C++ : Access Specifier and its usage in Inheritence

There are 3 access specifiers for a class/struct/Union in C++. These access specifiers define
how the members of the class can be accessed. Of course, any member of a class is accessible within
that class(Inside any member function of that same class). Moving ahead to type of access specifiers,
they are:
Public - The members declared as Public are accessible from outside the Class through an object of the
class.
Protected - The members declared as Protected are accessible from outside the class BUT only in a
class derived from it.
Private - These members are only accessible from within the class. No outside Access is allowed.
An Source Code Example:

class MyClass

{

public:

    int a;

protected:

    int b;

private:

    int c;

};

int main()

{

    MyClass obj;

    obj.a = 10; //Allowed

    obj.b = 20; //Not Allowed, gives compiler error

    obj.c = 30; //Not Allowed, gives compiler error

}

Inheritance and Access Specifiers
Inheritance is C++ can be one of the following types:
· Private Inheritance
· Public Inheritance
· Protected inheritance
Here are the member access rules with respect to each of these:
First and most important rule Private members of a class are never accessible from
anywhere except the members of the same class.

Public Inheritance:

All Public members of the Base Class become Public Members of the derived class &
All Protected members of the Base Class become Protected Members of the
Derived Class.
i.e. No change in the Access of the members. The access rules we discussed before are further then
applied to these members.
Code Example:
class Base

{

public:

    int a;

protected:

    int b;

private:

    int c;

};

class Derived:public Base

{

    void doSomething()

    {

        a = 10; //Allowed

        b = 20; //Allowed

        c = 30; //Not Allowed, Compiler Error

    }

};

int main()

{

    Derived obj;

    obj.a = 10; //Allowed

    obj.b = 20; //Not Allowed, Compiler Error

    obj.c = 30; //Not Allowed, Compiler Error

}

Private Inheritance:
All Public members of the Base Class become Private Members of the Derived class
&A
ll Protected members of the Base Class become Private Members of the Derived
Class.
An code Example:



class Base

{

public:

    int a;

protected:

    int b;

private:

    int c;

};

class Derived:private Base //Not mentioning private is OK because for classes it defaults to private

{

    void doSomething()

    {

        a = 10; //Allowed

        b = 20; //Allowed

        c = 30; //Not Allowed, Compiler Error

    }

};

class Derived2:public Derived

{

    void doSomethingMore()

    {

        a = 10; //Not Allowed, Compiler Error, a is private member of Derived now

        b = 20; //Not Allowed, Compiler Error, b is private member of Derived now

        c = 30; //Not Allowed, Compiler Error

    }

};

int main()

{

    Derived obj;

    obj.a = 10; //Not Allowed, Compiler Error

    obj.b = 20; //Not Allowed, Compiler Error

    obj.c = 30; //Not Allowed, Compiler Error

}

Protected Inheritance:

All Public members of the Base Class become Protected Members of the derived
class &
All Protected members of the Base Class become Protected Members of the
Derived Class.
A Code Example:

class Base

{

public:

    int a;

protected:

    int b;

private:

    int c;

};

class Derived:protected Base

{

    void doSomething()

    {

        a = 10; //Allowed

        b = 20; //Allowed

        c = 30; //Not Allowed, Compiler Error

    }

};

class Derived2:public Derived

{

    void doSomethingMore()

    {

        a = 10; //Allowed, a is protected member inside Derived & Derived2 is public derivation from

        //Derived, a is now protected member of Derived2

        b = 20; //Allowed, b is protected member inside Derived & Derived2 is public derivation from

        //Derived, b is now protected member of Derived2

        c = 30; //Not Allowed, Compiler Error

    }

};

int main()

{

    Derived obj;

    obj.a = 10; //Not Allowed, Compiler Error

    obj.b = 20; //Not Allowed, Compiler Error

    obj.c = 30; //Not Allowed, Compiler Error

}

Remember the same access rules apply to the classes and members down the inheritance hierarchy.