C h a p t e r 1 7 : V i r t u a l F u n c t i o n s a n d P o l y m o r p h i s m
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// point to derived2 p = &d2; p->vfunc(); // use derived1's vfunc()
} return 0;
The program displays the following:
This is base's vfunc().
This is derived1's vfunc().
This is derived1's vfunc().
Pure Virtual Functions
As the examples in the preceding section illustrate, when a virtual function is not redefined by a derived class, the version defined in the base class will be used.
However, in many situations there can be no meaningful definition of a virtual function within a base class. For example, a base class may not be able to define an object sufficiently to allow a base-class virtual function to be created. Further, in some situations you will want to ensure that all derived classes override a virtual function. To handle these two cases, C++ supports the pure virtual function.
A pure virtual function is a virtual function that has no definition within the base class. To declare a pure virtual function, use this general form: virtual type func-name(parameter-list) = 0;
When a virtual function is made pure, any derived class must provide its own definition. If the derived class fails to override the pure virtual function, a compile-time error will result.
The following program contains a simple example of a pure virtual function. The base class, number, contains an integer called val, the function setval( ), and the pure virtual function show( ). The derived classes hextype, dectype, and octtype inherit
number
and redefine show( ) so that it outputs the value of val in each respective number base (that is, hexadecimal, decimal, or octal).
#include <iostream> using namespace std; class number {
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C + + : T h e C o m p l e t e R e f e r e n c e protected: int val; public: void setval(int i) { val = i; }
};
// show() is a pure virtual function virtual void show() = 0; class hextype : public number { public: void show() { cout << hex << val << "\n";
};
} class dectype : public number { public: void show() { cout << val << "\n";
};
} class octtype : public number { public: void show() { cout << oct << val << "\n";
};
} int main()
{ dectype d; hextype h; octtype o; d.setval(20); d.show(); // displays 20 - decimal h.setval(20); h.show(); // displays 14 - hexadecimal
