[boost] Can SFINAE be used for testing some trait classes?

From: Daryle Walker (darylew_at_[hidden])
Date: 2007-08-29 17:35:36

We have some class templates in forms like:

//==========================================================
template <unsigned X> struct my_test;

template <> struct my_test<0u> {};

template <unsigned X>
struct my_test { typedef my_test<(X - 1u)> type; };
//==========================================================

IOW, the traits express their result as a type, but omit the type for 
out-of-domain values.  I was looking at the enable_if documentation and 
I was wondering if we can use SFINAE to create functions and/or function 
templates that will test manifestations of "my_test," but skip the test 
for out-of-domain values without have to explicitly exclude the values. 
  Here, it's easy to omit "0" from the test file, but for some templates 
it won't be so obvious which values to exclude.  (Worse, the omitted 
values may change between platforms!)

For example, could something like

//==========================================================
template < unsigned X >
typename my_test<X>::type  print_out( my_test<X> const &x )
{
        std::cout << "There's a specialization at " << x
         << '.' << std::endl;
        return my_test<X>::type();
}

template < typename T >
void  print_out( T const &x )
{ std::cout << "There's no 'type' here." << std::endl; }
//==========================================================

with some sort of compile-time list (maybe with MPL) that runs each 
version of "my_test" through the best version of "print_out" be made?

(This is all for replacing the macros in "integer_test.cpp" with 
something more up-to-date.)

-- 
Daryle Walker
Mac, Internet, and Video Game Junkie
darylew AT hotmail DOT com