How to check if void pointer points to NULL?

Question

So if you do:

void *ptr = NULL;

What is the best way to check if that void pointer is NULL?

My workaround for now is this:

if (*(void**)ptr == NULL) ... 

But this doesn't seem like the best way, as I'm implicitly assuming ptr is of type void** (which it isn't).

Solution 1

I'd simply write if (!ptr).

NULL is basically just 0 and !0 is true.

Solution 2

A NULL pointer is a pointer that isn't pointing anywhere. Its value is typically defined in stddef.h as follows:

#define NULL ((void*) 0)

or

#define NULL 0

Since NULL is zero, an if statement to check whether a pointer is NULL is checking whether that pointer is zero. Hence if (ptr) evaluates to 1 when the pointer is not NULL, and conversely, if (!ptr) evaluates to 1 when the pointer is NULL.

Your approach if (*(void**)ptr == NULL) casts the void pointer as a pointer to a pointer, then attempts to dereference it. A dereferenced pointer-to-pointer yields a pointer, so it might seem like a valid approach. However, since ptr is NULL, when you dereference it, you are invoking undefined behavior.

It's a lot simpler to check if (ptr == NULL) or, using terse notation, if (!ptr).

Comments

[verbose]

@EricPostpischil you're right of course, and I've edited accordingly. Thanks for the correction.

[fadedbee]

Not on all architectures, you will be writing non-portable code. It might have been the 286, when using segment addressing, where NULL != 0, but that was many decades ago.

[jacekmigacz]

I would compare ptr with NULL; like ptr == NULL. There is no guarantee that NULL is equal 0.

[Himanshu Shekhar]

Is that approach advisable in the present world, modern machines?

[user694733]

@jacekmigacz That makes no difference. ptr == NULL is same as ptr == 0 is same as !ptr. Even if implementation uses NULL which is not all bits zero, it's still guaranteed that compiler will make all three above behave the same.

[user694733]

This is C question. C doesn't have reinterpret_cast.