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

If non-monotonous values and an intptr_t are acceptable:

template
struct TypeID
{
private:
    static char id_ref;
public:
    static const intptr_t ID;
};

template
  char TypeID::id_ref;
template
  const intptr_t TypeID::ID = (intptr_t)&TypeID::id_ref;

If you must have ints, or must have monotonically incrementing values, I think using static constructors is the only way to go:

// put this in a namespace
extern int counter;

template
class Counter {
private:
  Counter() {
    ID_val = counter++;
  }
  static Counter init;
  static int ID_val;
public:
  static const int &ID;
};

template
  Counter Counter::init;
template
  int Counter::ID_val;
template
  const int &Counter::ID = Counter::ID_val;

// in a non-header file somewhere
int counter;

Note that neither of these techniques is safe if you are sharing them between shared libraries and your application!