Question

Suppose I define this structure:

struct Point {
   double x, y;
};

Now, suppose I create a dynamic array of this type:

Point *P = new Point[10];

Why do I use P[k].x and P[k].y instead of P[k]->x and P[k]->y to access the k-th point’s elements?

I thought you had to use the latter for pointers.

Answer 1

Actually, you use p[index].x and p[index].y to access elements of the struct inside an array, because in this case you are using a pointer to refer to a dynamically allocated array.

The ptr->member operator is simply a shorthand for (*ptr).member. In order to use it, you need a pointer on the left-hand side:

Point *p = new Point;
p->x = 12.34;
p->y = 56.78;

Note that even for a dynamically allocated array the -> operator would have worked:

Point *p = new Point[10];
p->x = 12.34;
p->y = 56.78;

This is equivalent to

p[0].x = 12.34;
p[0].y = 56.78;

because a pointer to an array is equal to the pointer to its first element.