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