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[Page 707 (continued)]

13.4. Type Fields and switch Statements

One way to determine the type of an object that is incorporated in a larger program is to use a switch statement. This allows us to distinguish among object types, then invoke an appropriate action for a particular object. For example, in a hierarchy of shapes in which each shape object has a shapeType attribute, a switch statement could check the object's shapeType to determine which print function to call.

[Page 708]

However, using switch logic exposes programs to a variety of potential problems. For example, the programmer might forget to include a type test when one is warranted, or might forget to test all possible cases in a switch statement. When modifying a switch-based system by adding new types, the programmer might forget to insert the new cases in all relevant switch statements. Every addition or deletion of a class requires the modification of every switch statement in the system; tracking these statements down can be time consuming and error prone.

Software Engineering Observation 13.6

Polymorphic programming can eliminate the need for unnecessary switch logic. By using the C++ polymorphism mechanism to perform the equivalent logic, programmers can avoid the kinds of errors typically associated with switch logic.

Software Engineering Observation 13.7

An interesting consequence of using polymorphism is that programs take on a simplified appearance. They contain less branching logic and more simple, sequential code. This simplification facilitates testing, debugging and program maintenance.

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