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14<div class="section" id="handling-placeholders">
15<h1><a class="toc-backref" href="./tutorial-metafunctions.html#id48" name="handling-placeholders">Handling Placeholders</a></h1>
16<p>Our implementation of <tt class="literal"><span class="pre">twice</span></tt> already works with metafunction
17classes.  Ideally, we would
18like it to work with placeholder expressions too, much the same as
19<tt class="literal"><span class="pre">mpl::transform</span></tt> allows us to pass either form.  For example, we
20would like to be able to write:</p>
21<pre class="literal-block">
22template &lt;class X&gt;
23struct two_pointers
24    : twice&lt;<strong>boost::add_pointer&lt;_1&gt;</strong>, X&gt;
25{};
26</pre>
27<!-- @ example.append('typedef two_pointers<int>::type intstar2;')
28compile('all', pop = 1, expect_error = True) -->
29<p>But when we look at the implementation of <tt class="literal"><span class="pre">boost::add_pointer</span></tt>,
30it becomes clear that the current definition of <tt class="literal"><span class="pre">twice</span></tt> can't
31work that way.</p>
32<pre class="literal-block">
33template &lt;class T&gt;
34struct add_pointer
35{
36    typedef T* type;
37};
38</pre>
39<!-- @ compile() -->
40<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
41to be a metafunction class, along the lines of <tt class="literal"><span class="pre">add_pointer_f</span></tt>.
42Instead, it's just a nullary metafunction returning the almost
43senseless 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
44fail when <tt class="literal"><span class="pre">apply1</span></tt> reaches for a nested <tt class="literal"><span class="pre">::apply</span></tt>
45metafunction in <tt class="literal"><span class="pre">boost::add_pointer&lt;_1&gt;</span></tt> and finds that it
46doesn't exist.</p>
47<p>We've determined that we don't get the behavior we want
48automatically, so what next?  Since <tt class="literal"><span class="pre">mpl::transform</span></tt> can do this
49sort of thing, there ought to be a way for us to do it too — and
50so there is.</p>
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52<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>
53<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>
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