Darren New wrote:
> By my reading, neither C nor C++ guarantee the following:
> 
> union {
>   long x;
>   char* y;
> } xyz;
> xyz.y = NULL;
> printf("%ld\n", xyz.x);
> 
> Neither C nor C++ guarantees that'll be zero (assuming 
> sizeof(char*)==sizeof(long)).  In other words, neither guarantees that 
> the bit pattern in memory for NULL is actually all zeros, right?

C/C++ unions are intended for allowing one piece of memory to store 
different types.  It has been used as a conversion mechanism by 
enterprising coders, but this requires excellent knowledge of the 
implementation of each type in the union.  Consequently code like this 
cannot be guaranteed portable.

Furthermore, NULL is a pre-processor #define, and thus is also 
implementation-specific.  Sure, NULL is defined as 0 on every platform 
for which I've ever coded (so that I often type "0" when I'm supposed to 
type "NULL") but I'm sure that an exception can be found at places like 
www.thedailywtf.com .

Regards,
John

Post a reply to this message