Explicit Template Confusion

[dude_1967]

Hi Gurus,

Howdy, I've been gone a while.

I would like to make specializations of a template subroutine within the specialization of a template struct.

• I know that the standard does not allow explicit template specilizations within a non-namespace scope.
• I also know that VC does allow it and does compiles it (non-standard).
• I know that GCC complains (correctly) about "error: explicit specialization in non-namespace scope".
• What I don't know is how to code it.

I have the code below. It works for VC and is non-standard. It doesn't work with GCC because GCC is conformant and the code is not.

Could anyone please tell me how to code it correctly?

Thanks. Sincerely, Chris.

#include <iostream>

namespace my_name
{
  // Interface of outer
  template<const unsigned A, const unsigned B>
  struct outer { };

  // Specialization for A = 1, B = ?.
  template<const unsigned B>
  struct outer<1u, B>
  {
    template<const unsigned C> static void inner(void)
    {
      std::cout << "Not yet implemented" << std::endl;
    }
    // Specialization for A = 1, B = ?, C = 0
    template<> static void inner<0u>(void)
    {
      std::cout << (1u + B + 0u) << std::endl;
    }
    // Specialization for A = 1, B = ?, C = 1
    template<> static void inner<1u>(void)
    {
      std::cout << (1u + B + 1u) << std::endl;
    }
    // Specialization for A = 1, B = ?, C = 2
    template<> static void inner<2u>(void)
    {
      std::cout << (1u + B + 2u) << std::endl;
    }
  };
}

int main(void)
{
  my_name::outer<1u, 2u>::inner<0u>(); // 3
  my_name::outer<1u, 2u>::inner<1u>(); // 4
  my_name::outer<1u, 2u>::inner<2u>(); // 5
  my_name::outer<1u, 2u>::inner<3u>(); // Not yet implemented
}

[superbonzo]

I'm quite sure you cannot specialize a member function template of a partially specialized class template unless outer<1u,B> is itself specialized ( for example, the code "template<> template<> void outer<1u, 0>::inner<0>(){}" at namespace scope placed after outer<1u,B> declaration defines a specialization of the member function template outer<1u,B>::inner() ).

a solution could be

#include <iostream>

namespace my_name
{
  // Interface of outer
  template<const unsigned A, const unsigned B>
  struct outer { };

  template<const unsigned A,const unsigned B,const unsigned C>
  struct outer_impl{};

  // Specialization for A = 1, B = ?.
  template<const unsigned B>
  struct outer<1u, B>
  {
    template<const unsigned C> static void inner(void)
	{
		outer_impl<1u,B,C>::inner();
	}
  };

  template<const unsigned B>
  struct outer_impl<1u,B,0u> { static inline void inner(){ /*whatever*/ } };

  template<const unsigned B>
  struct outer_impl<1u,B,1u> { static inline void inner(){ /*whatever*/ } };

  template<const unsigned B>
  struct outer_impl<1u,B,2u> { static inline void inner(){ /*whatever*/ } };

}

int main(void)
{
  my_name::outer<1u, 2u>::inner<0u>(); // 3
  my_name::outer<1u, 2u>::inner<1u>(); // 4
  my_name::outer<1u, 2u>::inner<2u>(); // 5
  // my_name::outer<1u, 2u>::inner<3u>(); // Not yet implemented
}

the only difference being that this time my_name:uter<1u, 2u>::inner<3u>(); gives a compiler error instead ... ( I've not put a default inner implementation in the primary outer_impl definition in such a way to retain outer_impl as a generic "ternary" specialization model to be used by other outer<> partial specializations that might or might not define an inner() member ... )

[dude_1967]

Thanks superbonzo.

I have used your suggestion, adapted to the application.
The language issues are working out!
The application still has a problem, but that is an unrelated issue.

Always a pleasure at codeguru!

Sincerely, Chris.