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| 14 | <div class="section" id="lambda-and-non"> |
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| 15 | <h1><a class="toc-backref" href="./lambda-details.html#id57" name="lambda-and-non">Lambda and Non-Metafunction Templates</a></h1> |
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| 16 | <p>There is just one detail of placeholder expressions that we haven't |
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| 17 | discussed yet. MPL uses a special rule to make it easier to |
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| 18 | integrate ordinary templates into metaprograms: After all of the |
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| 19 | placeholders have been replaced with actual |
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| 20 | arguments, if the resulting template specialization <em>X</em> doesn't |
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| 21 | have a nested <tt class="literal"><span class="pre">::type</span></tt>, the result is just |
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| 22 | <em>X</em> itself.</p> |
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| 23 | <p>For example, <tt class="literal"><span class="pre">mpl::apply<std::vector<_>,</span> <span class="pre">T></span></tt> is always just |
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| 24 | <tt class="literal"><span class="pre">std::vector<T></span></tt>. If it weren't for this behavior, we would |
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| 25 | have to build trivial metafunctions to create ordinary template |
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| 26 | specializations in lambda expressions:</p> |
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| 27 | <pre class="literal-block"> |
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| 28 | // trivial std::vector generator |
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| 29 | template<class U> |
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| 30 | struct make_vector { typedef std::vector<U> type; }; |
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| 31 | |
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| 32 | typedef mpl::apply<<strong>make_vector<_></strong>, T>::type vector_of_t; |
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| 33 | </pre> |
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| 34 | <p>Instead, we can simply write:</p> |
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| 35 | <pre class="literal-block"> |
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| 36 | typedef mpl::apply<<strong>std::vector<_></strong>, T>::type vector_of_t; |
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| 37 | </pre> |
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| 38 | <!-- @ # ensure indentation works. The ReST parser will push the 2nd line left |
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| 39 | while stack: |
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| 40 | stack[-1].prepend('#include <vector>\ntypedef int T;') |
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| 41 | compile() --> |
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