TITLE: Just when you thought it was safe to use NULL


PROBLEM: Many implementations use a #define for NULL in system
header files which could cause problems in your code.


RESPONSE: Jim Adcock (jimad@microsoft.com)

Given all these hassles, some old time C++ hackers just try avoiding use
of NULL.  Just use '0' everywhere instead.  Make sure that that '0'
is explicitly cast to the "right" type when used as a vararg -- just
as vararg need *always* be explicitly be cast to exact type in
vararg situations.

....Of course, use of NULL will probably be mandated by your local
C++ coding convention facists, in which case you'll get to discover all
these various failure modes for yourself!


RESPONSE: MfG Kai (???)

Or maybe, instead of NULL, use a NIL macro. I've seen several forms, two are

#define NIL(type)  ((type)0)

or

#define NIL(type)  ((type *)0)

Of course, too bad if you are not consistent with this. This "silently
guarantees" that you always cast 0 to the correct type.

You might consider persuading them to mandate one (but NOT both, for heaven's
sake!) of the above macros ... helps documentation as well!


RESPONSE: Marshall Cline (cline@sun.soe.clarkson.edu)

The most portable way to compare against the nil ptr is to compare against `0'.
Some programmers use a #define to set NULL to `0', but that can conflict with
the way the standard libraries #define NULL.  There is no portable way to
define a `const' ptr called `NULL' that can be compared against any arbitrary
ptr -- the literal `0' is acceptable for this however.