• If we want that operator + can support (int + MyTime) ```cpp MyTime t1(2, 40); 20 + t1;
    2. - Let a friend function to help
    3. - Friend functions
    4.    - Declare in a class body -> 在类内部声明
    5.    - Granted class access to members (including private members)  -> 有获取所有类内成员的能力
    6.    - But not members  -> 其不是类的成员
    8. :::info
    9. friend functions are not members! They just declared in the class body.
    10. :::
    11. ```cpp
    12. class MyTime
    13. {
    14.   // ...
    15.   public:
    16.     friend MyTime operator+(int m, const MyTime & t)
    17.     {
    18.         return t + m;
    19.     }
    20. };
    • A friend function is defined out of the class.
    • No MyTime:: before its function name ```cpp class MyTime { // … public: friend MyTime operator+(int m, const MyTime & t); };

    MyTime operator+(int m, const MyTime & t) { return t + m; }

    2. - Operator `<<` can also be overloaded.
    3. - But in (cout << t1; ) , the first operand is std::ostream, not MyTime.
    4. - To modify the definition of **std::ostream? No!**
    5. - Use a friend function
    6. ```cpp
    7. friend std::ostream & operator<<(std::ostream & os, const MyTime & t)
    8. {
    9.     std::string str = std::to_string(t.hours) + " hours and " 
    10.              + std::to_string(t.minutes) + " minutes.";
    11.     os << str;
    12.     return os;
    13. }
    15. friend std::istream & operator>>(std::istream & is, MyTime & t);