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