Class Foo
{
public:

  static unsigned std::set<unsigned long> s_usedID;

  static unsigned long generateID() // Generate a valid ID
  {
    static unsigned long id = 0;

     while (Foo::isIDUsed(id)) // If all ID are taken, create an infinite loop!
        ++id;

     return id;
  }

  static void addID(unsigned long id) // Add a given ID to the set of used ID
  {
    s_usedID.insert(id);
  }

  static void removeID(unsigned long id) // Remove a given ID from the set of used ID
  {
    s_usedID.erase(id);
  }

  static bool isIDUsed(unsigned long id)
  {
    return s_usedID.count(id) == 1 ? true:false;
  }

  explicit Foo() : m_id(Foo::generateID())
  {
    Foo::addID(m_id); // ID is now taken
  }

  virtual ~Foo()
  {
    Foo::removeID(m_id); // Free the ID
  }

private:

  unsigned long m_id;

};