Software Essentials

181

Chapter 7

Behavioral Design

P

   for automatic type checking greatly

advanced the pace and breadth of soware development. Chapter 6

explored this capacity for reuse as supported via the inheritance construct.

is chapter continues the examination of inheritance with a focus on its

support for dynamic function selection, also known as polymorphism.

We begin by dening three forms of polymorphism used in soware and

then examine subtype polymorphism in depth.

To highlight an eective design using subtype polymorphism, we

examine abstract classes and the resulting support of heterogeneous col-

lections and ease of soware maintenance. In addition to small, intuitive

examples, we dissect code from a real-world soware tool, a disassembler,

to illustrate appropriate use of abstract classes. To understand process and

cost, and to expose readers to a truly elegant soware solution, we care-

fully explain and illustrate virtual function tables. Such an examination

is unusual in a text; more oen, background processes are dismissed as

tedious. However, in the author’s experience, this knowledge successfully

reinforces an understanding of polymorphism. To assess unavoidable, but

common, design diculties, the chapter closes by looking at type intro-

spection and specic language requirements.

CHAPTER OBJECTIVES

• Dene common forms of polymorphism

• Overloading, generics, and subtyping

• Contrast static and dynamic binding

182 ◾ Software Essentials

7.1 INHERITANCE FOR FUNCTIONALITY

In the previous chapter, we examined structural design with an empha-

sis on inheritance, the hallmark construct of object-oriented design. As

noted there, the mere structural design of inheritance can be mimicked

with composition. A composing class can gain access to all the public data

and functionality of the “parent” component. If access to protected data

and functionality is desired, then a wrapper class can be dened with the

sole purpose of opening up the protected data and functionality. What

then is so important about inheritance, the major design construct of

OOD?

The true power of inheritance is not structural reuse but behavioral

modification. By defining an interface in the base class and provid-

ing variant behavior in descendant classes, a class designer can pro-

vide quite a range of functionality, all maintainable under a uniform

interface. Heterogeneous collections and substitutability, two design

touchstones of extensible code, are feasible only under a common

interface.

We examine the design of class hierarchies and how functional-

ity (behavior) can vary within class hierarchies. OOPLs support poly-

morphism, that is, dynamic binding of function calls so that a single

object handle can provide access to varying behavior at runtime. What

are the costs and benets of polymorphism? How does one design eec-

tively using polymorphism? We address these questions and also con-

sider abstract classes as the means to standardize form within a class

hierarchy.

7.2 POLYMORPHISM

Looking at the Greek roots of the word “polymorphism,” we discern its

meaning: many (poly) and form (morph). Polymorphism in soware then

• Illustrate effective use of dynamic binding

• Examine the structure and utility of virtual function tables

• Dene and demonstrate the utility of abstract classes

• Analyze production code for disassembler

• Demonstrate the use and benets of heterogeneous collections

• Assess the cost of type identication

• Identify relevant OOD principles