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From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: NULL pointers
Message-ID: <4375@mimsy.UUCP>
Date: Fri, 21-Nov-86 12:02:46 EST
Article-I.D.: mimsy.4375
Posted: Fri Nov 21 12:02:46 1986
Date-Received: Fri, 21-Nov-86 22:02:28 EST
References: <1696@ncoast.UUCP>
Reply-To: chris@mimsy.UUCP (Chris Torek)
Organization: University of Maryland, Dept. of Computer Sci.
Lines: 38

In article <1696@ncoast.UUCP> kent@ncoast.UUCP (Kent Williams) writes:
>... the use of NULL is a constant source of pain for which
>it seems there is a simple solution to wit
>
>#ifdef HASVOID
>#define NULL ((void *)0)
>#else
>#define NULL 0L
>#endif

This is neither necessary nor sufficient:

>If NULL is typed to have the size of the largest possible object that it
>will be assigned to, it will be 'narrowed' to fit into whatever object you
>are assigning it to.

If you are using `0' (the present proper definition for NULL) in
an assignment or comparison context, the narrowing or widening or
implicit cast conversion or whatever-is-required is done automatically.
If you are using 0 in some other context, the information as to
just what narrowing or widening or implicit cast conversion or
whatever-is-required is unavailable.  It is not difficult to add
an assignment context by using an explicit cast:

	foo((char *) NULL);	 /* or foo((char *) 0); */

In C++, or some other C-like language with function prototypes, the
prototype can provide the context:

	extern void foo(char *s);
	...
	foo(NULL);		/* or foo(0); */

In either case, an explicit cast is still legal, and may help the
reader as well as the compiler.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
UUCP:	seismo!mimsy!chris	ARPA/CSNet:	chris@mimsy.umd.edu