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<div class="section" id="handling-placeholders">
<h1><a class="toc-backref" href="./tutorial-metafunctions.html#id48" name="handling-placeholders">Handling Placeholders</a></h1>
<p>Our implementation of <tt class="literal"><span class="pre">twice</span></tt> already works with metafunction
classes.  Ideally, we would 
like it to work with placeholder expressions too, much the same as
<tt class="literal"><span class="pre">mpl::transform</span></tt> allows us to pass either form.  For example, we
would like to be able to write:</p>
<pre class="literal-block">
template &lt;class X&gt;
struct two_pointers
    : twice&lt;<strong>boost::add_pointer&lt;_1&gt;</strong>, X&gt;
{};
</pre>
<!-- @ example.append('typedef two_pointers<int>::type intstar2;')
compile('all', pop = 1, expect_error = True) -->
<p>But when we look at the implementation of <tt class="literal"><span class="pre">boost::add_pointer</span></tt>,
it becomes clear that the current definition of <tt class="literal"><span class="pre">twice</span></tt> can't
work that way.</p>
<pre class="literal-block">
template &lt;class T&gt;
struct add_pointer
{
    typedef T* type;
};
</pre>
<!-- @ compile() -->
<p>To be invokable by <tt class="literal"><span class="pre">twice</span></tt>, <tt class="literal"><span class="pre">boost::add_pointer&lt;_1&gt;</span></tt> would have
to be a metafunction class, along the lines of <tt class="literal"><span class="pre">add_pointer_f</span></tt>.
Instead, it's just a nullary metafunction returning the almost
senseless type <tt class="literal"><span class="pre">_1*</span></tt>.  Any attempt to use <tt class="literal"><span class="pre">two_pointers</span></tt> will
fail when <tt class="literal"><span class="pre">apply1</span></tt> reaches for a nested <tt class="literal"><span class="pre">::apply</span></tt>
metafunction in <tt class="literal"><span class="pre">boost::add_pointer&lt;_1&gt;</span></tt> and finds that it
doesn't exist.</p>
<p>We've determined that we don't get the behavior we want
automatically, so what next?  Since <tt class="literal"><span class="pre">mpl::transform</span></tt> can do this
sort of thing, there ought to be a way for us to do it too — and
so there is.</p>
<ul class="toc simple" id="outline">
<li><a class="reference" href="./the-lambda-metafunction.html" id="id49" name="id49">The <tt class="literal"><span class="pre">lambda</span></tt> Metafunction</a></li>
<li><a class="reference" href="./the-apply-metafunction.html" id="id50" name="id50">The <tt class="literal"><span class="pre">apply</span></tt> Metafunction</a></li>
</ul>
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