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Aleksey Gurtovoy
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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&lt;std::vector&lt;_&gt;,</span> <span class="pre">T&gt;</span></tt> is always just
<tt class="literal"><span class="pre">std::vector&lt;T&gt;</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&lt;class U&gt;
struct make_vector { typedef std::vector&lt;U&gt; type; };
typedef mpl::apply&lt;<strong>make_vector&lt;_&gt;</strong>, T&gt;::type vector_of_t;
</pre>
<p>Instead, we can simply write:</p>
<pre class="literal-block">
typedef mpl::apply&lt;<strong>std::vector&lt;_&gt;</strong>, T&gt;::type vector_of_t;
</pre>
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