What is a void pointer and what is a null pointer?

c++ pointers void-pointers null-pointer
Shouvik picture Shouvik · Dec 2, 2010 · Viewed 30.2k times · Source

So I was going through some interview questions and I came across one about void and null pointers, which claims:

a pointer with no return type is called a null pointer. It may be any kind of datatype.

This confused me thoroughly! It seems void and null could be used interchangeably according to this question, and I don't believe that to be correct. I assumed void to be a return type and null to be a value. But I am just a code-rookie and am not sure I am right.

Please express your views as to what a null pointer is and a void pointer is. I am not looking for difference between null and void.

Answer

Marcelo Cantos picture Marcelo Cantos · Dec 2, 2010

The two concepts are orthogonal:

  1. A void pointer, (void *) is a raw pointer to some memory location.
  2. A null pointer is a special pointer that doesn't point to anything, by definition. It can be a pointer to any type, void or otherwise.

A void pointer can be null or not:

void *void_ptr1 = nullptr;
void *void_ptr2 = malloc(42);
void *void_ptr3 = new Foo;               // void * can point to almost anything
void *void_ptr4 = (char*)void_ptr3 + 1;  // even somewhere inside an object

A non-void pointer can also be null or not:

Foo *f = nullptr;
Foo *g = new Foo;

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