Pointer to a Pointer

Big idea

A pointer is a variable too — so it has its own address, and another pointer can point to it. That's a pointer to a pointer: int **.

The idea

int x = 10;
int *p = &x;     // p holds x's address
int **pp = &p;   // pp holds p's address

Each * follows one arrow:

Why would you want one?

The same reason as any pointer: to let a function change something that belongs to the caller. If the thing you want to change is itself a pointer, the function needs a pointer to that pointer.

#include <stdio.h>

void point_at_bigger(int **target, int *a, int *b) {
    if (*a > *b) {
        *target = a;
    } else {
        *target = b;
    }
}

int main(void) {
    int left = 3, right = 8;
    int *best = NULL;
    point_at_bigger(&best, &left, &right);
    printf("best points at %d\n", *best);
    return 0;
}

output
best points at 8

You'll meet ** again with arrays of strings — main can even take char **argv, a list of the words typed on the command line.

Watch it

Think of it like

A treasure map that leads to a second map, which leads to the treasure. Two hops, but you still end up at the same chest.

🕷️ Spidey-sense: A note in my locker that says which rooftop has my backpack. The locker note → the backpack location → the backpack. ** is just "follow it twice".

Key terms

• Pointer to a pointer — A variable holding the address of a pointer: int **pp.
• Level of indirection — How many hops it takes to reach the value; each * is one hop.