NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part2/11465

Text File | 1996-08-05 | 3.1 KB | 73 lines

Path: kbad.eglin.af.mil!rpi!not-for-mail
From: [e-mail] (J. Kanze)
Newsgroups: comp.lang.c++,comp.lang.c++.moderated
Subject: Re: Enumerated type converted to pointer? Legal?
Followup-To: comp.lang.c++,comp.lang.c++.moderated
Date: 14 Mar 1996 17:56:34 -0000
Organization: GABI Software, Sarl.
Sender: [e-mail]
Approved: [e-mail]
Message-ID: <4i9mki$[e-mail]>
References: <4hobji$[e-mail]> <4i703u$[e-mail]>
NNTP-Posting-Host: netlab.cs.rpi.edu
X-Original-Date: 14 Mar 1996 11:58:25 GMT

In article <4i703u$[e-mail]> [e-mail]
(Shappir Dan) writes:

|> { comp.std.c++ removed from followups. -mod}

|> Brad Stowers ([e-mail]) wrote:

|> : Compile and run the above program.  Is it just me, or should the last line
|> : of the main() function ( FooBar(x, mdType1); ) have caused a type mismatch
|> : error?  I think I know why it didn't:  mdType1 evaluates to an integer with
|> : a value of 0, which is a valid value to pass as a pointer.  In support of
|> : this theory, the line:

|> This is due to one of the differences between C and C++. In C the NULL
|> pointer is defined as ((void*)0) while in C++ it's simply 0.

This is false.  In both C and C++, the exact definition of NULL depends
on the implementation.  On most Unix systems, for example, both are
defined as 0, but I have worked on systems (before C++) where NULL was
0L.

The only difference between C and C++ in this regard is that ((void*)0)
is legal in C, but not in C++.  The only reason that an implementor
would maintain different definitions is historical: he used ((void*)0)
in C before implementing C++, and is afraid that changing it would break
customer code.  (In fact, any code it broke was already broken, but it
is often easier for the implementor to support the broken code that to
explain to the customer why it worked in one release and not in the
next.)

|> This can raise
|> all sorts of type conversion problems, for example:

|> void foo(int bar) { ... }
|> void foo(void* bar) { ... }

|> ...
|> foo(0);
|> ...

|> The compiler will not be able to determine which version of foo to call.

The compiler had better call `foo( int )', which is an exact match.
More surprising, since NULL is generally defined as 0, `foo( NULL )'
also calls `foo( int )'.  (Fergus Henderson once modified g++ to have a
real null pointer, which would only match pointer types, but his
modifications are not part of the standard, and as far as I know, have
never been incorporated in a real g++ release.)
--
James Kanze           ([phone]          email: [e-mail]
GABI Software, Sarl., 8 rue des Francs Bourgeois, 67000 Strasbourg, France
Conseils, Θtudes et rΘalisations en logiciel orientΘ objet --
              -- A la recherche d'une activitΘ dans une region francophone

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