ceph/src/boost/libs/mpl/doc/tutorial/placeholder-expression.html

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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
  34 is a placeholder expression.</li>
  35 </ul>
  36 </blockquote>
  37 </blockquote>
  38 </div>
  39 <p>In other words, a placeholder expression always involves a
  40 placeholder.</p>
  41 <!-- DWA: I'm still not sure we shouldn't be at least mentioning the
  42 pitfall, but for now it's commented out.
  43
  44 Lambda and Nullary Metafunctions
  45 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  46
  47 The definition of *placeholder expression* above has an interesting
  48 implication: an ordinary nullary metafunction is never a placeholder
  49 expression.  In other words, even though ``add_pointer<int>`` is a
  50 nullary metafunction, it won't be invoked in the expression below;
  51 the 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
  62 In order to allow a nullary metafunction to be used as a lambda
  63 expression, 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
  77 When applied to a lambda expression's actual arguments, ``arg<F>``
  78 ignores them and simply returns ``F::type``.  In other words, if
  79 ``F`` is a nullary metafunction, ``arg<F>`` is a metafunction class
  80 that invokes ``F`` and returns the result.  So we can transform
  81 add_pointer<int> into a placeholder and get the desired result
  82 with:
  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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