How to specialize a class template with a template?
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
. I.e., I'd like to specialize the calc_flag()
method for such the case. How could this be done?
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.