Re: private types

[Maciej Sobczak <no.spam@no.spam.com>]

Peter C. Chapin wrote:

>>Note: the following is explicitly prohibited:
>>
>>#define NULL (void*)0
> 
> This defintion of NULL is not workable in C++.

Which we've already cleared. :)


> Thus in C++, NULL must be defined as the literal "0".

Not exactly. It can be anything that is an integral constant expression 
evalutating to zero. Thus, "0" is one obvious choice, but not any better 
than, say, "0L", "'\0'" or even "(125 - 5 * 5 * 5)".


> Because "NULL" just expands to "0" in C++, code using NULL can at times 
> be misleading. For example consider
> 
> void f(int);
> void f(char *);
> 
> f(NULL);  // Ambiguous, could call either f(int) or f(char *).
> 
> This is a surprising error.

That's why one of the sweeties in the upcoming C++ standard is a new 
keyword nullptr, which will be useable only in those contexts where a 
pointer type is expected, so that:

f(nullptr);

will always choose the second overload from the above.

> Other surprises come up when passing NULL to 
> functions taking a variable number of arguments.

Such functions are FUBARed anyway. :)

> For these reasons many C++ experts recommend using "0" explicitly to 
> represent NULL pointers and not the symbol NULL. This makes the problems 
> above more apparent in the source and thus more likely that the 
> programmer will notice them.

On the other hand, for the human reader "NULL" is immediately associated 
with some special pointer value, not with not-at-all-special integer, so 
that the following:

if (item != NULL) ...

is also immediately recognized as a test against *existence* of some 
item (which also implies that item is a pointer), not as a test against 
item's integer value. In such contexts I use NULL.

As you see, this subject has many faces.
I invite you to comp.lang.c++.moderated to discuss them in more depth.


-- 
Maciej Sobczak : http://www.msobczak.com/
Programming    : http://www.msobczak.com/prog/