How could the object access its private data members from outside?

**hefese** · 11-12-2013

Hi,
I want to ask you that how does an object access its private data members in copy constructor.

The relevant part of the code:

Code:

C::C(const C &obj)
{
    x = obj.x;
    y = obj.y;
}

Normally the object1 called "obj" cannot access its private data members outside. But in this situation it can access. How can it be explained?

Here are the complete code:

Code:

#include <iostream>




using namespace std;




class C{
    public:
        C(int,int);
        C(const C &);
        int area();
    private:
        int x;
        int y;
};




C::C(int a,int b)
{
    x = a;
    y = b;
}




C::C(const C &obj)
{
    x = obj.x;
    y = obj.y;
}




int C::area () {
    return x*y;
}




int main()
{
    C object1(2,3);
    C object2(object1);




    cout << "Object 1 : " << object1.area() << endl;
    cout << "Object 2 : " << object2.area() << endl;




    return 0;
}

**laserlight** · 11-12-2013

Originally Posted by hefese

Normally the object1 called "obj" cannot access its private data members outside. But in this situation it can access. How can it be explained?

It is permitted because this access is done within a member function of the class. After all, if you are writing the core implementation of the class, then it is to be expected that you will access the implementation details directly.

By the way, you should use the initialiser list, e.g.,

Code:

C::C(const C &obj) : x(obj.x), y(obj.y) {}

But then you don't even need to declare the copy constructor here since the compiler generated one will suffice.

**Elysia** · 11-12-2013

Good advice: SourceForge.net: Do not remove parameter names - cpwiki

**hefese** · 11-13-2013

Originally Posted by laserlight

It is permitted because this access is done within a member function of the class.

I don't understand this point: This access is done within a member function of object2, not member function of object1. Because of this my mind is confusing.

After the process of object2(object1), the copy constructor of object2 is called, isnt it? So data members of object2 can be used by object2 in copy constructor, but how do data members of object1 in this scope(in object2 scope) can access its data members. This scope is outside for object1, isnt it?

Originally Posted by Elysia

Good advice: SourceForge.net: Do not remove parameter names - cpwiki

Sorry, I don't understand why you have shared this link.

**laserlight** · 11-13-2013

Originally Posted by hefese

This access is done within a member function of object2, not member function of object1.

Both object2 and object1 are instances of the same class, therefore there is no problem.

Originally Posted by hefese

After the process of object2(object1), the copy constructor of object2 is called, isnt it? So data members of object2 can use in copy constructor, but data members of object1 in this scope(in object2 scope) can access its data members. How?

Because it is permitted. It is as simple as that. The idea here is that access control is used to restrict access to implementation detail. Therefore, we are not concerned with the fact that the copy constructor of object2, not object1, is invoked here: either way, the copy constructor of the class is invoked, hence the private members of objects of the class can be accessed so as to implement the copy constructor.

**hefese** · 11-13-2013

I understand now, thank you for responding.

**Elysia** · 11-13-2013

Originally Posted by hefese

Sorry, I don't understand why you have shared this link.

Because you are doing this:

C(int,int);
C(const C &);

(i.e. removing the name of the parameters.)
This is exactly what the article is asking you not to do.

**hefese** · 11-20-2013

Okay, thanks for your reply.

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