What is the difference between public, private, and protected inheritance in C++?

 

In C++, when you derive a class from a base class, you have three access specifiers for inheritance: public, private, and protected. These access specifiers determine how the members of the base class are inherited by the derived class. Let's explore the differences using examples:

1. Public Inheritance:
   • Public inheritance preserves the access levels of the base class members in the derived class.
   • Public members of the base class remain public in the derived class.
   • Protected members of the base class become protected in the derived class.
   • Private members of the base class are not accessible in the derived class.

Example:

cpp

class Base {
public:
    int publicMember;
protected:
    int protectedMember;
private:
    int privateMember;
};

class Derived : public Base {
    // Members from Base are inherited with access levels preserved.
};

int main() {
    Derived d;
    d.publicMember = 1;   // Accessible
    d.protectedMember = 2; // Accessible
    // d.privateMember = 3; // Not accessible

    return 0;
}

2. Private Inheritance:
   • Private inheritance makes all the members of the base class private in the derived class.
   • The public and protected members of the base class are inaccessible outside the base and derived classes.

Example:

cpp

class Base {
public:
    int publicMember;
protected:
    int protectedMember;
private:
    int privateMember;
};

class Derived : private Base {
    // All members from Base become private in Derived.
};

int main() {
    Derived d;
    // d.publicMember = 1;   // Not accessible
    // d.protectedMember = 2; // Not accessible
    // d.privateMember = 3;   // Not accessible

    return 0;
}

3. Protected Inheritance:
   • Protected inheritance makes all the members of the base class protected in the derived class.
   • The public members of the base class become protected in the derived class.

Example:

cpp

class Base {
public:
    int publicMember;
protected:
    int protectedMember;
private:
    int privateMember;
};

class Derived : protected Base {
    // All members from Base become protected in Derived.
};

int main() {
    Derived d;
    // d.publicMember = 1;   // Not accessible
    // d.protectedMember = 2; // Not accessible
    // d.privateMember = 3;   // Not accessible

    return 0;
}

Choose the appropriate inheritance specifier based on your design needs. Public inheritance is the most common and generally adheres to the "is-a" relationship. Private and protected inheritance are less common and used in specific cases where you want to achieve "has-a" or special access relationships.