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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<std::vector<_>,</span> <span class="pre">T></span></tt> is always just | |
27 | <tt class="literal"><span class="pre">std::vector<T></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<class U> | |
33 | struct make_vector { typedef std::vector<U> type; }; | |
34 | ||
35 | typedef mpl::apply<<strong>make_vector<_></strong>, T>::type vector_of_t; | |
36 | </pre> | |
37 | <p>Instead, we can simply write:</p> | |
38 | <pre class="literal-block"> | |
39 | typedef mpl::apply<<strong>std::vector<_></strong>, T>::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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