c - Modifying head pointer in a linked list

Question

I am having trouble understanding this code. All I really need is to modify the head pointer to point to the first element. So why won't *head work ? Changing the value of *head changes where this pointer points to and that should work, right ? I have read the pass by reference/pass by value, but am finding it hard to understand. Can someone help clarify this ? Appreciate your help. Thanks.

In C/C++ it’s easier to make mistakes with pointer misuse. Consider this C/C++ code for inserting an element at the front of a list:

bool insertInFront( IntElement *head, int data ){
  IntElement *newElem = new IntElement;
  if( !newElem ) return false;

  newElem->data = data;
  head = newElem; // Incorrect!
  return true;
}

The preceding code is incorrect because it only updates the local copy of the head pointer. The correct version passes in a pointer to the head pointer:

bool insertInFront( IntElement **head, int data ){
  IntElement *newElem = new IntElement;
  if( !newElem ) return false;

  newElen->data = data;
  *head = newElem; // Correctly updates head
  return true;
}

Answer 1

You need help understanding the difference right?

Imagine the caller of the function in the first case:

IntElement *head;
int data;
...
insertInFront (head, data);

Now, in this case, the address pointed to by head is placed on the stack and passed in as an argument to insertInFront. When insertInFront does head = newElement; only the argument (on the stack) is modified.

In the second case, the caller would be:

IntElement *head;
int data;
...
insertInFront (&head, data);

In this case, the address of head is placed on the stack and passed in as an argument to insertInFront. When you do *head = newElement, this passed in address is de-referenced to get the address of the original list head, and that is modified.