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<div class="section" id="the-apply-metafunction">
<h1><a class="toc-backref" href="./handling-placeholders.html#id50" name="the-apply-metafunction">The <tt class="literal"><span class="pre">apply</span></tt> Metafunction</a></h1>
<p>Invoking the result of <tt class="literal"><span class="pre">lambda</span></tt> is such a common pattern
that MPL provides an <tt class="literal"><span class="pre">apply</span></tt> metafunction to do just
that. Using <tt class="literal"><span class="pre">mpl::apply</span></tt>, our flexible version of <tt class="literal"><span class="pre">twice</span></tt>
becomes:</p>
<pre class="literal-block">
#include <boost/mpl/apply.hpp>
template <class F, class X>
struct twice
: mpl::apply<F, typename mpl::apply<F,X>::type>
{};
</pre>
<!-- @ example.append(twice_test + '''
BOOST_MPL_ASSERT((boost::is_same<twice<boost::add_pointer<_1>,int>::type,int**>));
''')
compile()
prefix.append('#include <boost/mpl/apply.hpp>') -->
<p>You can think of <tt class="literal"><span class="pre">mpl::apply</span></tt> as being just like the <tt class="literal"><span class="pre">apply1</span></tt>
template that we wrote, with two additional features:</p>
<ol class="arabic">
<li><p class="first">While <tt class="literal"><span class="pre">apply1</span></tt> operates only on metafunction classes, the first
argument to <tt class="literal"><span class="pre">mpl::apply</span></tt> can be any lambda expression
(including those built with placeholders).</p>
</li>
<li><p class="first">While <tt class="literal"><span class="pre">apply1</span></tt> accepts only one additional argument to which
the metafunction class will be applied, <tt class="literal"><span class="pre">mpl::apply</span></tt> can
invoke its first argument on any number from zero to five
additional arguments. <a class="footnote-reference" href="#arity" id="id11" name="id11">[5]</a> For example:</p>
<pre class="literal-block">
// binary lambda expression applied to 2 additional arguments
mpl::apply<
mpl::plus<_1,_2>
, <strong>mpl::int_<6></strong>
, <strong>mpl::int_<7></strong>
>::type::value // == 13
</pre>
</li>
</ol>
<table class="footnote" frame="void" id="arity" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id11" name="arity">[5]</a></td><td>See the Configuration Macros section of the <a class="reference" href="./reference-manual.html">the MPL reference manual</a>
for a description of how to change the maximum number of
arguments handled by <tt class="literal"><span class="pre">mpl::apply</span></tt>.</td></tr>
</tbody>
</table>
<!-- @ prefix+=['#include <boost/mpl/plus.hpp>']
example.wrap('enum { is13 = ','''};
BOOST_STATIC_ASSERT(is13 == 13);''')
compile() -->
<div class="admonition-guideline admonition">
<p class="admonition-title first">Guideline</p>
<p>When writing a metafunction that invokes one of its arguments,
use <tt class="literal"><span class="pre">mpl::apply</span></tt> so that it works with lambda expressions.</p>
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