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[Page 678 (continued)]

12.7. Software Engineering with Inheritance

In this section, we discuss the use of inheritance to customize existing software. When we use inheritance to create a new class from an existing one, the new class inherits the data members and member functions of the existing class, as described in Fig. 12.27. We can customize the new class to meet our needs by including additional members and by redefining base-class members. The derived-class programmer does this in C++ without accessing the base class's source code. The derived class must be able to link to the base class's object code. This powerful capability is attractive to independent software vendors (ISVs). ISVs can develop proprietary classes for sale or license and make these classes available to users in object-code format. Users then can derive new classes from these library classes rapidly and without accessing the ISVs' proprietary source code. All the ISVs need to supply with the object code are the header files.

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Sometimes it is difficult for students to appreciate the scope of problems faced by designers who work on large-scale software projects in industry. People experienced with such projects say that effective software reuse improves the software development process. Object-oriented programming facilitates software reuse, thus shortening development times and enhancing software quality.

The availability of substantial and useful class libraries delivers the maximum benefits of software reuse through inheritance. Just as shrink-wrapped software produced by independent software vendors became an explosive-growth industry with the arrival of the personal computer, so, too, interest in the creation and sale of class libraries is growing exponentially. Application designers build their applications with these libraries, and library designers are being rewarded by having their libraries included with the applications. The standard C++ libraries that are shipped with C++ compilers tend to be rather general purpose and limited in scope. However, there is massive worldwide commitment to the development of class libraries for a huge variety of applications arenas.

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Software Engineering Observation 12.9

At the design stage in an object-oriented system, the designer often determines that certain classes are closely related. The designer should "factor out" common attributes and behaviors and place these in a base class, then use inheritance to form derived classes, endowing them with capabilities beyond those inherited from the base class.

Software Engineering Observation 12.10

The creation of a derived class does not affect its base class's source code. Inheritance preserves the integrity of a base class.

Software Engineering Observation 12.11

Just as designers of non-object-oriented systems should avoid proliferation of functions, designers of object-oriented systems should avoid proliferation of classes. Proliferation of classes creates management problems and can hinder software reusability, because it becomes difficult for a client to locate the most appropriate class of a huge class library. The alternative is to create fewer classes that provide more substantial functionality, but such classes might provide too much functionality.

Performance Tip 12.3

If classes produced through inheritance are larger than they need to be (i.e., contain too much functionality), memory and processing resources might be wasted. Inherit from the class whose functionality is "closest" to what is needed.

Reading derived-class definitions can be confusing, because inherited members are not shown physically in the derived classes, but nevertheless are present. A similar problem exists when documenting derived-class members.

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