Conditional compile-time inclusion/exclusion of code based on template argument(s)?

Question

Consider the following class, with the inner struct Y being used as a type, eg. in templates, later on:

template
class X{
  templat         


        

Answer 1

There are two problems here:

1. enable_if works with partial specialization, not primary templates.
2. The number of externally-visible arguments is determined by the primary template, of which there may be only one.

Answer 1.

As you suggested in chat, a linked list of templates can emulate the variadic parameter pack.

template
class X{
  template
  struct Y;

  template
  struct Y< list, typename std::enable_if::type > {
      typedef typename list::type t1;
  };

  template
  struct Y< list, typename std::enable_if::type > {
      typedef typename list::type t1;
      typedef typename list::next::type t2;
  };
};

If you end up with next::next::next garbage, it's easy to write a metafunction, or use Boost MPL.

---

Answer 2.

The different-arity templates can be named similarly but still stay distinct if they are nested inside the SFINAE-controlled type.

template
class X{
  template
  struct Z;

  template
  struct Z< v, typename std::enable_if::type > {
      template
      struct Y{};
  };

  template
  struct Z< v, typename std::enable_if::type > {
      template
      struct Y{};
  };
};

X<1>::Z<>::Y< int > a;
X<2>::Z<>::Y< char, double > b;