access private members in inheritance

Question

I have a class A, which have a field val declared as private. I want to declare a class B, that inherit from A and have an access to val. Is there a way to do it on C++?

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Answer 1

Private members of a base class can only be accessed by base member functions (not derived classes). So you have no rights not even a chance to do so :)

class Base

• public: can be accessed by anybody
• private: can only be accessed by only base member functions (not derived classes)
• protected: can be accessed by both base member functions and derived classes

Answer 2

Well, if you have access to base class, you can declare class B as friend class. But as others explained it: because you can, it does not mean it's good idea. Use protected members, if you want derived classes to be able to access them.

Answer 3

You can access the private members of a base class say A through an inherited member function of A

#include<iostream>
using namespace std;

class A{
    int a;
    
    public:
    A(){};
    A(int val){a=val;};
    
    int get_a(){//To access a private variable it must be initialized here
        a=10;
        return a;
    }
};

class B: public A{
    int b;
    
    public:
    B(){};
    B(int val){b=val;};
    
    void get(){
        cout<<get_a();
    }
};

int main(){
    A ob1(2);
    cout<<ob1.get_a()<<endl;
    
    B ob2(4);
    ob2.get();
    
    return 0;
}

Answer 4

Quick answer: You don't. Thats what the protected key-word is for, which you want to use if you want to grant access to subclasses but no-one else.

private means that no-one has access to those variables, not even subclasses.

If you cannot change code in A at all, maybe there is a public/protected access method for that variable. Otherwise these variables are not meant to be accessed from subclasses and only hacks can help (which I don't encourage!).

Answer 5

You need to define it as protected. Protected members are inherited to child classes but are not accessible from the outside world.

Answer 6

It is doable as describe in this Guru of the Week - GotW #76 - Uses and Abuses of Access Rights. But it's should be considered a last resort.