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[QUOTE="Ivan Riis Nielsen, post: 4055560"]
Hello everyone,

I have been playing around with object oriented techniques in C. 
Specifically, I wanted to implement generic reference counting.  This 
would be implemented by a base class which would always be the first 
field in all derived classes.  The reference counting would then be 
handled by a universal assignment macro.  Below is some trimmed down 
code demonstrating my issue:

gcc (4.4.3) complains about strict-aliasing rules.

% gcc -O3 -Wall t1.c
t1.c: In function ‘main’:
t1.c:28: warning: dereferencing pointer ‘_pdst’ does break 
strict-aliasing rules
t1.c:28: note: initialized from here

The output from the program is

% ./a.out
d1 = {{1},2}
d1 = 0x9b53008, d2 = (nil)
d1 = {{2},2}
d1 = 0x9b53008, d2 = 0x9b53008

The compiled program does as expected.  I don't understand exactly what 
the problem is, can anyone shed some light on this?  Or is this a false 
warning?

Thanks
Ivan

<code>

#include <stdio.h>
#include <stdlib.h>

typedef struct {
   int count;
} base_struct, *base;

typedef struct {
   base_struct b;
   int deriv_field;
} deriv_struct,*deriv;

#define UNIVERSAL_ASSIGN(dst,src)    \
   do {				     \
     base* _pdst=(base*)&(dst);	     \
     base _src=(base)(src);	     \
     if (_src) ++(_src->count);	     \
     if (*_pdst) --((*_pdst)->count); \
     *_pdst=_src;		     \
   } while (0)

int main(void) {
   deriv d1=(deriv)malloc(sizeof(deriv_struct)),d2=0;
   d1->b.count = 1;
   d1->deriv_field = 2;
   printf("d1 = {{%d},%d}\n",d1->b.count,d1->deriv_field);
   printf("d1 = %p, d2 = %p\n",d1,d2);
   UNIVERSAL_ASSIGN(d2,d1);
   printf("d1 = {{%d},%d}\n",d1->b.count,d1->deriv_field);
   printf("d1 = %p, d2 = %p\n",d1,d2);
   return 0;
}

</code>
[/QUOTE]

Verification