C++ Template Specialization and Subclassing

Question

I was wondering if it is possible to have a template specialization accept a class and its subclasses. Like so:

class A {};

class B : public A {};

template <typename T>
void foo(const T& t) {
  printf("T");
}

template <>
void foo(const A& t) {
  printf("A");
}

int main(int argc, char** argv) {
  B b;
  foo(b);

  return 0;
}

Currently it output 'T' because b doesn't have its own template specialization, so it defaults to printing 'T'. I was wondering if it was possible for B to use the template specialization of A since B is a subclass of A. Or is that just not a thing?

Note: Because of some requirement, I can't use copy/move.

Note: I would also prefer if I didn't need to change A or B, but lets see what is possible first.

Answer 1

The probelm is, the primary template is an exact match when T being deduced as B; it's a better match than the specialization.

You can use template overloading instead; with SFINAE.

template <typename T>
std::enable_if_t<!std::is_base_of_v<A, T>> foo(const T& t) {
  printf("T");
}

template <typename T>
std::enable_if_t<std::is_base_of_v<A, T>> foo(const T& t) {
  printf("A");
}

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