Real-World Examples of Friend Functions

1#include <iostream>
2
3class Point {
4  friend std::ostream& operator<<(
5    std::ostream &os, const Point &p); 
6
7public:
8  Point(int x, int y) : x{x}, y{y} {}
9
10private:
11  int x, y;
12};
13
14std::ostream& operator<<(
15  std::ostream &os, const Point &p) {
16  os << "Point(" << p.x << ", " << p.y << ")";
17  return os;
18}
19
20int main() {
21  Point p(3, 4);
22  std::cout << p;
23}

1#include <iostream>
2
3class Server {
4  friend void logServerState(const Server &server);  
5
6 public:
7  Server(int activeConnections, int maxConnections)
8    : activeConnections{activeConnections},
9      maxConnections{maxConnections} {}
10
11 private:
12  int activeConnections;
13  int maxConnections;
14};
15
16void logServerState(const Server &server) {
17  std::cout << "Active Connections: "
18    << server.activeConnections << "\n";
19  std::cout << "Max Connections: "
20    << server.maxConnections << "\n";
21}
22
23int main() {
24  Server server(120, 200);
25  logServerState(server);
26}

1#include <iostream>
2
3class Rectangle {
4  friend int calculateArea(const Rectangle &rect);  
5
6 public:
7  Rectangle(int width, int height)
8    : width{width}, height{height} {}
9
10 private:
11  int width, height;
12};
13
14int calculateArea(const Rectangle &rect) {
15  return rect.width * rect.height;
16}
17
18int main() {
19  Rectangle rect(10, 20);
20  std::cout << "Area: " << calculateArea(rect);
21}