literal"><span class="pre">x</span></tt> and <tt class="literal"><span class="pre">tag</span></tt>:</p>
<pre class="literal-block">
typedef <a href="./lambda.html" class="identifier">lambda</a>&lt;x&gt;::type f;
</pre>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first"><a class="reference internal" href="./metafunction-class.html">Metafunction Class</a>.</p>
</td>
</tr>
<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">If <tt class="literal"><span class="pre">x</span></tt> is a <a class="reference internal" href="./placeholder-expression.html">Placeholder Expression</a> in a general form <tt class="literal"><span class="pre">X&lt;a1,...an&gt;</span></tt>, where
<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, equivalent
to</p>
<pre class="literal-block">
typedef <a href="./protect.html" class="identifier">protect</a>&lt; <a href="./bind.html" class="identifier">bind</a>&lt;
      <a href="./quote.html" class="identifier">quote</a><em>n</em>&lt;X&gt;
    , <a href="./lambda.html" class="identifier">lambda</a>&lt;a1&gt;::type,<em>...</em> <a href="./lambda.html" class="identifier">lambda</a>&lt;a<em>n</em>&gt;::type
    &gt; &gt; f;
</pre>
<p class="last">otherwise, <tt class="literal"><span class="pre">f</span></tt> is identical to <tt class="literal"><span class="pre">x</span></tt>.</p>
</td>
</tr>
</tbody>
</table>
<!-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -->
<pre class="literal-block">
typedef <a href="./lambda.html" class="identifier">lambda</a>&lt;x,tag&gt;::type f;
</pre>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first"><a class="reference internal" href="./metafunction-class.html">Metafunction Class</a>.</p>
</td>
</tr>
<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">If <tt class="literal"><span class="pre">x</span></tt> is a <a class="reference internal" href="./placeholder-expression.html">Placeholder Expression</a> in a general form <tt class="literal"><span class="pre">X&lt;a1,...an&gt;</span></tt>, where
<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, equivalent
to</p>
<pre class="literal-block">
typedef <a href="./protect.html" class="identifier">protect</a>&lt; <a href="./bind.html" class="identifier">bind</a>&lt;
      <a href="./quote.html" class="identifier">quote</a><em>n</em>&lt;X,tag&gt;
    , <a href="./lambda.html" class="identifier">lambda</a>&lt;a1,tag&gt;::type,<em>...</em> <a href="./lambda.html" class="identifier">lambda</a>&lt;a<em>n</em>,tag&gt;::type
    &gt; &gt; f;
</pre>
<p class="last">otherwise, <tt class="literal"><span class="pre">f</span></tt> is identical to <tt class="literal"><span class="pre">x</span></tt>.</p>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="id927">
<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
<pre class="literal-block">
template&lt; typename N1, typename N2 &gt; struct int_plus
    : <a href="./int.html" class="identifier">int_</a>&lt;( N1::value + N2::value )&gt;
{
};

typedef <a href="./lambda.html" class="identifier">lambda</a>&lt; int_plus&lt;<a href="./placeholders.html" class="identifier">_1</a>, <a href="./int.html" class="identifier">int_</a>&lt;42&gt; &gt; &gt;::type f1;
typedef <a href="./bind.html" class="identifier">bind</a>&lt; <a href="./quote.html" class="identifier">quote</a><tt class="literal"><span class="pre">2</span></tt>&lt;int_plus&gt;, <a href="./placeholders.html" class="identifier">_1</a>, <a href="./int.html" class="identifier">int_</a>&lt;42&gt; &gt; f2;

typedef f1::<a href="./apply.html" class="identifier">apply</a>&lt;42&gt;::type r1;
typedef f2::<a href="./apply.html" class="identifier">apply</a>&lt;42&gt;::type r2;

<a href="./assert-relation.html" class="identifier">BOOST_MPL_ASSERT_RELATION</a>( r1::value, ==, 84 );
<a href="./assert-relation.html" class="identifier">BOOST_MPL_ASSERT_RELATION</a>( r2::value, ==, 84 );
</pre>
</div>
<div class="section" id="id928">
<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
<p><a class="reference internal" href="./composition-and-argument-binding.html">Composition and Argument Binding</a>, <a class="reference internal" href="./invocation.html">invocation</a>, <a class="reference internal" href="./placeholders.html">Placeholders</a>, <a class="reference internal" href="./bind.html">bind</a>, <a class="reference internal" href="./quote.html">quote</a>, <a class="reference internal" href="./protect.html">protect</a>, <a class="reference internal" href="./apply.html">apply</a></p>
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