Editing Pointer (computer programming) (section)

====Multiple indirection====
In some languages, a pointer can reference another pointer, requiring multiple dereference operations to get to the original value. While each level of indirection may add a performance cost, it is sometimes necessary in order to provide correct behavior for complex [[data structures]]. For example, in C it is typical to define a [[linked list]] in terms of an element that contains a pointer to the next element of the list:

<syntaxhighlight lang="C">
struct element {
    struct element *next;
    int            value;
};

struct element *head = NULL;
</syntaxhighlight>

This implementation uses a pointer to the first element in the list as a surrogate for the entire list. If a new value is added to the beginning of the list, <code>head</code> has to be changed to point to the new element. Since C arguments are always passed by value, using double indirection allows the insertion to be implemented correctly, and has the desirable side-effect of eliminating special case code to deal with insertions at the front of the list:

<syntaxhighlight lang="C">
// Given a sorted list at *head, insert the element item at the first
// location where all earlier elements have lesser or equal value.
void insert(struct element **head, struct element *item) {
    struct element **p;  // p points to a pointer to an element
    for (p = head; *p != NULL; p = &(*p)->next) {
        if (item->value <= (*p)->value)
            break;
    }
    item->next = *p;
    *p = item;
}

// Caller does this:
insert(&head, item);
</syntaxhighlight>

In this case, if the value of <code>item</code> is less than that of <code>head</code>, the caller's <code>head</code> is properly updated to the address of the new item.

A basic example is in the [[argv]] argument to the [[main function#C and C++|main function in C (and C++)]], which is given in the prototype as <code>char **argv</code>—this is because the variable <code>argv</code> itself is a pointer to an array of strings (an array of arrays), so <code>*argv</code> is a pointer to the 0th string (by convention the name of the program), and <code>**argv</code> is the 0th character of the 0th string.