Friends are functions or classes declared as such. ความสัมพันธ์มีฟังก์ชันหรือคลาสที่ประกาศเช่น
If we want to declare an external function as friend of a class, thus allowing this function to have access to the
private and protected members of this class, we do it by declaring a prototype of this external function within the
class, and preceding it with the keyword friend: