Re: union and overstamping zero values

From:
Thomas Richter <thor@mersenne.math.TU-Berlin.DE>
Newsgroups:
comp.lang.c++.moderated
Date:
Thu, 22 Mar 2007 16:39:28 CST
Message-ID:
<56g2ciF28rk0sU1@mid.dfncis.de>
andrew_nuss@yahoo.com <andrew_nuss@yahoo.com> wrote:

Not portably. The NULL pointer need not to be represented
by a bit pattern that also represents the zero integer


If so, then why does this work?:

main {
   void* p = 0;
   if (p == 0)
       p = malloc(2000);
   if (p)
       free(p);
}


Because the NULL pointer is written as 0. It is the matter of the
compiler to figure out that this specific 0 is a NULL pointer constant
and not the all-zero bit pattern. Or rather, and to be more precise,
every numerical constant that evaluates to zero can be promoted to a
pointer, and is then the NULL pointer. This conversion is trivial on
platforms where the NULL pointer has the all-zero bit pattern, but not
all platforms are like that.

E.g.:

void *p = !!!!0;

is equivalent to the above, and no cast is required.

Similar, the implicit conversion from pointer to bool tests against the NULL
pointer and not against the zero-bit pattern.

I have chosen to use zero with pointers rather than the #define NULL
according to some recommendation that I saw once upon a time.


This is because some (old) platforms choose to define NULL as

#define NULL (void *)0

but this will cause trouble in C++ because there is no implicit
conversion from void * to any other pointer, unlike in C.

Some compilers offer a specific _null built-in key-word that allows
the compiler to check whether that NULL is erratically assigned to
a numerical (non-pointer) variable, it might be thus still a good
idea to use NULL, and just avoid broken compilers. But all that IMHO.

So long,
    Thomas

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