Re: linux libio status

[Joe Buck <jbuck at synopsys dot com>]

> 
> On Tue, Oct 14, 1997 at 11:23:49PM -0700, Joe Buck wrote:
> > We need to silence these warnings.  For the NULL, I suggest a global
> > s/NULL/0/ in the C++ iostreams headers.  There is no reason to write
> > NULL, it is just a source of problems like this (*especially* since
> > we have changed C++ to be strict about void*).
> 
> Don't use '0' but '0L'. This way you will be OK with NULL in stdargs
> when int=32b and pointer=64b.

And you'll be broken on platforms where sizeof(long) != sizeof(pointer),
or at least you would if it weren't for the fact that C++ requires
prototypes for all functions.  Remember, we are talking about C++ here,
not C.

Stroustrup says (The C++ Programming Language, 3rd Edition, p. 88)
"Because of C++'s tighter type checking, the use of plain 0, rather than
any suggested NULL macro, leads to fewer problems.  If you feel that you
must define NULL, use

const int NULL = 0;

..."

There is a similar discussion in the comp.lang.c++ FAQ.

In C++, there are always prototypes.  Thus 0 or 0L makes no difference,
and in fact the "standard null pointer" is essentially 0 in C++.  The
argument is coerced to the proper pointer.  However, standard C++ does
not let you convert (void *) to (T *) without a cast, so defining NULL
as (void *)0 is completely broken in C++.  It used to work in g++, but
that was an extension (furthermore this extension prevented g++ from
implementing the standard overloading rules correctly).  Hence

The only exception is functions with ... in their spec.  Here, neither 0
nor 0L will result in portable code.  Your suggestion switches the
erroneous assumption that sizeof(T *) == sizeof(int) with another
erroneous assumption, that sizeof(T *) == sizeof(long).  Worse, you leave
users with the idea that they can safely use NULL for any null pointer
type, even in variadic functions.  Sorry, you can't do that in standard
C++.  For variadic functions, users must declare any optional arguments
correctly, period.

Conclusion: we should deprecate the use of NULL in C++ code.  If we
must define NULL for C++ code, it must be 0.  Nothing else.