Public, Private, Protected inheritance

Public inheritance

Public inheritance is by far the most commonly used type of inheritance. In fact, very rarely will you use the other types of inheritance, so your primary focus should be on understanding this section. Fortunately, public inheritance is also the easiest to understand. When you inherit a base class publicly, all members keep their original access specifications. Private members stay private, protected members stay protected, and public members stay public.

class Base

{

public: intm_nPublic;

private: intm_nPrivate;

protected: intm_nProtected;

};

class Pub: public Base

{

//  Public inheritance means:

//  m_nPublic stays public

//  m_nPrivate stays private

//  m_nProtected stays protected Pub()

{

//  The derived class always uses the immediate parent's class access specifications

//  Thus, Pub uses Base's access specifiers

m_nPublic = 1; // okay: anybody can access public members

m_nPrivate = 2; // not okay: derived classes can't access private members in the base class!

m_nProtected = 3; // okay: derived classes can access protected members

}

};

int main()

{

//  Outside access uses the access specifiers of the class being accessed.

//  In this case, the access specifiers of cPub. Because Pub has inherited publicly from Base,

//  no access specifiers have been changed.

Pub cPub;

cPub.m_nPublic = 1; // okay: anybody can access public members

cPub.m_nPrivate = 2; // not okay: can not access private members from outside class cPub.m_nProtected = 3; // not okay: can not access protected members from outside class

}

This is fairly straightforward. The things worth noting are:

1. Derived classes can not directly access private members of the base class.

2. The protected access specifier allows derived classes to directly access members of the base class while not exposing those members to the public.

3. The derived class uses access specifiers from the base class.

4. The outside uses access specifiers from the derived class.

To summarize in table form:

Private inheritance

With private inheritance, all members from the base class are inherited as private. This means private members stay private, and protected and public members become private. Note that this does not affect that way that the derived class accesses members inherited from its parent! It only affects the code trying to access those members through the derived class.

class Base

{

public: intm_nPublic; private: intm_nPrivate; protected: intm_nProtected; };

class Pri: private Base

{

//  Private inheritance means:

//  m_nPublic becomes private

//  m_nPrivate stays private

//  m_nProtected becomes private Pri()

{

//The derived class always uses the immediate parent's class access specifications

//Thus, Pub uses Base's access specifiers

m_nPublic = 1; // okay: anybody can access public members

m_nPrivate = 2; // not okay: derived classes can't access private members in the base class!

m_nProtected = 3; // okay: derived classes can access protected members

}

};

int main()

{

//  Outside access uses the access specifiers of the class being accessed.

//  Note that because Pri has inherited privately from Base,

//  all members of Base have become private when access through Pri. PricPri;

cPri.m_nPublic = 1; // not okay: m_nPublic is now a private member when accessed through Pri

cPri.m_nPrivate = 2; // not okay: can not access private members from outside class cPri.m_nProtected = 3; // not okay: m_nProtected is now a private member when accessed through Pri

//  However, we can still access Base members as normal through Base:

Base cBase;

cBase.m_nPublic = 1; // okay, m_nPublic is public

cBase.m_nPrivate = 2; // not okay, m_nPrivate is private cBase.m_nProtected = 3; // not okay, m_nProtected is protected

}

To summarize in table form:

Protected inheritance

Protected inheritance is the last method of inheritance. It is almost never used, except in very particular cases. With protected inheritance, the public and protected members become protected, and private members stay private.

To summarize in table form:

Protected inheritance is similar to private inheritance. However, classes derived from the derived class still have access to the public and protected members directly. The public (stuff outside the class) does not.