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

It is possible to generate a compile time HASH from a string using the code from this answer.

If you can modify the template to include one extra integer and use a macro to declare the variable:

template struct A
{
    static const int id = ID;
};

#define DECLARE_A(x) A

Using this macro for the type declaration, the id member contains a hash of the type name. For example:

int main() 
{
    DECLARE_A(int) a;
    DECLARE_A(double) b;
    DECLARE_A(float) c;
    switch(a.id)
    {
    case DECLARE_A(int)::id:
        cout << "int" << endl;
        break;
    case DECLARE_A(double)::id:
        cout << "double" << endl;
        break;
    case DECLARE_A(float)::id:
        cout << "float" << endl;
        break;
    };
    return 0;
}

As the type name is converted to a string, any modification to the type name text results on a different id. For example:

static_assert(DECLARE_A(size_t)::id != DECLARE_A(std::size_t)::id, "");

Another drawback is due to the possibility for a hash collision to occur.