Substitution failure is not an error (SFINAE) for enum

Question

Is there a way to use Substitution failure is not an error (SFINAE) for enum?

template 
struct Traits
{
}
template <>
struct Traits

        

Answer 1

If you can use C++11, this is completely trivial:

template<class T>
struct has_nested_option{
  typedef char yes;
  typedef yes (&no)[2];

  template<class U>
  static yes test(decltype(U::option)*);
  template<class U>
  static no  test(...);

  static bool const value = sizeof(test<T>(0)) == sizeof(yes);
};

The C++03 version is (surprisingly) similar:

template<class T>
struct has_nested_option{
  typedef char yes;
  typedef yes (&no)[2];

  template<int>
  struct test2;

  template<class U>
  static yes test(test2<U::option>*);
  template<class U>
  static no  test(...);

  static bool const value = sizeof(test<T>(0)) == sizeof(yes);
};

Usage:

struct foo{
  enum { option = 1 };
};

struct bar{};

#include <type_traits>

template<class T>
typename std::enable_if<
  has_nested_option<T>::value
>::type Do(){
}

int main(){
  Do<foo>();
  Do<bar>(); // error here, since you provided no other viable overload
}