Virtual members

Virtual members

A virtual member is a member function that can be redefined in a derived class, while preserving its calling properties through references. The syntax for a function to become virtual is to precede its declaration with the virtual keyword:

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// virtual members #include <iostream> using namespace std; class Polygon { protected: int width, height; public: void set_values (int a, int b) { width=a; height=b; } virtual int area () { return 0; } }; class Rectangle: public Polygon { public: int area () { return width * height; } }; class Triangle: public Polygon { public: int area () { return (width * height / 2); } }; int main () { Rectangle rect; Triangle trgl; Polygon poly; Polygon * ppoly1 = &rect; Polygon * ppoly2 = &trgl; Polygon * ppoly3 = &poly; ppoly1->set_values (4,5); ppoly2->set_values (4,5); ppoly3->set_values (4,5); cout << ppoly1->area() << '\n'; cout << ppoly2->area() << '\n'; cout << ppoly3->area() << '\n'; return 0; }

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In this example, all three classes (Polygon, Rectangle and Triangle) have the same members: width, height, and functions set_values and area.

The member function area has been declared as virtual in the base class because it is later redefined in each of the derived classes. Non-virtual members can also be redefined in derived classes, but non-virtual members of derived classes cannot be accessed through a reference of the base class: i.e., if virtual is removed from the declaration of area in the example above, all three calls to area would return zero, because in all cases, the version of the base class would have been called instead.

Therefore, essentially, what the virtual keyword does is to allow a member of a derived class with the same name as one in the base class to be appropriately called from a pointer, and more precisely when the type of the pointer is a pointer to the base class that is pointing to an object of the derived class, as in the above example.

A class that declares or inherits a virtual function is called a polymorphic class.

Note that despite of the virtuality of one of its members, Polygon was a regular class, of which even an object was instantiated (poly), with its own definition of member area that always returns 0.