c++ - How can an object contained by another can mutate private data members of container object?

Question

Lets say we have an object elevator Elevator have states for open and closed doors, elevators door is a different object ie,

Class Elevator{

private:
ElevatorDoor door;
int state
};

What I want to do is, when a person tries to open elevator door, she has to interact with door object not elevator itself, but the door object should be able to mutate a private data member (which is state). How is this possible?

Answer 1

首先，移动state到 ElevatorDoor 类本身：

class ElevatorDoor
{
public:
  ElevatorDoor() { state = DoorClosed; }
  void Open() { state = DoorOpen; }
  void Close() { state = DoorClosed; }
  bool IsClosed() { return (state == DoorClosed); }
private:
  int state;
};

您可以拥有一个产生对 的引用的函数ElevatorDoor，或者您可以与 交Person朋友Elevator，这样Person就可以访问 的私有成员Elevator。

例如：

class Elevator{
public:
  ElevatorDoor& GetDoor() { return door; }

private:
  ElevatorDoor door;
};

或者

class Elevator{
  friend Person;
private:
  ElevatorDoor door;
};

在第二种情况下，Person当然可以触摸Elevator.

另一个选项，可以更精细地控制可以做什么，是在电梯上具有门操纵器功能：

class Elevator{
public:
  void OpenDoor() { door->Open(); }
  void CloseDoor() { door->Close(); }
private:
  ElevatorDoor door;
};