Template method specialization for multiple types

Question

I have a class “A” which exposes the template method foo. Foo has a standard implementation which works fine with B,C. It also has a special implementation for D.

class A
{
  template<typename T>
  void foo()
  {
    //standard implementation
  }

  template<>
  void foo<D>
  {
    //special implementation
  }
}

class B{};
class C{};
class D{};

int main()
{
  A<B> a1;
  A<C> a2;
  A<D> a3;
}

Now, I need to add the class E, which requires for "foo" the same special implementation as D. Is there a way to say something like: For all the types use the standard foo. For D,E (and so on) the special implementation.

class A
{
  template<typename T>
  void foo()
  {
    //standard implementation
  }

  template<>
  void foo<D && E>  <-- PseudoCode - It doesn't work
  {
    //special implementation
  }
}

class B{};
class C{};
class D{};
class E{};

int main()
{
  A<B> a1;
  A<C> a2;
  A<D> a3;
  A<E> a4;
}

I was thinking to use the trait classes. But I was hoping there is something simpler to achieve this. Thanks

Answer 1

Using Walter Brown's (C++1z) void_t.

#include <iostream>
#include <type_traits>

template <typename...>
using void_t = void;

template <typename T, typename = void>
struct has_bar 
  : std::false_type { };

template <typename T>
struct has_bar<T, void_t<decltype( std::declval<T&>().bar() ) > >
  : std::true_type { };

class A {
  public:
    void foo() { };
};

class B {
  public:
    void bar() { };
};

class C {
  public:
    void bar() { };
};

template <typename T> 
typename std::enable_if<!has_bar<T>::value, void>::type
fun(T t) {
  std::cout << "fun" << std::endl;
}

template <typename T>
typename std::enable_if<has_bar<T>::value, void>::type
fun(T t) {
  std::cout << "special fun" << std::endl;
}

The code...

int main(const int argc, const char* argv[argc]) {

  A a;
  B b;
  C c;

  fun(a);
  fun(b);
  fun(c);

  return 0;
}

prints out

fun
special fun
special fun

Note, that does not check any type semantics, so it may be better declaring bar() as an interface and using std::is_base_of.