ceph/src/boost/libs/mpl/doc/tutorial/lambda-and-non.html

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  10 <title>THE BOOST MPL LIBRARY: Lambda and Non-Metafunction Templates</title>
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  17 <div class="section" id="lambda-and-non">
  18 <h1><a class="toc-backref" href="./lambda-details.html#id57" name="lambda-and-non">Lambda and Non-Metafunction Templates</a></h1>
  19 <p>There is just one detail of placeholder expressions that we haven't
  20 discussed yet.  MPL uses a special rule to make it easier to
  21 integrate ordinary templates into metaprograms:  After all of the
  22 placeholders have been replaced with actual
  23 arguments, if the resulting template specialization <em>X</em> doesn't
  24 have a nested <tt class="literal"><span class="pre">::type</span></tt>, the result is just
  25 <em>X</em> itself.</p>
  26 <p>For example, <tt class="literal"><span class="pre">mpl::apply&lt;std::vector&lt;_&gt;,</span> <span class="pre">T&gt;</span></tt> is always just
  27 <tt class="literal"><span class="pre">std::vector&lt;T&gt;</span></tt>.  If it weren't for this behavior, we would
  28 have to build trivial metafunctions to create ordinary template
  29 specializations in lambda expressions:</p>
  30 <pre class="literal-block">
  31 // trivial std::vector generator
  32 template&lt;class U&gt;
  33 struct make_vector { typedef std::vector&lt;U&gt; type; };
  34
  35 typedef mpl::apply&lt;<strong>make_vector&lt;_&gt;</strong>, T&gt;::type vector_of_t;
  36 </pre>
  37 <p>Instead, we can simply write:</p>
  38 <pre class="literal-block">
  39 typedef mpl::apply&lt;<strong>std::vector&lt;_&gt;</strong>, T&gt;::type vector_of_t;
  40 </pre>
  41 <!-- @ # ensure indentation works.  The ReST parser will push the 2nd line left
  42 while stack:
  43    stack[-1].prepend('#include <vector>\ntypedef int T;')
  44    compile() -->
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