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10<title>THE BOOST MPL LIBRARY: Placeholder Expression Definition</title>
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17<div class="section" id="placeholder-expression">
18<h1><a class="toc-backref" href="./lambda-details.html#id56" name="placeholder-expression">Placeholder Expression Definition</a></h1>
19<p>Now that you know just what <em>placeholder</em> means, we can define
20<em>placeholder expression</em>:</p>
21<div class="admonition-definition admonition">
22<p class="admonition-title first">Definition</p>
23<p>A placeholder expression is either:</p>
24<blockquote>
25<blockquote>
26<ul class="simple">
27<li>a placeholder</li>
28</ul>
29</blockquote>
30<p><em>or</em></p>
31<blockquote>
32<ul class="simple">
33<li>a template specialization with at least one argument that
34is a placeholder expression.</li>
35</ul>
36</blockquote>
37</blockquote>
38</div>
39<p>In other words, a placeholder expression always involves a
40placeholder.</p>
41<!-- DWA: I'm still not sure we shouldn't be at least mentioning the
42pitfall, but for now it's commented out.
43
44Lambda and Nullary Metafunctions
45- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
46
47The definition of *placeholder expression* above has an interesting
48implication: an ordinary nullary metafunction is never a placeholder
49expression.  In other words, even though ``add_pointer<int>`` is a
50nullary metafunction, it won't be invoked in the expression below;
51the assertion will always fail:
52
53.. parsed-literal::
54
55   BOOST_STATIC_ASSERT((
56       mpl::apply<
57           boost::is_same<**boost::add_pointer<int>**,_1>
58         , int\*
59       >::type::value
60   ));
61
62In order to allow a nullary metafunction to be used as a lambda
63expression, MPL provides this definition of ``arg``:
64
65.. parsed-literal::
66
67    // primary template definition (not a specialization)
68    template <class F>
69    struct arg
70    {
71        template <class A1 = void\_, class A2 = void\_, ... class *Am* = void\_>
72        struct apply : F
73        {
74        };
75    };
76
77When applied to a lambda expression's actual arguments, ``arg<F>``
78ignores them and simply returns ``F::type``.  In other words, if
79``F`` is a nullary metafunction, ``arg<F>`` is a metafunction class
80that invokes ``F`` and returns the result.  So we can transform
81add_pointer<int> into a placeholder and get the desired result
82with:
83
84.. parsed-literal::
85
86   BOOST_STATIC_ASSERT((
87       mpl::apply<
88           boost::is_same<
89                **mpl::arg<boost::add_pointer<int> >**
90              , _1
91           >
92         , int\*
93       >::type::value
94   )); -->
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