apply_if.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="applyif">
<title>apply_if</>

<para>
In run-time &Cxx;, it is guaranteed that when we reach an <literal>if</> statement, only one branch will be executed. Executing the branch for which the result is not required would be unnecessary and inefficient. More importantly, frequently the non-required branch is invalid, and executing it would cause an error. For instance, the following code would be badly broken if both branches of the statement were evaluated:
</>

<programlisting>
<![CDATA[
void fun(giraffe* g)
{
    if (g)
        cout << g->name();
    else
        cout << "no giraffe";
}
]]>
</> 

<para>
In compile-time world, things are different. Which parameters to <literal>if_</> template are instantiated is determined by the form of each template parameter and the corresponding language rules (<citation><xref linkend="ref.ISO98"></>, section 14.7.1), not by the value of the compile-time expression being switched on. That means that if, in attempt to process a particular <literal>if_</> construct, the compiler determines that one of its <quote>branch</> template parameters is ill-formed, it will issue a diagnostics even if the value of compile-time expression would lead to <quote>choosing</> the other, valid parameter type.
</>

<para>
To clarify what we just said, here is a broken first attempt at writing a <literal>pointed_type</> metafunction, that when instantiated for a <literal>T</> that is either a plain pointer or a smart pointer, <quote>returns</> the pointed type:
</>

<programlisting>
<![CDATA[
template< typename T >
struct pointed_type
{
    typedef typename mpl::if_<
          boost::is_pointer<T>
        , typename boost::remove_pointer<T>::type
        , typename T::element_type // #1
        >::type type;
};

typedef pointed_type< std::auto_ptr<int> >::type int_ptr; // ok
typedef pointed_type<char*>::type char_ptr; // error in line #1!
]]>
</>

<para>
If we try to compile the above, we will get something like this: 
</>

<programlisting>
Error: name followed by "::" must be a class or namespace name
</>

<para>
because the expression <literal>typename T::element_type</> is not valid in case of <literal>T == char*</>.
</>

<para>
Here's what we need to do to make <literal>pointed_type</> work for plain pointers:

<footnote><para>
It would be easy to implement <literal>pointed_type</> using partial specialization to distinguish the case where <literal>T</> is a pointer. <literal>if_</> is used here to avoid creating a complicated example.
</></>

instead of instantiating our two potential results before passing them to <literal>if_</>, we need to write metafunctions that can be used to instantiate the results; then we can use <literal>if_</> to choose a metafunction, and only then should we use that function to get the result.
</>

<para>
<literal>boost::remove_pointer</> already is a metafunction. We just need to write an auxiliary function to return the <literal>element_type</> of a pointer type:
</>

<programlisting>
<![CDATA[
namespace aux {
template< typename T >
struct element_type
{
     typedef typename T::element_type type;
};
}
]]>
</>

<para>
Now we can select the metafunction to call based on the result of <literal>boost::is_pointer</>, and then <emphasis>apply</> it to form the result:
</>

<programlisting>
<![CDATA[
template< typename T >
struct pointed_type
{
 private:
    // pick a metafunction
    typedef typename mpl::if_<
          boost::is_pointer<T>
        , boost::remove_pointer<T>
        , aux::element_type<T>
        >::type func_; // #1

 public:
    // apply the metafunction
    typedef typename func_::type type;
};
]]>
</>

<para>
The key knowledge that makes the above viable is that in line #1 the compiler is <emphasis>guaranteed</> not to instantiate <literal>boost::remove_pointer&lt;T&gt;</> and <literal>aux::element_type&lt;T&gt;</> templates, - even although they are passed as actual arguments to the <literal>if_</>. 
</>

<para>
The described technique is so common in template metaprograms, that it makes sense to facilitate the selection of the nested <literal>type</> member by introducing a high level equivalent to <literal>if_</> that will do <literal>func_::type</> operation as a part of its invocation.  The MPL provides such a template - it's called <literal>apply_if</>. Using it, we can re-write the above code as simply as:
</>

<programlisting>
[<![CDATA[
template< typename T >
struct pointed_type
{
    typedef typename mpl::apply_if<
          boost::is_pointer<T>
        , boost::remove_pointer<T>
        , aux::element_type<T>
        >::type type;
};
]]>
</>

</section>