How to prevent a template class from being derived more than once?

Question

I have the following template class:

template
class T : public I
{
    // ...
};

This template class need to be derived once (an

Answer 1

Since you mention that it's okay to pass A, B and C to T then how about this runtime solution?

#include 
#include 

//This class will check to see each T is only instantiated with a unique Y
template 
struct T_helper
{
    template 
    static void check()
    {
        if(derived_type)
            assert(*derived_type == typeid(Y));
        else
            derived_type = &typeid(Y);
    }
    static const std::type_info * derived_type;
};

template 
const std::type_info * T_helper::derived_type = nullptr;

template 
struct T
{
    T()
    {
        T_helper::template check();
    }
};

struct A : T {};
struct B : T {};

int main()
{
    A a1, a2, a3; // These are all okay
    B b1;         // This one will trigger the assert
}