Question

I am looking at a textbook example of a linked list that implements a stack. I don’t understand why using a pointer to a pointer to the stack is necessary for the push operation. See the following example:

bool push( Element **stack, void *data)
{
    Element *elem = new Element;
    if(!elem) return false;

    elem->data = data;
    elem->next = *stack;
    *stack = elem;
    return true;
}

If anyone can help clarify why the first parameter of the push method is a pointer to a pointer, I would greatly appreciate it. Thanks.

Amazing, thank you for all of the excellent help.

Answer 1

The function needs to modify the value of the Element pointer, so it needs a pointer to that pointer.

Put it another way: a function takes a pointer of something when it needs to modify that thing.

In this case, that something is a pointer itself. So the function ends up taking a pointer to a pointer.