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14<div class="section" id="the-apply-metafunction">
15<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>
16<p>Invoking the result of <tt class="literal"><span class="pre">lambda</span></tt> is such a common pattern
17that MPL provides an <tt class="literal"><span class="pre">apply</span></tt> metafunction to do just
18that.  Using <tt class="literal"><span class="pre">mpl::apply</span></tt>, our flexible version of <tt class="literal"><span class="pre">twice</span></tt>
19becomes:</p>
20<pre class="literal-block">
21#include &lt;boost/mpl/apply.hpp&gt;
22
23template &lt;class F, class X&gt;
24struct twice
25   : mpl::apply&lt;F, typename mpl::apply&lt;F,X&gt;::type&gt;
26{};
27</pre>
28<!-- @ example.append(twice_test + '''
29BOOST_MPL_ASSERT((boost::is_same<twice<boost::add_pointer<_1>,int>::type,int**>));
30''')
31compile()
32prefix.append('#include <boost/mpl/apply.hpp>') -->
33<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>
34template that we wrote, with two additional features:</p>
35<ol class="arabic">
36<li><p class="first">While <tt class="literal"><span class="pre">apply1</span></tt> operates only on metafunction classes, the first
37argument to <tt class="literal"><span class="pre">mpl::apply</span></tt> can be any lambda expression
38(including those built with placeholders).</p>
39</li>
40<li><p class="first">While <tt class="literal"><span class="pre">apply1</span></tt> accepts only one additional argument to which
41the metafunction class will be applied, <tt class="literal"><span class="pre">mpl::apply</span></tt> can
42invoke its first argument on any number from zero to five
43additional arguments. <a class="footnote-reference" href="#arity" id="id11" name="id11">[5]</a>  For example:</p>
44<pre class="literal-block">
45// binary lambda expression applied to 2 additional arguments
46mpl::apply&lt;
47    mpl::plus&lt;_1,_2&gt;   
48  , <strong>mpl::int_&lt;6&gt;</strong>
49  , <strong>mpl::int_&lt;7&gt;</strong>
50&gt;::type::value // == 13
51</pre>
52</li>
53</ol>
54<table class="footnote" frame="void" id="arity" rules="none">
55<colgroup><col class="label" /><col /></colgroup>
56<tbody valign="top">
57<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>
58for a description of how to change the maximum number of
59arguments handled by <tt class="literal"><span class="pre">mpl::apply</span></tt>.</td></tr>
60</tbody>
61</table>
62<!-- @ prefix+=['#include <boost/mpl/plus.hpp>']
63example.wrap('enum { is13 = ','''};
64BOOST_STATIC_ASSERT(is13 == 13);''')
65compile() -->
66<div class="admonition-guideline admonition">
67<p class="admonition-title first">Guideline</p>
68<p>When writing a metafunction that invokes one of its arguments,
69use <tt class="literal"><span class="pre">mpl::apply</span></tt> so that it works with lambda expressions.</p>
70</div>
71</div>
72
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