Object-oriented Software in Ada 95

10 Inheritance
This chapter introduces the concept of inheritance in which an existing class can be specialized without
modifying the original class. By using this technique software re-use can become a practical consideration
when developing software. Thus a programmer can become a builder of software using previously
developed components.
10.1 Introduction
Inheritance is the ability to create a new class by using the methods and instance attributes from an existing class
in the creation of a new class. For example, a class Account that provides the methods deposit, withdraw,
balance, and statement can be used as the base for a new class that provides the ability to pay interest on the
outstanding balance in the account. The new class Interest_Account inherits all the methods and instance
attributes from the class Account and adds to these the new methods of Calc_Interest, Add_Interest,
and Set_Rate plus the instance attribute Accumulated_Interest. This is illustrated in Figure 10.1.
Class Account Class Interest_Account Actual components of
Class Interest_Account
Account
Deposit
Withdraw
Balance
Balance_Of
Interest_Account
Add_Interest
Calc_Interest
Set_Rate
Accumulated_Interest
Interest_Account
Deposit
Withdraw
Balance_Of
Add_Interest
Calc_Interest
Set_Rate
Balance_Of
Accumulated_Interest
Figure 10.1 components of Account and the derived class Interest_account.
Note:
The class Interest_Account has the same visibility of components in the base class Account as
would a client of the class. In particular, it has no access to the private instance attributes of the base
class. Thus methods in the class Interest_Account cannot access the base class instance
attribute Balance_Of.
However, for a class type to be fully extended it must be declared as tagged. If a class type is not tagged
then it can only be extended by adding new methods. New instance attributes may not be added.
The consequence of this is that an implementor of a class must explicitly declare the class record type
tagged if new classes are to be derived from it.
© M A Smith - May not be reproduced without permission