problem_20.md

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Problem 20 - I want a class with no objects

2017-10-26

class Student {
    public:
        virtual float fees() const;
};

class RegularStudent: public Student {
    public:
        float fees() const override;    // Regular student frees
}

class CoopStudent: public Student {
    public:
        float fees() const override;    // Co-op student frees
}

What should Student::fees do?

Don't know - every Student should either be RegularStudent or CoopStudent.

Solution: explicitly give Student::fees no implementation.

class Student {
    public:
        virtual float fees() const = 0;
};

Student is an abstract class
fees() is a pure virtual method

Abstract classes cannot be instantiated:

Student s;  // ERROR
Student *s = new Student;   // ERROR

Can point to instances of concrete classes (non-abstract classes):

Student *s = new RegularStudent;

Subclasses of abstract classes are abstract are abstract, unless they implement every pure virtual method in the superclass.

Abstract classes

• used to organize concrete classes
• can contain common fields and default methods (not need to be overrided)

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