Re: How to make every derived class to return a different int

Stefano Sabatini wrote:
[snip]


Objects or classes?

Assuming classes, this poses an interesting problem. Here are two attempts:

a)

class reg_base {
protected:

  static
  unsigned int &
  the_count ( void ) {
    static unsigned int c = 0;
    return ( c );
  }

  static
  unsigned int count ( void ) {
    return ( the_count() ++ );
  }

public:

  static
  unsigned int num_classes ( void ) {
    return ( the_count() );
  }
  
  virtual
  unsigned int get_id ( void ) const = 0;

  virtual
  ~reg_base ( void ) {}

};

template < typename D >
class reg : public reg_base {

  static unsigned int const the_id;
  
public:

  unsigned int get_id ( void ) const {
    return ( the_id );
  }

  static
  unsigned int id ( void ) {
    return ( the_id );
  }

  virtual
  ~reg ( void ) {}
  
};

template < typename D >
unsigned int const reg<D>::the_id = reg<D>::reg_base::count();

struct X : public reg<X> {};
struct Y : public reg<Y> {};
struct Z : public reg<Z> {};

#include <iostream>

int main ( void ) {
  std::cout << "Classes: " << reg_base::num_classes() << '\n';
  
  X x;
  Y y1;
  Y y2;
  reg_base * px = new X ();
  reg_base * py = new Y ();
  
  std::cout << "X " << x.get_id() << '\n';
  std::cout << "Y " << y1.get_id() << '\n';
  std::cout << "Y " << y2.get_id() << '\n';
  
  std::cout << "X " << px->get_id() << '\n';
  std::cout << "Y " << py->get_id() << '\n';
  
  std::cout << "X " << X::id() << '\n';
  std::cout << "Y " << Y::id() << '\n';

  delete ( px );
  delete ( py );
}

This one does not count the class Z. Honestly, I don't know the reason.

b)

class reg_base {
protected:

  static
  unsigned int &
  the_count ( void ) {
    static unsigned int c = 0;
    return ( c );
  }

  static
  unsigned int count ( void ) {
    return ( the_count() ++ );
  }

public:

  static
  unsigned int num_classes ( void ) {
    return ( the_count() );
  }
  
  virtual
  unsigned int get_id ( void ) const = 0;

  virtual
  ~reg_base ( void ) {}

};

template < typename D >
class reg : public reg_base {

  static
  unsigned int & the_id ( void ) {
    static unsigned int dummy = 0;
    return ( dummy );
  }

  struct do_register {

    do_register ( void ) {
      the_id() = reg<D>::reg_base::count();
    }
    
  };
  
public:

  reg ( void ) {
    static do_register dummy;
  }
  
  unsigned int get_id ( void ) const {
    return ( the_id() );
  }

  static
  unsigned int id ( void ) {
    return ( the_id() );
  }

  virtual
  ~reg ( void ) {}
  
};

struct X : public reg<X> {};
struct Y : public reg<Y> {};
struct Z : public reg<Z> {};

#include <iostream>

int main ( void ) {
  std::cout << "Classes: " << reg_base::num_classes() << '\n';
  X x;
  std::cout << "Classes: " << reg_base::num_classes() << '\n';
  Y y1;
  Y y2;
  std::cout << "Classes: " << reg_base::num_classes() << '\n';
  Z z;
  std::cout << "Classes: " << reg_base::num_classes() << '\n';
  reg_base * px = new X ();
  reg_base * py = new Y ();
  
  std::cout << "X " << x.get_id() << '\n';
  std::cout << "Y " << y1.get_id() << '\n';
  std::cout << "Y " << y2.get_id() << '\n';
  
  std::cout << "X " << px->get_id() << '\n';
  std::cout << "Y " << py->get_id() << '\n';
  
  std::cout << "X " << X::id() << '\n';
  std::cout << "Y " << Y::id() << '\n';

  delete ( px );
  delete ( py );
  std::cout << "Classes: " << reg_base::num_classes() << '\n';
}

This one counts each class, once the first object of that class has been
constructed.

I don't know, which semantics you actually need.

Best

Kai-Uwe Bux