Re: conditional type selection (templates)

From:

Carl Barron <cbarron413@adelphia.net>

Newsgroups:

comp.lang.c++.moderated

Date:

Tue, 3 Jun 2008 09:47:07 CST

Message-ID:

<030620080513298886%cbarron413@adelphia.net>

In article
<727bd12b-cdc1-4809-8ffc-e2bf3346e120@w8g2000prd.googlegroups.com>,
Michael Kilburn <crusader.mike@gmail.com> wrote:

I am trying to do smth like this (which of course refuses to compile):

[code]
template<class T> struct Void { typedef void type; };

template <class T, template<class> class X, class Enable=void>
struct Select { typedef T type; };

template <class T, template<class> class X>
struct Select<T, X, typename Void<typename X<T>::type>::type >
{
     typedef typename X<T>::type type;
};

struct TestYes { typedef int State; };
struct TestNo { };
void foo(TestNo) { cout << "selected Test" << endl; }
void foo(int) { cout << "selected Test::State" << endl; }

template<class T> struct SelectState { typedef typename T::State
type; };

void main()
{
     foo(Select<TestYes, SelectState>::type());
     foo(Select<TestNo, SelectState>::type());
}
[/code]

i.e. user required to provide Select_Subtype template and framework
will attempt to return Select_Subtype<T>::type, but in case of error
will fallback to type T itself. My compiler (MSVC can't compile this).

Sincerely yours,
Michael.

if boost::mpl is used [recomended to simplify TMP]

#include <boost/mpl/has_xxx.hpp>
#include <boost/mpl/if.hpp>

namespace Private
{
/* creates template <class T>
   struct has_type
   {
      typedef bool_const type;
   };
*/
   BOOST_MPL_HAS_XXX_TRAIT_DEF(type)

   template <class T>
   struct select_type:boost::mpl::if_
   <
      has_type<T>,
      typename T::type,
      T
   >
   {};

}

template <class T,template <class> class X>
struct Select:Private::select_type<X<T> >
{};

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