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

Here is a pragmatic solution, if you are ok with writing a single additional line DECLARE_ID(type) for each type you want to use:

 #include 

 template struct my_id_helper;
 #define DECLARE_ID(C) template<> struct my_id_helper { enum {value = __COUNTER__ }; }

 // actually declare ids:
 DECLARE_ID(int);
 DECLARE_ID(double);
 // this would result in a compile error: redefinition of struct my_id_helper’
 // DECLARE_ID(int);

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

 int main()
 {
     switch(A::ID)
     {
     case A::ID:    std::cout << "it's an int!\n"; break;
     case A::ID: std::cout << "it's a double!\n"; break;
     // case A::ID: // error: incomplete type ‘my_id_helper’
     default: std::cout << "it's something else\n"; break;
     }
 }