C++ Primer, Fifth Edition

7.1. Defining Abstract Data Types

The Sales_item class that we used in Chapter 1 is an abstract data type. We use a Sales_item object by using its interface (i.e., the operations described in § 1.5.1 (p. 20)). We have no access to the data members stored in a Sales_item object. Indeed, we don’t even know what data members that class has.

Our Sales_data class (§ 2.6.1, p. 72) is not an abstract data type. It lets users of the class access its data members and forces users to write their own operations. To make Sales_data an abstract type, we need to define operations for users of Sales_data to use. Once Sales_data defines its own operations, we can encapsulate (that is, hide) its data members.