Compile-time constant id

Question

Given the following:

template
class A
{
public:
    static const unsigned int ID = ?;
};

I want ID to generate a unique c

Answer 1

Using this constant expression counter:

template 
class A
{
public:
    static constexpr int ID() { return next(); }
};
class DUMMY { };
int main() {
    std::cout << A::ID() << std::endl;
    std::cout << A::ID() << std::endl;
    std::cout << A::ID() << std::endl;
    std::cout << A::ID() << std::endl;
    return 0;
}

output: (GCC, C++14)

1
2
3
3

The downside is you will need to guess an upper bound on the number of derived classes for the constant expression counter to work.