Using a function in a class to change the value of a variable in another class c++

Question

Is it possible to Use a function in a class to change the value of a variable in another class c++. I want to use the manger class to change the price in a events class or the price in the function.

#include <fstream>
#include <iostream>
#include <string>
#include <iomanip>
#include <cstdio>
#include <sstream>

using namespace std;

class Manager {
public:
    void Changevalue() {
        //Changes the value of a variable some where in events class.
    }
};

class Event {
    //variable
    void price() {
        int price;
    }
};

int main() {
    Manager a;
    Event b;
    a.Changevalue();// use to change the value of a variable in the events class

    return 0;
}

Answer 1

Yes.

1. If the variable in Event class is public static , you can directly access it

    void Changevalue() { Event::someVar = someValue; } 

2. If it is not static but public , you will need an object of Event of which you change the value

    void Changevalue(Event& evt) { evt.someVar = someValue; } 

3. If it is neither static nor public , you will need an object of Event and a public method in Event through which you change the value

    void Changevalue(Event& evt) { evt.ChangeSomeValue( someValue); //this is the best approach in OOP world } 

Where Event class will be like

class Event {

  pulbic:
     void ChangeSomeValue( someType someValue);
  //other code
};

Answer 2

You can add class Manager as a friend class of the class event. Just add a member to class event:

class Event
{
public:
  friend class Manager;
  int price;
  void price()
  {

  }

But the price should be a member of class event too, or you can not change the value of a variable in a function. Because it's a local variable.

Answer 3

You can likely pass the Event to the ChangeValue function as an argument. You could then use any getter or setter methods the class provides.

Answer 4

Have Manager take Event as an argument:

Event e;
Manager m(e);

Then, add a member function on Event that Manager can call to change its value.

Answer 5

I am making a few assumptions about what you are trying to do based on the current structure of your code. For example, I assume that you want to set variables of objects, and not of classes, because you declared objects.

#include <fstream>
#include <iostream>
#include <string>
#include <iomanip>
#include <cstdio>
#include <sstream>

using namespace std;

class Event {
    int price;
    public:
    void setPrice(int p) {
        price = p;
    }
};

class Manager {
    Event *event;
    public:
    Manager(Event * e) : event(e) {}
    void ChangeValue() {
        //Changes the value of a variable some where in events class.
        event->setPrice(5);
    }
};


int main() {
    Event b;
    Manager a(&b);
    a.ChangeValue();// use to change the value of a variable in the events class

    return 0;
}

Answer 6

Q: Is it possible to Use a function in a class to change the value of a variable in another class?

A: Sure. There are lots of ways. For example:

1) You can simply make "price" a public member of "Event" - so anybody can change it, at any time. This is usually Bad...

2) You can make Manager.ChangeValue() a friend of Event:

http://www.cprogramming.com/tutorial/friends.html

3) Or, arguably best, you can create a getter/setter method in "Event"

CAVEAT:

// Questionable
class Event {
public:
    void price() {
        int myPrice;  // The scope of this variable exists *ONLY INSIDE THE FUNCTION*
    }
};

// Better
class Event
private:
  int price;
public:
  int getPrice() { return price; }
  void setPrice (int p) { price = p; }