Thread: Void Pointer and Pointer to Void

Originally Posted by OnionKnight

"void" is not a type qualifier, so "void pointer" makes no sense.

Not quite.

void pointers are generic pointers. They can't be used without being cast to something else first.

Code:

void *MemLocation; //LEGAL

Yes, a void pointer and a pointer to void are the same thing.

Const pointers (const type *) and pointers to const (type *const) are different because const is a type qualifier. const can go before or after the asterisks, where it means different things, as you have discovered. void is an ordinary variable type, just like int and char and all the rest. (Well, not quite, because you can declare an int and you can't declare a void.) Anyway, type *int or type *void is invalid. You can only put the pointer type before the asterisks, not afterwards, where you can put const.

Originally Posted by dwks

void is an ordinary variable type, just like int and char and all the rest. (Well, not quite, because you can declare an int and you can't declare a void.)

I strongly disagree with "not quite." It's a HUGE difference. "void" is not a type. It sometimes seems to act like a type but the very fact that you cannot declare a variable of type "void" proves that it is not a type.

Void serves two completely different purposes in the language. One, you use it to declare that a function takes no arguments or has no return value. Two (completely unrelated), you use it to specify a pointer to a generic type. In one case, it indicates "nothingness." In the other case it indicates genericness.

It does NOT mean that this generic type is "void." There is absolutely NO RELATIONSHIP between "void" and "void *."

In spoken conversation I call it a "void star."

But void has a lot more in common with int than it does with if or while. That's all I was trying to get across.

In spoken conversation I call it a "void star."

I say that if I'm pronouncing a variable name, like "void star p". But I refer to it as a "void pointer" otherwise. I'm sure other people refer to it differently.

Originally Posted by brewbuck

I strongly disagree with "not quite." It's a HUGE difference. "void" is not a type. It sometimes seems to act like a type but the very fact that you cannot declare a variable of type "void" proves that it is not a type.

Being able to manipulate pointers to undeclarable types is a pretty fundamental feature of the language that extends beyond void. Except for the syntactic trick of a function which returns or accepts the void type, there's really nothing differentiating void from any other incomplete type.

void is an incomplete type that can not be completed... and that's it. You can't really think of it as something too far outside the type system without memorizing a lot of things twice.

Code:

#include <stdlib.h>

// Types which can not be declared are normal and necessary to the language.
struct foo;

struct foo * foomalloc (size_t size) {
   return (struct foo *) malloc(size);
}

int main (void) {
   struct foo * f = foomalloc(5);
   return 0;
}

And if you ask me, the fact that void is the 'nothing' when used as a parameter type makes perfect sense. The only possible type you could unambiguously use for such a notation would be a type which can not be declared, which is more or less the definition of void.

You can even cast something to void, like lint likes:

Code:

(void)printf("Hello, World!\n");

You just can't declare a variable of type void, because what would it store?

I personally read "void pointer" and "pointer to void" as synonymous. Much like "int pointer" and "pointer to int".