How to know if optional template parameter was not given?

Question

Is is possible to know if an optional template parameter was not given/set? Made an attempt with std::is_empty to no avail.

template<class T, class U = int> 
class A {
    public:
        T x;
        U y;
  
        void check() { 
            if (std::is_empty<U>::value)
                std::cout << "not given\n";
            else
                std::cout << "given\n";
        }
};

int main()
{
    A<float> a(1.5);    
    a.check();
}

Answer 1

If it is not supplied by the user, it is int:

template<class T, class U = int> 
                          ^^^^^

that is what = int means.

There is no way to tell within the template if int was passed explicitly or if it was set implicitly by skipping it.

You can make A<float> different from A<float, int> in a number of ways.

You could U = void then set RealU within the template to U unless it is void, and int if it is void.

You could take class...U and restrict your code to sizeof...(U)<=1. Then do the same RealU trick.

You could make a wrapper template that does this stuff, and have the real template get passed T, U and a bool saying if U was passed.

In every case, the type of A<float> will not match the type of A<float, int> because you are implicitly demanding that to be the case.