🚀 go-pugleaf

In article <4imak1$e0j@solutions.solon.com>
ANH@KUHUB.CC.UKANS.EDU writes:
<snip>
A:  > 	printf( "%p\n", (void*) NULL); /* OK */
A:  > 	printf( "%p\n", (Type*) NULL); /* not OK */
A:  > 	printf( "%p\n",  NULL);        /* not OK */
<snip>
A: Really?  Seems like I need to update my 'beliefs' :-)  Actually, (TYPE*)0
A: has been recommended to me by several professors of mine who worked as
A: software engineer as places such as Bell Labs.

I think that the basic problem is that there is actually no
alternative to learning a language: such rules of thumb are often
misleading. %p needs a pointer of type void*, and unless Type is same
as void, (Type*)NULL is not such a pointer. In a prototyped function
which specifies void* as a parameter type, (Type*)NULL will be
converted to void*. printf is not such a function. So, passing
(Type*)NULL to printf is plain wrong (when the format specifies %p
that is).

Note, however, that this has nothing to do with NULL, (void*)NULL and
(Type*)NULL have different types, and one may need one or the
other. The usual mistake is, in fact,
Type x;
printf( "%p\n", &x); /* wrong */
printf( "%p\n", (void*) &x); /* OK */

Plain NULL is wrong because of a similar reason. In fact, NULL is not
guaranteed to have a pointer type at all. The only useful thing that
is guaranteed is that when cast, compared, passed as a visibily
prototyped parameter or assigned to a pointer (let's not get technical
with the actual wording here: it is not comp.std.c), NULL becomes a
null pointer of appropriate type. None of these applies in the current
situation, and so use of unadorned NULL is incorrect.

A: 
A: Just to re-iterate,
A: 
A: tMyType *ptr;
A: if (ptr==NULL) {
A: 	... 
A: }
A: 
A: is a perfectly safe construct?

Yes, see the last para.

A: 
A: [Yes, and in fact, so is
A: 	if (ptr == 0)
A:  and it is guaranteed equivalent. -mod]
A: 
A: Anh
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