A member function with private visibility can only be invoked by other member functions in the class in which it is defined, but not in the subclasses.
Making it private is the "old" way of doing it. The constructor still exists, but it is private, and can only be invoked from within another class member function.
使其私人的“旧”的方式这样做。构造函数仍然存在，但它是私人的，只能从内部调用另一个类的成员函数。
3
The constructor still exists, but it is private, and can only be invoked from within another class member function.