Inheritance

Inheritance is a form of software reuse where a new class is created to

– absorb an existing class’s data and behaviors, and

– enhance them with new capabilities

The new class, the derived class, inherits the members of the existing class, known as the base class

A direct base class is the base class from which a derived class explicitly inherits. An indirect base class is inherited from two or more levels up in the class hierarchy.

In single inheritance, a class is derived from one base class. With multiple inheritance, a derived class inherits from multiple base classes. public:: every object of a derived class is also an object of its base class

• Note, base-class objects are NOT objects of their derived classes. private:: is essentially an alternative to composition

• I.e., derived class members not accessible from outside

protected:: is rarely used

class Employee {

string givenName, familyName;

date hiringDate;

short department;

...

};

class Manager: public Employee {

set <Employee *> group;

short level;

...

}

Member functions of derived class cannot directly access private members of base class

• Example:–

Manager member functions in previous example cannot read manager’s own name! Because data members of a class are by default private

A base class’s protected members can be accessed by members and friends of the base class, and

members and friends of any class derived from the base class.

Derived-class member functions can refer to public and protected members of the base class. By simply using their names

is-a relationship:: inheritance

– e.g., derived class object, car, is an object of the base class vehicle

– e.g., derived class object, Manager, is an object of the base class Employee

– has-a relationship:: composition

– e.g., a TreeNode object has (i.e., contains) a member object of type string Base classes typically represent larger sets of objects than derived classes Example

– Base class: vehicle

• Includes cars, trucks, boats, bicycles, etc.

– Derived class: car

a smaller, more-specific subset of vehiclesI.e., base classes have more objects

• But fewer data and function members

• Derived classes have only subsets of the objects

• Hence the term subclass

• But a derived class has more data and function members