C++ – Does protected inheritance allow the derived class access the private members of its base class

c

I am really confused about private inheritance and protected inheritance.

1) in protected inheritance, the public and protected members become protected members in the derived class. In the private inheritance, everything is private. However, the derived class can never access the private members of the base class, is that right? The derived class can access the public and protected members in both cases. Is that right?

2) I noticed that the private members of the base class will never be touched by the derived class. So why are the private members inherited?

Best Answer

You are correct on point #1. Specifying private, protected or public when inheriting from a base class does not change anything access-wise on the derived class itself. Those access specifiers tell the compiler how to treat the base-class members when instances of the derived class are used elsewhere, or if the derived class happens to be used as a base class for other classes.

UPDATE: The following may help to illustrate the differences:

class Base
{
    private:   int base_pri;
    protected: int base_pro;
    public:    int base_pub;
};

For classes derived from base:

class With_Private_Base :   private Base   { void memberFn(); };
class With_Protected_Base : protected Base { void memberFn(); };
class With_Public_Base :    public Base    { void memberFn(); };

// this would be the same for all of the above 3 classes:
void With_PXXX_Base::memberFn()
{
    base_pri = 1; // error: `int Base::base_pri' is private
    base_pro = 1; // OK
    base_pub = 1; // OK
}

For classes derived from the 3 derived classes:

class A : public With_Private_Base { void memberFn(); }
void A::memberFn()
{
    base_pri = 1; // error: `int Base::base_pri' is private
    base_pro = 1; // error: `int Base::base_pro' is protected
    base_pub = 1; // error: `int Base::base_pub' is inaccessible
}

class B : public With_Protected_Base { void memberFn(); }
void B::memberFn()
{
    base_pri = 1; // error: `int Base::base_pri' is private
    base_pro = 1; // OK
    base_pub = 1; // OK
}

class C : public With_Public_Base { void memberFn(); }
void C::memberFn()
{
    base_pri = 1; // error: `int Base::base_pri' is private
    base_pro = 1; // OK
    base_pub = 1; // OK
}

External access to the first three derived classes:

void main()
{
    With_Private_Base   pri_base;
    pri_base.base_pri = 1; // error: `int Base::base_pri' is private
    pri_base.base_pro = 1; // error: `int Base::base_pro' is protected
    pri_base.base_pub = 1; // error: `int Base::base_pub' is inaccessible

    With_Protected_Base pro_base;
    pro_base.base_pri = 1; // error: `int Base::base_pri' is private
    pro_base.base_pro = 1; // error: `int Base::base_pro' is protected
    pro_base.base_pub = 1; // error: `int Base::base_pub' is inaccessible

    With_Public_Base    pub_base;
    pub_base.base_pri = 1; // error: `int Base::base_pri' is private
    pub_base.base_pro = 1; // error: `int Base::base_pro' is protected
    pub_base.base_pub = 1; // OK
}
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