1. COP 3330 Object-oriented Programming in C++
Polymorphism
Spring 2019

2. A Motivational Example
At FSU, we may have many sub-categories of Students
Undergrads, graduates, online, part-time, ……
Thus, with many different types of grade reports for different type of students
It would be great to store one aray of students, and print out ALL grade reports with a loop
Student list[42000];
for (int j = 0; j < size; j++)
list[j].gradeReport();

3. Problem The items in the array are base-class Student objects
Base class objects do not know about subtypes (Grads, Undergrads, or others)
Everything in an array needs to be the same type and size
The gradeReport() function being called is the Student class version
However, we want to call the Grad version for graduate students and Undergrad version for undergraduate students
What about creating separate arrays for different student types?
Not realistic if there are many subtypes

4. A Base Class Pointer Property
We can get around the first problem through a special property of inheritance
Normally, a pointer can only point to one type
However, there is a special rule for inheritance
A pointer to a base class type can be pointed at an object derived from the base class
Similarly, a base class reference variable can refer to an object derived from that base class
Examples
Student s; Grad g; Undergrad u;
Student *sp1, *sp2, *sp3;
sp1 = &s; // pointing at Student object
sp2 = &g; // pointing at Grad object
sp3 = &u; // pointing at Undergrad object

5. Heterogeneous List A single array of pointers to a base class Example
These pointers can point to ANY objects derived from that base
Thus, we have created a list of various types of objects
Without breaking the array rules!
All pointers have the same type—a pointer to the base class
All pointers have the same size
Example
Student *list[42000];
Student s; Grad g; Undergrad u;
list[0] = &g;
list[1] = &u;
list[2] = new Grad;
list[3] = &s; …

6. Virtual Functions
The heterogenous list solves the first problem. How about the second problem?
Which version of gradeReport() will be run?
for (j = 0; j < size; j++)
list[j]->gradeReport();
This still calls the Student version of gradeReport due to a concept known as binding
list[j].gradeReport is bound to the Student version at compile time
Compiler cannot guess the version of objects at runtime
If we want to run the correct version at runtime, we need to achieve late (dynamic) binding

7. Virtual Functions The keyword virtual will do the trick
To override a base class function when it is called through a pointer, declare the function to be virtual
class Student {
public:
virtual void gradeReport(); };
Now when gradeReport()is called through a base-class pointer, the program will run the appropriate version (late binding)

8. Example
Student s; Grad g; Undergrad u; Student *sp1, *sp2, *sp3; sp1 = &s; // pointing at Student object sp2 = &g; // pointing at Grad object sp3 = &u; // pointing at Undergrad object sp1->gradeReport(); // runs Student’s version sp2->gradeReport(); // runs Grad’s version sp3->gradeReport(); // runs Undergrad’s version

9. Example
With the heterogeneous list, and the use of virtual functions, we can put all Students in one list, and print all grade reports:
for (int i = 0; i < size; i++)
list[i]->GradeReport();

10. Pure Virtual Function
Suppose you do not want to do anything for the Student’s version of gradeReport()
You can omit the definition
virtual void gradeReport()=0;
A virtual function without a definition is a pure virtual function
has no function definition in .cpp file, only declaration in .h file
For instance, class Student might not have enough information for the full grade reports -- we need to know what kind of student (i.e. which subtype)

11. Abstract Class
Any class that has at least one pure virtual function is an abstract class
An abstract class cannot be instantiated
Abstract classes are generally used as base classes
They are intended to be a place to declare data and functions common to classes derived from them
Abstract class can still be used to build pointers, to take advantage of virtual functions
Example abstract class Shape
Shape s; // illegal
Shape *sptr; // legal

12. virtual void printCheck()=0
Code Review: Employee
Employee
virtual void printCheck()=0
Temporary
void printCheck()
Permanent
void printCheck() = 0
Hourly
Salaried

13. Questions