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

From:

Triple-DES <DenPlettfrie@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 23 Sep 2008 20:57:58 -0700 (PDT)

Message-ID:

<1bf92a60-e356-4a54-84fc-335ede7e6e29@59g2000hsb.googlegroups.com>

On 23 Sep, 19:26, Kai-Uwe Bux <jkherci...@gmx.net> wrote:

Stefano Sabatini wrote:

[snip]

Yes, I tried it and it is indeed what I need. Only what I also need
would be a static method which returns the total number of objects
already registered.

Objects or classes?

Assuming classes, this poses an interesting problem. Here are two attempt=

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.

I think the reason is that even if reg<Z> is implicitly instantiated
by the declaration of
struct Z : public reg<Z>, this does not cause the members of reg<Z> to
be instantiated.

Consider the following, from [temp.inst]/1: (annotations in brackets)

(...)in particular, the initialization (and any associated side-
effects) [in this case, increasing the counter] of a static data
member [of a class template] does not occur unless the static data
member is itself used in a way that requires the definition of the
static data member to exist. [never happens, since Z::the_id is never
referenced]

DP