using SFINAE for template class specialisation

Question

suppose I have these declarations

template class User;
template class Data;

and want to implement

Answer 1

As you only want to implement it when a single condition is true, the easiest solution is to use a static assertion. It does not require SFINAE, gives a clear compile error if used incorrectly and the declaration of User<> does not need to be adapted:

template class User {
  static_assert(is_Data::value, "T is not (a subclass of) Data<>");
  /** Implementation. **/
};

See also: When to use static_assert instead of SFINAE?. The static_assert is a c++11 construct, however there are plenty workarounds available for pre-c++11 compilers, like:

#define STATIC_ASSERT(consdition,name) \
  typedef char[(condition)?1:-1] STATIC_ASSERT_ ## name

---

If the declaration of user<> can be changed and you want two implementations depending on the value of is_Data, then there is also a solution that does not use SFINAE:

template::value> class User;

template class User {
  static_assert(is_Data::value, "T is not (a subclass of) Data<>"); // Optional
  /* Data implementation */
};

template class User {
  static_assert(!is_Data::value, "T is (a subclass of) Data<>"); // Optional
  /* Non-data implementation */
};

The static assertions only checks whether the user did not accidentally specify the template argument D incorrectly. If D is not specified explicitly, then the static assertions can be omitted.