On Wed, Jan 9, 2019 at 5:32 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> (because NULL literally has special meaning even in C, even if the
> special meaning is less than the special meaning in C++).
Just to clarify: a lot of people seem to think that
#define NULL ((void *)0)
means that NULL just behaves as a "void *" and is thus compatible with
all other pointers, and that's it.
Nope. Even in C, NULL is actually special in ways that have nothing to
do with "void *" part. You should think of the "(void *)" part as just
syntactic sugar, that makes sure it cannot be mistaken for a regular
integer, and helps turn it into the special thing that NULL is.
It's just way less _noticeably_ special than it is in C++.
In particular, it's special wrt the ternary operator.
If you have
void *a;
int *b;
then the expression
cond ? a : b;
has type 'void *'.
But
cond ? NULL : b;
has type "int *".
Notice how NULL - even in C - doesn't act as "just another void
pointer". It really is its own special value, and has its own rules
for typing, _outside_ of the rules of "void *" pointers.
This was always true, btw. The C++ people who told you that you couldn't do
#define NULL ((void *)0)
in C++, because NULL is not just another void pointer were full of
shit. ANSI C did it, and did it right. Long long long before the C++
people invented a completely pointless new syntax for the same thing.
It's sad that this has to be stated even in the year 2018.
Linus
