12.3. Polymorphism In Depth

Polymorphism is the third and final characteristic that defines the object-oriented paradigm. Admittedly, polymorphism doesn't allow us to solve more problems, but it does allow us to solve them more compactly, elegantly, and clearly. If the following discussion seems confusing, remember that any alternative solution will involve either maintaining numerous variables, passing them as function arguments, duplicating large amounts of code, or creating, maintaining, passing, and using arrays of function pointers. Polymorphism is simple by comparison.

A key concept to remember throughout the following discussion is that the compiler uses different algorithms to locate polymorphic functions than it uses to find member variables and non-polymorphic functions. In the latter case, the compiler creates simple expressions. But, polymorphism requires pointers, tables, and address arithmetic.

Function Binding, Part 2

Programmers say that "normal" (i.e., non-polymorphic) functions have compile time binding, early binding, or static binding. These terms suggest that the compiler system can connect a function call to the function's machine instructions when programmers compile and link the program. Polymorphism delays function binding from compile-time to run-time. Computer scientists use several different terms to denote this kind of binding:

• Run-time binding
• Late binding
• Dynamic binding
• Dynamic dispatch (calling a function is sometimes called dispatching the function)

Drawing Shapes With Polymorphism

The first and most obvious change that we must make to the previous problem is to introduce a new superclass named Shape and to make the original classes subclasses of this new class. Figure 1 illustrates the UML class diagram that describes the class hierarchy for the restated problem:

Finally, we have all of the pieces in place to make the final step from a member function to a polymorphic function - that is, we are ready to switch on polymorphism, which we do by adding the virtual keyword to a function's prototype. The code appearing in Figure 2 illustrates making "virtual" functions and building an inheritance relationship.

class Shape
{
    public:
	virtual void draw();
};

class Circle : public Shape
{
    public:
	virtual void draw();
};

class Rectangle : public Shape
{
    public:
	virtual void draw();
};

class Triangle : public Shape
{
    public:
	virtual void draw();
};

The introduction of the Shape class, along with upcasting, allows us to simplify the code from the previous version of the shape drawing problem. The previous version required three separate variables, one for each kind of shape that the program can create and draw. Furthermore, if we added a new shape, an Ellipse, for example, the new shape would require adding a new variable to the program. Using inheritance and upcasting, we can have a Shape pointer variable that can point to any shape, including shapes that we create in the future.

cout << "C:\tCircle" << endl;
cout << "S:\tRectangle" << endl;
cout << "T:\tTriangle" << endl;

cout << "Please choose a shape: ";

char	choice;
cin >> choice;
cin.ignore();

Circle*		c;
Rectangle*	r;
Triangle*	t;
Shape*		s = nullptr;

switch (choice)
{
	case 'C' :
	case 'c' :
		s = new Circle();
		break;

	case 'S' :
	case 's' :
		s = new Rectangle();
		break;

	case 'T' :
	case 't' :
		s = new Triangle();
		break;
}

The most profound changes occur in the third block of code, where the shape is dawn. The previous version required a switch statement to determine which shape the user created so the program could call the appropriate draw function (see Drawing the shape). This version dispenses with the switch statement and replaces it with a polymorphic function call.

if (s != nullptr)
	s->draw();

Comparing Figure 4 above to Figure 3 in the original shape drawing problem gives a sense of polymorphism's computational power and potential. The two code fragments do the same thing, drawing the user's selected shape. The polymorphic version above does not bind to the correct draw function until the time of the function call. By the time the program makes the polymorphic function call, the user has selected which shape to draw, and the program has instantiated the appropriate object. The object determines which function to call: the program "sends the draw message to the object, and it responds appropriately depending on what kind of object it is."

Shape* s = new Circle(...);
 

void render(Shape* s) {...}
render(new Circle(...));

Let's return to the drawing problem introduced at the beginning of the chapter. With polymorphism in place, what must we do if the user wishes to add an Ellipse shape?

1. We must create a new Ellipse class, a subclass of Shape, similar to the Circle, Rectangle, and Triangle classes illustrated in Figures 1 and 2. The new Ellipse class must override the draw function. That draw function is virtual automatically, making the virtual keyword optional.
2. We must add a new case to the prompt and instantiation code shown in Figure 3.
3. The function call illustrated in Figure 4 requires no modification!

The polymorphic solution has the same three steps as our initial solution, so it may not seem like much of an improvement. But step 3 of the polymorphic solution differs substantially from the original. An actual program typically uses objects in many locations throughout the code - potentially spread across thousands of lines of code and hundreds of files. Without polymorphism, programmers must find and update every place the program uses the object, and the odds of missing one of these are quite high. Alternatively, the polymorphic solution doesn't require any changes.

Accessing Member Variables Without Downcasting

When programming with objects, it's common to "hide" variables by making them private class members. Programs access private variables through the class's public interface - its public member functions. For example, Figure 7 illustrates different kinds of employees with different member variables and ways of calculating their pay. If salaried employees are paid twice a month, their pay is salary / 24. A sales employee is a salaried employee by inheritance, earning a salary plus a commission. Logically, a sales employee's pay is salary / 24 + commission.

However, salary is private in the SalariedEmployee class, so a SalesEmployee object cannot access it. The proper object-oriented solution is to call a member function in the superclass. All three classes define a public member function named calc_pay() that calculates the pay for a specific kind of employee. The SalesEmployee calc_pay() function can call the SalariedEmployee calc_pay() function and add the commission. This scheme works well until we introduce an upcast.

If we upcast a SalesEmployee object to SalariedEmployee, it won't be able to access commission without a downcast. Without saying why we may want to do the upcast, let's note that we can achieve an elegant solution by making calc_pay() polymorphic. Compare the requirements for polymorphism to Figures 7 and 8, and verify that all polymorphism requirements are satisfied.

virtual double calc_pay()
{
	return salary / 24;
}

virtual double calc_pay()
{
	return SalariedEmployee::calc_pay() + commission;
}

Safe Downcasting With Polymorphism

Previously, we saw that an an upcast followed by an incorrect downcast was potentially meaningless and dangerous. Although polymorphism greatly reduces the need to downcast to access the member variables in an object, it is sometimes necessary. In these cases, polymorphism provides a way to ensure that the downcast is meaningful and safe. For example, suppose that sometimes, after an object is selected and instantiated (Figure 3 above), we need to downcast pointer s to a specific shape class. If the user selected a Circle, we don't want to downcast s to a Rectangle or a Triangle. How do we know to which shape to safely downcast s? Polymorphism gives us two different ways of solving this problem:

#include <iostream>
#include <typeinfo>
using namespace std;

int main()
{
    Shape*	s = new Circle;
    Circle*	c;
    Rectangle*	r;
    Triangle*	t;

    if (typeid(*s) == typeid(Circle))
	c = (Circle *)s;
    else if (typeid(*s) == typeid(Rectangle))
	r = (Rectangle *)s;
    else if (typeid(*s) == typeid(Triangle))
	t = (Triangle *)s;

    return 0;
}

#include <iostream>
using namespace std;

int main()
{
    Shape*	s = new Circle;
    Circle*	c;
    Rectangle*	r;
    Triangle*	t;

    if (dynamic_cast<Circle*>(s) != nullptr)
	c = dynamic_cast<Circle*>(s);
    else if (dynamic_cast<Rectangle*>(s) != nullptr)
	r = dynamic_cast<Rectangle*>(s);
    else if (dynamic_cast<Triangle*>(s) != nullptr)
	t = dynamic_cast<Triangle*>(s);

    return 0;
}