Question

Note: I’m a experienced C++ programmer, so I don’t need any pointer basics. It’s just that I never worked with void** and have kind of a hard time getting my mental model adjusted to void* vs. void**. I am hoping someone can explain this in a good way, so that I can remember the semantics more easily.

Consider the following code: (compiles with e.g. VC++ 2005)

int main() {
  int obj = 42;
  void* ptr_to_obj = &obj;
  void* addr_of_ptr_to_obj = &ptr_to_obj;
  void** ptr_to_ptr_to_obj = &ptr_to_obj;
  void* another_addr = ptr_to_ptr_to_obj[0];
  // another_addr+1; // not allowed : 'void*' unknown size
  ptr_to_ptr_to_obj+1; // allowed
}

Answer 1

void* is a pointer to something, but you don’t know what. Because you don’t know what it is, you don’t know how much room it takes up, so you can’t increment the pointer.

void** is a pointer to void*, so it’s a pointer to a pointer. We know how much room pointers take up, so we can increment the void** pointer to point to the next pointer.