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| <div class="section" id="lambda-and-non"> |
| <h1><a class="toc-backref" href="./lambda-details.html#id57" name="lambda-and-non">Lambda and Non-Metafunction Templates</a></h1> |
| <p>There is just one detail of placeholder expressions that we haven't |
| discussed yet. MPL uses a special rule to make it easier to |
| integrate ordinary templates into metaprograms: After all of the |
| placeholders have been replaced with actual |
| arguments, if the resulting template specialization <em>X</em> doesn't |
| have a nested <tt class="literal"><span class="pre">::type</span></tt>, the result is just |
| <em>X</em> itself.</p> |
| <p>For example, <tt class="literal"><span class="pre">mpl::apply<std::vector<_>,</span> <span class="pre">T></span></tt> is always just |
| <tt class="literal"><span class="pre">std::vector<T></span></tt>. If it weren't for this behavior, we would |
| have to build trivial metafunctions to create ordinary template |
| specializations in lambda expressions:</p> |
| <pre class="literal-block"> |
| // trivial std::vector generator |
| template<class U> |
| struct make_vector { typedef std::vector<U> type; }; |
| |
| typedef mpl::apply<<strong>make_vector<_></strong>, T>::type vector_of_t; |
| </pre> |
| <p>Instead, we can simply write:</p> |
| <pre class="literal-block"> |
| typedef mpl::apply<<strong>std::vector<_></strong>, T>::type vector_of_t; |
| </pre> |
| <!-- @ # ensure indentation works. The ReST parser will push the 2nd line left |
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