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on() =0; off() =0; record() =0; stop() =0; play() =0;
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and the implementations are the concrete derived classes below:
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class Panasonic : public VCR { public: void on(); void off(); void record(); void stop(); void play(); }; class Sony : public VCR { public: void on(); void off(); void record(); void stop(); void play(); }; class Mitsubishi : public VCR { public: void on(); void off(); void record(); void stop();
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CHAP. 12]
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COMPOSITION AND INHERITANCE
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void play(); };
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One important advantage of object-oriented systems is extensibility. This refers to the ease with which the system can be extended. In the example above, the VCR controller would be called extensible if it automatically works the same way on new VCRs that we might add in the future. The controller should not have to be modified when we extend our collection of VCRs, adding a Toshiba or replacing the Sony with an RCA. In the object-oriented programming, we imagine two distinct points of view of the system: the view of the consumer (i.e., the client or user) that shows what is to be done, and the view of the manufacturer (i.e., the server or implementor) that shows how it is to be done. The consumer sees only the abstract base class, while the manufacturer sees the concrete derived classes. The customer s actions are generally called operations, as opposed to the manufacturer s implementations of these actions which are called generally methods. In C++, the actions are the pure virtual functions, and the methods are their implementations in the concrete derived classes. In this context, the abstract base class (the user s view) is called the system interface, and the concrete derived classes (the implementor s view) are called the system implementation: The Two Views in an Object-Oriented Program The System Interface (user s view) shows what is done abstract base class operations pure virtual functions The System Implementation (implementor s view) shows how it is done concrete derived classes methods functions
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This dichotomy is most effective when we use pointers to objects, as in Example 12.13. Then we can exploit dynamic binding to make the system interface even more independent from the system implementation. Extensibility is facilitated by the fact that only the newly added methods need to be compiled. Review Questions
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12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9 12.10 12.11 12.12 What is the difference between composition and inheritance What is the difference between protected and private members How do the default constructors and destructors behave in an inheritance hierarchy What is a virtual member function What is a pure virtual member function What is a memory leak How can virtual destructors plug a memory leak What is an abstract base class What is a concrete derived class What is the difference between static binding and dynamic binding What is polymorphism How does polymorphism promote extensibility
COMPOSITION AND INHERITANCE
[CHAP. 12
12.13 What is wrong with the following definitions:
class X { protected: int a; }; class Y : public X { public: void set(X x, int c) { x.a = c; } };
Problems
12.1 12.2 Implement a Card class, a composite Hand class, and a composite Deck class for playing poker. Implement the following class hierarchy:
Shape
TwoDimensional
ThreeDimensional
Triangle
Rectangle
Circle
Cone
Cylinder
Sphere
Define and test a Name class whose objects looks like the diagram at the top of the next page. Then modify the Person class so that name has type Name instead of type string.
x last Tudor first Mary middle title Queen suffix I nick Bloody Mary Name
Answers to Review Questions
12.1 Composition of classes refers to using one class to declare members of another class. Inheritance refers to deriving a subclass from a base class.
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CHAP. 12]
COMPOSITION AND INHERITANCE
12.4 12.5
12.6 12.7 12.8 12.9 12.10
12.12 12.13
A private member is inaccessible from anywhere outside its class definition. A protected member is inaccessible from anywhere outside its class definition, with the exception that it is accessible from the definitions of derived classes. In an inheritance hierarchy, each default constructor invokes its parent s default constructor before it executes itself, and each destructor invokes its parent s destructor after it executes itself. The effect is that all the parent default constructors execute in top-down order, and all the parent destructors execute in bottom-up order. A virtual member function is a member function that can be overridden in a subclass. A pure virtual function is a virtual member function that cannot be called directly; only its overridden functions in derived classes can be called. A pure virtual function is identified by the initializer =0 at the end of its declaration. A memory leak is the loss of access to memory in a program due to the wrong destructor being invoked. See Example 12.12 on page 285. By declaring a base class destructor virtual, memory leaks as in Example 12.12 on page 285 can be prevented because after it is invoked its indicated subclass destructor(s) will also be invoked. An abstract base class is a base class which includes at least one pure virtual function. Abstract base classes cannot be instantiated. A concrete derived class is a subclass of an abstract base class that can be instantiated; i.e., one which contains no pure virtual functions. Static binding refers to the linking of a member function call to the function itself during compile time, in contrast to dynamic binding which postpones that linking until run time. Dynamic is possible in C++ by using virtual functions and by passing pointers to objects. Polymorphism refers to the run-time binding that occurs when pointers to objects are used in classes that have virtual functions. The expressions p->f() will invoke the functions f() that is defined in the object to which p points. However, that object could belong to any one of a series of subclasses, and the selection of subclass could be made at run time. If the base-class function is virtual, then the selection (the binding ) of which f() to invoke is made at run time. So the expression p->f() can take many forms. Polymorphism promotes extensibility by allowing new subclasses and methods to be added to a class hierarchy without having to modify application programs that already use the hierarchy s interface. The protected data member a can be accessed from the derived Y only if it is the member of the current object (i.e. only if it is this->a). Y cannot access x.a for any other object x.
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