TITLE: definition of NULL

(Newsgroups: comp.lang.c++.moderated)

PROBLEM: shappir@libra.math.tau.ac.il (Shappir Dan)

I've encountered several differences between GNU C++ (ver 2.7.2)
and Watcom C++ (ver 10.0) and I wonder which is correct:

1. Watcom C++ defines NULL as 0 (or 0L) while GNU C++ defines
NULL as ((void*)0).


RESPONSE: clamage@Eng.Sun.COM (Steve Clamage), 2 Jul 96

The draft standard requires NULL to be a null pointer constant.
A null pointer constant is "an integral constant expression rvalue of
integer type that evaluates to zero". 0 and 0L are therefore valid,
but ((void*)0) is not valid. The point of NULL is that you can write
things like
	char* p = NULL;
which will not work in C++ if NULL has type void*. In C, ((void*)0)
is the preferred definition, because a void* can be converted
implicitly to other pointer types. Not so in C++.

NULL is still defined to be a macro for compatibility with C. It
is required to be a macro in C, and if C++ changed the requirement
it would break existing code like "#ifdef NULL".