CODEWITHSUNDEEP
c++
Vishnu CS
Vishnu CS

Reputation: 851

Accessing a class variable in another class shows a different behavior in C++

I am a newbie in C++. Here in my case I have created two classes, class One and class Two. _x is declared as a private member variable of class One. So here I am trying to learn different methods to access this private variable(_x) from any other class(Here in this case it is class Two). This is my scenario basically. So one method I tried is ,making class Two as a friend class of class One and for accessing this variable, I have used directValOfOne function to print the value of this variable and another one I tried was passing the address of the class One instance in another member function (getDataTwo) of class Two. Everything works well until here.

As you can see, I have initialized the _x using a parameterized constructor and printing this variable gives me the value as 10 using both the methods. Again I modified this variable using a setter function (setDataOne) to 220 and when I print it using the getDataTwo function it prints the value as 220. But when I tried to print the value using directValOfOne, it still showing the old initialized value (10) which I am really confused. I have printed the address of class One object in this directValOfOne funtion and it shows same. If so then why the new value(220) is not updated here.

Source code is placed below.

#include <iostream>

using namespace std;

class One{
    int _x;
public:
    One(int a):_x{a} {}
    //setters and getters
    void setDataOne(int a) { _x = a; }
    int getDataOne() const { return _x; }
    //print values
    void printOne() { cout<<"_x - "<<_x<<endl; }
    friend class Two;
};

class Two{
    One _a;
    int id_one = 0;
public:
    Two(One a):_a{a} {}
    //setters and getters
    void setDataTwo(int a) {  }
    int getDataTwo(One *obj) {
        id_one = obj->getDataOne();
        return id_one;
    }
    void printTwo() { printf("id_one = %d\n",id_one); }
    void directValOfOne() {
        printf("address  = %p\n",&_a);
        printf("_a._x = %d\n",_a._x);
    }
};

int main(){
    One one(10);
    Two two(one);
    one.printOne();
    two.getDataTwo(&one);
    two.printTwo();
    two.directValOfOne();
    printf(" *********** \n");
    one.setDataOne(222);
    one.printOne();
    two.getDataTwo(&one);
    two.printTwo();
    two.directValOfOne();

    return 0;
}

and the console output is,

_x - 10
id_one = 10
address  = 006dfee4
_a._x = 10
 ***********
_x - 222
id_one = 222
address  = 006dfee4
_a._x = 10

As you can see the _a._x still prints the value 10. But actually the _x is updated to new value 220. Sorry if I explained it so detail. Just to give you an overview I detailed like this.

So my que is ,

Why it is not showing the updated value since the addresses are same?

Am I missing any important concept here ? If so please guide me to understand this problem.

Upvotes: 1

Views: 955

Answers (1)

Gal
Gal

Reputation: 504

The problem in your code is that you pass to Two an instance of class One by value. This means that Two stores a copy of the instance of One you pass to it. When you change the value of _x in one it does not affect the value of _x in _a (inside two).

In order for your code to work you need to pass a reference to One in Two's constructor and store this reference. Now every change to one will also affect _a.

Modified code:

class Two{
    One& _a;
    int id_one = 0;
public:
    Two(One& a):_a(a) {}
    // ...

Regarding the address issue, the address of _a will stay the same since there's no reason for it to change once initialized. If you'll print the address of one in your current code (before the suggested modifications) it should be in a different address than _a (since _a is a copy that's located somewhere else).

In the modified version of the code you should see the same address since it's the same object.

Upvotes: 3

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