tempate-d using.

tempate-d using.

#include <iostream>

enum class Selector : int { A, B };

struct ImplA {
  void foo() {
    std::cout << "A::foo()" << std::endl;
  }
  int a;
  constexpr static bool is_a = true;
  constexpr static bool is_b = false;
};

struct ImplB {
  void foo() {
    std::cout << "B::foo()" << std::endl;
  }
  int b;
  constexpr static bool is_a = false;
  constexpr static bool is_b = true;
};

template<Selector> struct ImplAB {
};

template<> struct ImplAB<Selector::A> {
  typedef ImplA type;
};

template<> struct ImplAB<Selector::B> {
  typedef ImplB type;
};

template<Selector T = Selector::A> using Impl = typename ImplAB<T>::type;

int main() {
  Impl<Selector::A> a;
  a.foo();
  a.a = 42;
  static_assert(a.is_a, "");
  static_assert(!a.is_b, "");

  Impl<Selector::B> b;
  b.foo();
  b.b = 100;
  static_assert(!b.is_a, "");
  static_assert(b.is_b, "");

  Impl<> d;
  d.foo();
  static_assert(d.is_a, "");
  static_assert(!d.is_b, "");
}