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 12:28:43 -0400
Message-ID:
<gb8h3s$96p$1@aioe.org>
Stefano Sabatini wrote:

Hi all,

I'm facing this design problem.

I have a table which defines the behaviour of an Object in a given
state according to the type of event it is receiving.

So for each couple event,state I want I set in the table a different
handler.

My trivial implementation make use of defines (but I could have used
an enum as well).

So I have:
#define EVT_FOO 0
#define EVT_BAR 1
...
#define EVT_FOOBAR N

Then I define for each event a virtual GetType() which returns the
event type.

This is ugly and awkward, since everytime I add an event I have to
update the enum/list of defines, while I would like to be able to
automatically get a different integer for every derived class of
Event.

Mayeb I could get this result with some kind of run-time registration
mechanism.


What about something like this:

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';
}

You could also add a static method that yields the id of a type.

I think this problem should be quite common, so I thought maybe
someone can suggest a better design than that which requires the
enum/defines list.


Best

Kai-Uwe Bux

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