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

From:
Kai-Uwe Bux <jkherciueh@gmx.net>
Newsgroups:
comp.lang.c++
Date:
Tue, 23 Sep 2008 20:53:13 -0400
Message-ID:
<gbc31r$et3$1@aioe.org>
Victor Bazarov wrote:

Kai-Uwe Bux wrote:

[snip]

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.


Most likely, the optimizer throws Z away (and the instantiation of the
base class as well), since it isn't used anywhere in the program. Built
without optimization with VC++ 2008 (the "Debug" configuration), the
program reports 3 classes, built optimized it reports only 2.

[snip]

Hm, interesting.

That indicates, that the output is either undefined or implementation
defined. That in turn poses the question, where in the program I invoke
undefined or implementation defined behavior. Got to take a closer look; as
of now, I am quite puzzled.

Best

Kai-Uwe Bux

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