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Why Virtual Functions
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As stated at the start of this chapter, virtual functions in combination with derived types allow C++ to support run-time polymorphism Polymorphism is essential to object-oriented programming because it allows a generalized class to specify those functions that will be common to any derivative of that class, while allowing a derived class to specify the exact implementation of those functions In other words, the base class dictates the general interface that any object derived from that class will have, but it lets the derived class define the actual method This is why the phrase one interface, multiple methods is often used to describe polymorphism Part of the key to successfully applying polymorphism is understanding that base and derived classes form a hierarchy that moves from greater to lesser generalization (base to derived) Hence, when used correctly, the base class provides all elements that a derived class can use directly plus the basis for those functions that the derived class must implement on its own Having a consistent interface with multiple implementations is important because it helps the programmer handle increasingly complex programs For example, when you develop a program, all objects you derive from a particular base class are accessed in the same general way, even if the specific actions vary from one derived class to the next This means that you need to remember only one interface rather than several Further, the separation of interface and implementation allows the creation of class libraries, which can be provided by a third party If these libraries are implemented correctly, they provide a common interface that you can use to derive your own specific classes
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To get an idea of the power of the one interface, multiple methods concept, examine this short program It creates a base class called figure This class is used to store the dimensions of various two-dimensional objects and to compute their areas The function set_dim( ) is a standard member function because its operation is common to all derived classes However, show_area( ) is declared as virtual because the way the area of each object is computed varies The program uses figure to derive two specific classes, called square and triangle
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#include <iostream> using namespace std; class figure { protected: double x, y; public: void set_dim(double i, double j) { x = i; y = j; } virtual void show_area() { cout << "No area computation defined "; cout << "for this class\n"; } } ; class triangle : public figure { public: void show_area() { cout << "Triangle with height "; cout << x << " and base " << y; cout << " has an area of "; cout << x * 05 * y << "\n"; } }; class square : public figure { public: void show_area() { cout << "Square with dimensions "; cout << x << "x" << y; cout << " has an area of "; cout << x * y << "\n";
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} }; int main() { figure *p; /* create a pointer to base type */ triangle t; /* create objects of derived types */ square s; p = &t; p->set_dim(100, 50); p->show_area(); p = &s; p->set_dim(100, 50); p->show_area(); return 0; }
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As you can see by examining this program, the interface to both square and triangle is the same even though both provide their own methods for computing the area of each of their objects Given the declaration for figure, it is possible to derive a class called circle that computes the area of a circle given its radius To do so, you must create a new derived type that computes the area of a circle The power of virtual functions is based in the fact that you can easily derive a new type that shares the same common interface as other related objects For example, here is one way to do it:
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class circle : public figure { public: void show_area() { cout << "Circle with radius "; cout << x; cout << " has an area of "; cout << 314 * x * x; } } ;
Before trying to use circle, look closely at the definition of show_area( ) Notice that it uses only the value of x, which is assumed to hold the radius (Remember that the area of a circle is computed using the formula R2) However, the function set_dim( )
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