encapsulation.c

/* point.c */
#include "point.h"
#include <stdlib.h>
#include <math.h>

struct Point {
    double x, y;
}

struct Point* makePoint(double x, double y) {
    struct Point* p = malloc(sizeof(struct Point));
    p->x = x;
    p->y = y;
    return p;
}

double getDistance(struct Point* p1, struct Point *p2) {
    double dx = p1->x - p2->x;
    double dy = p1->y - p2->y;
    return sqrt(dx*dx + dy*dy);
}

encapsulation.h

/* point.h */

struct Point;
struct Point* makePoint(double x, double y);
double getDistance(struct Point *p1, struct Point *p2);