Change member of a class from another class

[iciwi]

Hey there,
I have a problem: I want to change the member of a class A from class B. So I made class B to a friend of class A. There is a function in class B that changes the variable "state". And in class A there is a function that checks if the variable "state" is 1. If true it will turn on an Led. I wrote this code:

class A {
  friend class B;
  
  private:
  int state;
  int Led = 13;
  A(){
  pinMode(Led, OUTPUT);
  }

  void update(){
  if (state == 1){
  digitalWrite (Led, HIGH);
  }
 }
};

class B{
  public:
  void update(){
    state = 1;
  }

};

A first;
B second;

void setup() {
  // put your setup code here, to run once:

}

void loop() {
  // put your main code here, to run repeatedly:
  first.update();
  second.update();
}

But it doesnt work. Where is the mistake? I hope somebody can help me.

Best regards

Maurice W.

[Mxt]

Hi,

the friend declaration does only say that B has access to A's private and protected elements.

But an instance of B does not know in which instance of A it should change the value of state. You could have something like this

A a, b, c;

there are three objects of type A, which one do you want to change ?

One possible solution would be giving B's update a parameter

class B{
  public:
  void update(A& a){
    a.state = 1;
  }

};

and using it this way

void loop() {
  // put your main code here, to run repeatedly:
  first.update();
  second.update(first);
}

Much better C++ would be giving A a changeState method, instead of using friend.