How to specialize a class template with a template?

Question

Consider the class template:

template<typename T>
struct A
{
  T data;

  static const bool flag;
  static bool calc_flag()
  {
    // Default value;
    return false;
  }
};

template<typename T>
const bool A<T>::flag = A<T>::calc_flag();

And now I'd like to specialize this template for the following class template:

template<char N>
struct B
{
  static const bool is_a;
};

template<char N>
const bool B<N>::is_a = N == 'a';

in order to the A<B<N>>::flag will be initialized with the B<N>::is_a . Ie, I'd like to specialize the calc_flag() method for such the case. How could this be done?

Answer 1

You could separate the calculation to an implementation struct and only specialize that

template<class T>
struct calc_flag_impl {
    static bool calc_flag() { return false; }
};

template<typename T>
struct A
{
  T data;

  static const bool flag = calc_flag_impl<T>::calc_flag();
};

Now you can specialize calc_flag_impl

template<char N>
struct calc_flag_impl<B<N>> {
    static bool calc_flag() { return B<N>::is_a; }
};

Note: The specialization must exist prior to instantiation of the class template for your type for it to work with static class variables.