NULL and 0

I just read in the time(0) thread that 0 is to be preferred over NULL in C++.

What is the reasoning for this? I'm all for it, but I might need to convince others.

If I remember correctly, its because NULL isn't automatically defined as zero, even though most of the time it is. If a compiler wanted to define NULL as -13 it can, which is why it's simply safer to just use zero when that is what you want.

NULL can be defined as:

Code:

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#define NULL 0

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or

Code:

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#define NULL ((void*)0)

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so you can always use 0 when a pointer is required, but not the other way around.

as for the time function:
http://www.cppreference.com/stddate_details.html#time

Code:

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time_t time(time_t* time);

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it takes a pointer, so you can use any of 0 or NULL.

EDIT: missed ()

Quote:

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Originally posted by PJYelton
If I remember correctly, its because NULL isn't automatically defined as zero, even though most of the time it is. If a compiler wanted to define NULL as -13 it can, which is why it's simply safer to just use zero when that is what you want.

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0 is used as NULL because almost every OS uses the first memory address for its own stuff, so a program can use it to indicate an invalid address.

I was referring to this particular comment by Prelude:

Quote:

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In C++ the use of the macro NULL is not recommended, so the integer literal 0 is used because in pointer context it is treated as a null pointer.

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NULL is a macro that in C is usuallly either 0 or ((void *)0). in C the cast makes sense, as you can always assign a void* to any pointer type without an explict cast, but not to an int (or float for that matter). In some C implementations (and this may still be valid) NULL would have a numeric value other than zero so NULL is prefered. In C++ 0 the literal is magical and part of the language. If you are on a platform where you need some other binary value for an invalid address it is the resposibility of the language to provide that value. Same deal with an implied cast to a bool. while(p->next) p = p->next; Could break in K&R C on oddball wierdo platforms where NULL is non-zero, in C++ the language makes sure that a pointer to NULL evaluates to false. This may also be true of C99 and later.

type *p=0; // always cool in C++
(int)p == 0; // MAY NOT BE TRUE IN C++
if(p==0 && !p) // always true in C++

Got it, thanks!