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<div class="section" id="placeholder-expression">
<h1><a class="toc-backref" href="./lambda-details.html#id56" name="placeholder-expression">Placeholder Expression Definition</a></h1>
<p>Now that you know just what <em>placeholder</em> means, we can define
<em>placeholder expression</em>:</p>
<div class="admonition-definition admonition">
<p class="admonition-title first">Definition</p>
<p>A placeholder expression is either:</p>
<blockquote>
<blockquote>
<ul class="simple">
<li>a placeholder</li>
</ul>
</blockquote>
<p><em>or</em></p>
<blockquote>
<ul class="simple">
<li>a template specialization with at least one argument that
is a placeholder expression.</li>
</ul>
</blockquote>
</blockquote>
</div>
<p>In other words, a placeholder expression always involves a
placeholder.</p>
<!-- DWA: I'm still not sure we shouldn't be at least mentioning the
pitfall, but for now it's commented out.

Lambda and Nullary Metafunctions
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

The definition of *placeholder expression* above has an interesting
implication: an ordinary nullary metafunction is never a placeholder
expression.  In other words, even though ``add_pointer<int>`` is a
nullary metafunction, it won't be invoked in the expression below;
the assertion will always fail:

.. parsed-literal::

   BOOST_STATIC_ASSERT((
       mpl::apply<
           boost::is_same<**boost::add_pointer<int>**,_1>
         , int\*
       >::type::value
   ));

In order to allow a nullary metafunction to be used as a lambda
expression, MPL provides this definition of ``arg``:

.. parsed-literal::

    // primary template definition (not a specialization)
    template <class F>
    struct arg
    {
        template <class A1 = void\_, class A2 = void\_, ... class *Am* = void\_>
        struct apply : F
        {
        };
    };

When applied to a lambda expression's actual arguments, ``arg<F>``
ignores them and simply returns ``F::type``.  In other words, if
``F`` is a nullary metafunction, ``arg<F>`` is a metafunction class
that invokes ``F`` and returns the result.  So we can transform
add_pointer<int> into a placeholder and get the desired result
with:

.. parsed-literal::

   BOOST_STATIC_ASSERT((
       mpl::apply<
           boost::is_same<
                **mpl::arg<boost::add_pointer<int> >**
              , _1
           >
         , int\*
       >::type::value
   )); -->
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