Structs and Functions

Passing by value (copy)

int area(struct Rect r) {          // r is a COPY
    return r.size.x * r.size.y;
}

Small structs pass cheaply and safely — the function cannot touch the caller's original.

Returning structs

struct Point make_point(int x, int y) {
    struct Point p = {x, y};
    return p;                      // a copy comes back
}

Passing a pointer to modify

void move_by(struct Point *p, int dx, int dy) {
    p->x += dx;                    // arrow: dereference + member
    p->y += dy;
}
...
struct Point pos = {0, 0};
move_by(&pos, 3, 4);              // pos is now (3, 4)

p->x is exactly shorthand for (*p).x — dereference then take the member. The arrow exists because . binds tighter than *, making *p.x mean the wrong thing.

Choosing the shape

• read-only use of a small struct → pass by value
• must modify, or the struct is large → pass struct S *