Question

I was just trying to grasp better understanding of pointers to pointers and on internet I found an example of Binary search where the developer used pointers to pointers in an insert function.

My question is:

1. What edge does the below code have over other BST code which uses single pointers?
2. At the end of insert function, the code uses a recursive method but I don’t understand the syntax of using & in insert(&(*tree)->right, item);.

The insert function looks like this:

 void insert(node ** tree, node * item)  
 {   
   if(!(*tree))  
   {  
        *tree = item;  
        return;  
   }  
   if(item->val<(*tree)->val)  
      insert(&(*tree)->left, item);  
   else if(item->val>(*tree)->val)  
      insert(&(*tree)->right, item);  
}  

Answer 1

Because of this line, which assigns tree to item if it is null, i.e., it starts a new tree.

*tree = item;

If you only took a node* then you would not be able to assign a new value to it that would be visible from outside of the function. This is because the pointer is passed by value, so the insert function would simply be modifying a copy of the original.

2) at the end of insert function the code uses recursive method but I didnt understadn teh syntax of using “&” insert(&(*tree)->right, item);

The -> operator has a igher precedence than the & operator does, so it is passing in the address of *tree->right, i.e., the node** next in line.