apply_if2.sgml

boost库提供标准的C++ API 配合dev c++使用,功能更加强大

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<section id="applyif2">
<title>apply_if, part 2</>

<para>
Besides solving the <quote>making the code compile</> problem, the <literal>apply_if</> technique we've just learned can be also used to improve metaprogram efficiency.
</>

<para>
Suppose we want to define a high-level wrapper around <literal>boost::remove_pointer</> traits template, which will strip the pointer qualification conditionally. We will call it <literal>remove_pointer_if</>:
</>

<programlisting>
<![CDATA[
template<
      typename Condition
    , typename T
    >
struct remove_pointer_if
{
    typedef typename mpl::if_<
          Condition
        , typename boost::remove_pointer<T>::type
        , T
        >::type type;
};
]]>
</>

<para>
The above works the first time, but it's not the most optimal implementation. Similar to our previous examples, <literal>boost::remove_pointer&lt;T&gt;</> gets instantiated even if its result is never used. In the metaprogramming world compilation time is an important resource <citation><xref linkend="ref.Abr01"></>, and it is wasted by unnecessary template instantiations. 
</>

<para>
Let's see what we need to substitute <literal>if_</> by <literal>apply_if</> here. We already have one metafunction to pass to <literal>apply_if</> - <literal>boost::remove_pointer&lt;T&gt;</>, but we need a second one, - let's call it <literal>f</>, - such as <literal>f&lt;T&gt;::type == T</>. We could write this one ourselves, but fortunately &MPL; already provides us with a template that matches this exact definition - it's called <literal>identity</>. Applying this knowledge, we get:
</>

<programlisting>
<![CDATA[
template<
      typename Condition
    , typename T
    >
struct remove_pointer_if
{
    typedef typename mpl::apply_if<
          Condition
        , boost::remove_pointer<T>
        , mpl::identity<T>
        >::type type;
};
]]>
</>

</section>