c++ doubt

vivek.m

#include <iostream>
using namespace std;

class B
{
public:
    B(int r){a = r;}
    int getValue(){return a;}
private:
    int a;
};

class A
{
public:
    A(int r){a = r;}
    int add(B& e2)
    {
        e2.a = 7;
        return (a + e2.a);
    }
    int getValue(){return a;}
private:
    int a;
};

int main()
{
    A e1(9);
    B e2(15);
    cout<<e1.getValue()<<"\n";
    cout<<e2.getValue()<<"\n";
    cout<<e1.add(e2)<<"\n";
    cout<<e1.getValue()<<"\n";
    cout<<e2.getValue()<<"\n";
    return 0;
}

Here's the result is obvious compiler error where e2 does not allow access to private member inside e1.

not able to figure out.. 😔

As i specified in below post
#-Link-Snipped-#
that the variable 'a' of class 'B' is private and can't be accessed directly outside the class. Hence you are getting compile time error.

To fix this either of the below approach can be taken:
1. Specify the access specifier for the variable 'a' as public (not a good practice).
2. Create a public getter/setter function that return/set the value of the variable 'a' and then use these functions to do operations outside class.

Here in class 'B' you have defined a public getter function 'getValue' to get value of variable 'a' outside the class 'B' but you are not using it. Also you have not defined any setter for variable 'a'.

Your class 'A' and 'B' should be modified as:

class B
{
public:
    B(int r){a = r;}
    int getValue(){return a;}
    void setValue(int r) {a = r;}
private:
    int a;
};

class A
{
public:
    A(int r){a = r;}
    int add(B& e2)
    {
        e2.setValue(7);
        return (a + e2.getValue());
    }
    int getValue(){return a;}
private:
    int a;
};

Let me know if you face issue with above changes.

-Pradeep

Hi Deesha,

I have got the answer. Let's go back to the definition of 'private' variable
A 'private' data member or member function can only be accessed by functions that are members of that class. It cannot be accessed by any function that is not a member function of that class.

note the use of word 'class' which means that 'private' is class private and NOT object private as both of us were assuming till now.

Probably, the rationale behind this is that any objects of that class can easily access that variable by simply using a get function for that variable. Moreover, in case of operator overloading, just look at the advantage of this:

class A
{
public:
    A(int _a){a = _a; b=0;}
    A(int _a, int _b){a = _a; b = _b;}
    int modify(A& obj2)
    {
        obj2.a = 707; //this no longer surprises me. :happy:
        return obj2.a;
    }
    bool operator== (const A& obj2)
    {
        if( a == obj2.a && b == obj2.b)
            return true;
        return false;
    }
    A& operator+ (const A& obj2)
    {
        a += obj2.a;
        b += obj2.b;
        return *this;
    }
    int getValueA(){return a;}
    int getValueB(){return b;}
private:
    int a, b;
};

void main()
{
    A obj1(2);
    A obj2(10, 18);
    if(obj1 == obj2)
        cout<<"Equal";

    cout<<"a = "<<obj1.getValueA()<<"\n";
    cout<<"b = "<<obj1.getValueB()<<"\n";
    obj1 = obj1+obj2;
    cout<<"a = "<<obj1.getValueA()<<"\n";
    cout<<"b = "<<obj1.getValueB()<<"\n";
}

Just look how in-convenient, not to mention the unnecessary function calls, it would have been to call getter function for each of the private variables inside operator overloading functions.

Also, making private vars as class private may also depend on internal implementation of private variables also.

More often than not, we are so much engrossed with public accessor/mutator that we use them even to access variables among objects!! At least, that's what I was doing for past so many years. 😒

You sure are on right track to learning C++. you may find #-Link-Snipped-# helpful.

Happy Learning and keep questioning (on CE )!