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

From:
Kai-Uwe Bux <jkherciueh@gmx.net>
Newsgroups:
comp.lang.c++
Date:
Mon, 22 Sep 2008 19:10:37 -0400
Message-ID:
<gb98lf$3bt$1@aioe.org>
puzzlecracker wrote:

unsigned int
count ( void ) {
static unsigned int c = 0;
return ( c++ );

}

struct reg_base {

virtual
unsigned int get_id ( void ) = 0;

};

template < typename D >
class reg : public reg_base {

static unsigned int const id;

public:

unsigned int get_id ( void ) {
return ( id );
}

};

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

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

#include <iostream>

int main ( void ) {
X x;
Y y1;
Y y2;
reg_base * p = 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 << "Y " << p->get_id() << '\n';

}


I think it's better to eliminate count function altogether and put the
count functionality into class/struct such as, in your case, reg_base.
struct counter{

    static unsigned int count getId(return id++;)

    private:
      static unsigned int id;

 };
unsigned int counter::id=0;

Or some cleaner variation of that. I don't like member-less function
in the code. At the very least, we ought to namespace it if we're to
have this function, given that it's short to use and easier to code
than an alternative I've just mentioned.


Yup. Also, get_id() wasn't const. Here is a cleaner version:

class reg_base {
protected:

  static
  unsigned int
  count ( void ) {
    static unsigned int c = 0;
    return ( c++ );
  }

public:
  
  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> {};

#include <iostream>

int main ( void ) {
  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 );
}

Best

Kai-Uwe Bux

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