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11.6 private ACCESS VERSES protected ACCESS
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The difference between private and pro tee ted class members is that subclasses can access protected members of a parent class but not private members. Since protected is more flexible, when would you want to make members private.3 The answer lies at the heart of the principle of information hiding: restrict access now to facilitate changes later. If you think you may want to modify the implementation of a data member in the future, then declaring it private will obviate the need to make any corollary changes in subclasses. Subclasses are independent of private data members.
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COMPOSITION AND INHERITANCE
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EXAMPLE 11.9 The person Class with protected and private Data Members Suppose could just add to replace it for now, we that we need to know whether people (i.e., Person objects) are high school graduates. We a pro tee ted data member like sex that stores either 0 or 1. But we might decide later with data member(s) that contain more detailed information about the person s education. So, set up a private data member hs to prevent derived classes from accessing it directly:
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class Person { public: Person(char* n="", int s=O, char* nat="U.S.A.") : name(n), sex(s), nationality(nat) { } // protectedl String name, nationality; Date dob, dod; // date of birth, date of death int sex; // 0 = female, 1 = male void setHSgraduate(int g) { hs \= g; } int isHSgraduate() { return hs; } private: int hs; // = 1 if high school graduate 1;
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We include pro tee ted access functions to allow subclasses to access the information. If we do later replace the hs data member with something else, we need only modify the implementations of these two access functions without affecting any subclasses. 11.7 virtual FUNCTIONS AND POLYMORPHISM
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One of the most powerful features of C++ is that it allows objects of different types to respond differently to the same function call. This is called polymorphism and it is achieved by means of virtual functions. Polymorphism is rendered possible by the fact that a pointer to a base class instance may also point to any subclass instance:
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class X {
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class Y : public X { // . . .
// Y is a subclass if X
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x* p;
y y; p = &lr;
// p is a pointer to objects of base class X // p can also point to objects of subclass Y
So if p has type x* ( pointer to type x ), then p can also point to any object whose type is a subclass of x. However, even when p is pointing to an instance of a subclass Y, its type is still x* . So an expression like p- >f ( > would invoke the function f ( > defined in the base class.
COMPOSITION AND INHERITANCE
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Recall that p- > f ( ) is an alternate notation for *p . f ( ) . This invokes the member function f ( > of the object to which p points. But what if p is actually pointing to an object y of a subclass of the class to which p points, and what if that subclass Y has its own overriding version of f ( ) Which f ( > gets executed: X: : f ( ) or Y : : f ( ) The answer is that p- >f ( ) will execute x : : f ( ) because p had type X* . The fact that p happens to be pointing at that moment to an instance of subclass Y is irrelevant; it s the statically defined type x * of p that normally determines its behavior.
EXAMPLE 11.10 Using virtual Functions
This demo program declares p to be a pointer to objects of the base class X. First it assigns p to point to an instance x of class X. Then it assigns p to point to an instance y of the derived class Y.
class X { public: void f() >;
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