Structs and the arrow operator

p1 is one box holding two fields, x and y. move_by gets a pointer to it, so pt->x += dx changes p1 in main. pt->x is shorthand for (*pt).x.

Passing a pointer also avoids copying the whole struct, which matters once structs get big.

#include <stdio.h>

struct point {
    int x;
    int y;
};

void move_by(struct point *pt, int dx, int dy) {
    pt->x += dx;
    pt->y += dy;
}

int main(void) {
    struct point p1 = {1, 2};
    move_by(&p1, 3, 4);
    printf("(%d, %d)\n", p1.x, p1.y);
    return 0;
}

Output:

(4, 6)

From the lesson: Structs, enums and typedef