Friend c++

gistfile1.txt

#include <iostream>


using namespace std;

class Square;

class Rectangle {
    int width, height;
public:
    int area() {
        return (width * height);
    }

    void convert(Square a);
};

class Square {
    friend class Rectangle;

private:
    int side;
public:
    explicit Square(int a) : side(a) {}
};

void Rectangle::convert(Square a) {
    width = a.side;
    height = a.side;
}

int main() {
    Rectangle rect{};
    Square sqr(4);
    rect.convert(sqr);
    cout << rect.area();
    return 0;
}