Friend Functions and Classes

1class Box {
2    double width;
3
4public:
5    Box(double w) : width(w) {}
6
7    // Friend function declaration
8    friend void printWidth(const Box& b);
9
10    // Friend class
11    friend class BoxPrinter;
12
13    // Friend operator
14    friend ostream& operator<<(ostream& os, const Box& b);
15};
16
17// Friend function definition
18void printWidth(const Box& b) {
19    cout << b.width << endl;  // Access private member
20}
21
22// Friend class
23class BoxPrinter {
24public:
25    void print(const Box& b) {
26        cout << "Width: " << b.width << endl;
27    }
28};
29
30// Friend operator
31ostream& operator<<(ostream& os, const Box& b) {
32    os << "Box(" << b.width << ")";
33    return os;
34}
35
36Box b(5.0);
37printWidth(b);     // 5
38cout << b << endl; // Box(5)