An Object-oriented Point: Example 2

// point_class2.cpp

#include "cmpt_error.h"
#include <iostream>
#include <cmath>

using namespace std;

//
// This is based on point_class1.cpp, and differs as follows:
//
// - a destructor with a debugging message has been added
//
// - the destructor uses *this to use operator<< for printing
//
// - declarations of both Point and operator<< are needed for the code to
//   compile
//

// declaration
//
// must declare Point to be a struct so that the declaration of operator<<
// that follows will compile
struct Point;

// declaration
//
// must declare operator<< here because it is used in ~Point below
ostream& operator<<(ostream& out, const Point& p);


struct Point {
        double x = 0;  // member initialization
        double y = 0;  // i.e. x and y are set to 0 by default

        // default constructor
        Point() // x and y are initialized above
    { }

        Point(double a, double b)
        : x(a), y(b)         // initializer list
        { }

        // copy constructor
        Point(const Point& other)
        : Point(other.x, other.y)  // constructor delegation
        { }


    // this is a special variable added to every object by the C++ compiler;
    // this points to the object itself, and so here this has type Point*
        ~Point() {
                cout << "Point destructor called: " << *this << "\n";
        }

        // multiply x and y by r
        void scale(double r) {
                x *= r;
                y *= r;
        }

}; // struct Point

ostream& operator<<(ostream& out, const Point& p) {
        out << "(" << p.x << ", " << p.y << ")";
        return out;
}

// returns a new Point that is the sum of a and b
Point operator+(const Point& a, const Point& b) {
        return Point{a.x + b.x, a.y + b.y};
}

// returns the distance between p and q
double dist(const Point& p, const Point& q) {
        double dx = p.x - q.x;
        double dy = p.y - q.y;
        return sqrt(dx*dx + dy*dy);
}

void test1() {
        Point p;
        cout << "p = " << p << "\n";
        cout << "  p.x: " << p.x << "\n";
        cout << "  p.y: " << p.y << "\n";

        Point c(p);
        cout << "c = " << c << "\n";
        cout << "  c.x: " << c.x << "\n";
        cout << "  c.y: " << c.y << "\n";

        Point pts[5];  // only compiles if Point has a default constructor
        for(int i = 0; i < 5; ++i) {
                cout << "pts[" << i << "] = " << pts[i] << "\n";
        }
}

void test2() {
        Point a(1, 2);
        Point b(5, -2);

        cout << "distance between " << a << " and " << b << " = "
             << dist(a, b) << "\n";

        // a + b returns a Point, and after it is printed its destructor is called
        cout << a << " + " << b << " = " << a + b << "\n";

        // b's destructor is called
        // a's destructor is called
}

int main() {
        // test1();
        test2();
}