ceph/src/boost/libs/mpl/doc/tutorial/handling-placeholders.html

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  10 <title>THE BOOST MPL LIBRARY: Handling Placeholders</title>
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  17 <div class="section" id="handling-placeholders">
  18 <h1><a class="toc-backref" href="./tutorial-metafunctions.html#id48" name="handling-placeholders">Handling Placeholders</a></h1>
  19 <p>Our implementation of <tt class="literal"><span class="pre">twice</span></tt> already works with metafunction
  20 classes.  Ideally, we would
  21 like it to work with placeholder expressions too, much the same as
  22 <tt class="literal"><span class="pre">mpl::transform</span></tt> allows us to pass either form.  For example, we
  23 would like to be able to write:</p>
  24 <pre class="literal-block">
  25 template &lt;class X&gt;
  26 struct two_pointers
  27     : twice&lt;<strong>boost::add_pointer&lt;_1&gt;</strong>, X&gt;
  28 {};
  29 </pre>
  30 <!-- @ example.append('typedef two_pointers<int>::type intstar2;')
  31 compile('all', pop = 1, expect_error = True) -->
  32 <p>But when we look at the implementation of <tt class="literal"><span class="pre">boost::add_pointer</span></tt>,
  33 it becomes clear that the current definition of <tt class="literal"><span class="pre">twice</span></tt> can't
  34 work that way.</p>
  35 <pre class="literal-block">
  36 template &lt;class T&gt;
  37 struct add_pointer
  38 {
  39     typedef T* type;
  40 };
  41 </pre>
  42 <!-- @ compile() -->
  43 <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
  44 to be a metafunction class, along the lines of <tt class="literal"><span class="pre">add_pointer_f</span></tt>.
  45 Instead, it's just a nullary metafunction returning the almost
  46 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
  47 fail when <tt class="literal"><span class="pre">apply1</span></tt> reaches for a nested <tt class="literal"><span class="pre">::apply</span></tt>
  48 metafunction in <tt class="literal"><span class="pre">boost::add_pointer&lt;_1&gt;</span></tt> and finds that it
  49 doesn't exist.</p>
  50 <p>We've determined that we don't get the behavior we want
  51 automatically, so what next?  Since <tt class="literal"><span class="pre">mpl::transform</span></tt> can do this
  52 sort of thing, there ought to be a way for us to do it too — and
  53 so there is.</p>
  54 <ul class="toc simple" id="outline">
  55 <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>
  56 <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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