Lecture 7 - Polymorphism

# Lecture 7 - Polymorphism - Monday,January30th Polymorphism...

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Monday, January 30th  Polymorphism Virtual Functions Virtual Destructors Pure Virtual Functions

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Polymorphism Consider a function that accepts a  Person  as an  argument void LemonadeStand( Person &p) { cout << “Hello “ << p.getName(); cout << “How many cups of ”; cout << “lemonade do you want?”; } Can we also pass a  Student  as a  parameter to it? void main(void) { Person p; LemonadeStand(p); } We know we can do this: void main(void) { Student s; LemonadeStand(s); } But can we do this?
Polymorphism Consider a function that accepts a  Person  as an  argument void LemonadeStand( Person &p) { cout << “Hello “ << p.getName(); cout << “How many cups of ”; cout << “lemonade do you want?”; } Can we also pass a  Student  as a  parameter to it? Person I’d like to buy some  lemonade. We only serve people.  Are you a person? Yes. I’m a person.  I have a  name and everything. class Person { public: string getName(void); ... private: string m_sName; int m_nAge; }; Ok.  How many cups of  lemonade would you  like? Mom. I think that’s  Lucy Lu! Shhhh.

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Student Polymorphism Consider a function that accepts a  Person  as an  argument void LemonadeStand( Person &p) { cout << “Hello “ << p.getName(); cout << “How many cups of ”; cout << “lemonade do you want?”; } Can we also pass a  Student  as a  parameter to it? I’d like to buy some  lemonade. We only serve people.  Are you a person? Prove it to us! class Student : public Person { public: // new stuff: int getStudentID(); private: // new stuff: int m_nStudentID; }; Hmm. I’m a student but  as far as I know, all  students are people!  Well, you can see by my  class declaration  that all  students  are just a more  specific sub-class of  people . public Person But do you have a  name like a person? Since I’m based on a  Person , I  have every-thing a Person  has… Including a name!  Look! class Person { public: string getName(void); ... private: string m_sName; int m_nAge; Well, as long as you’re a  person, we can serve  you. Mom. It’s a  NERD!!! Uh huh. getName
class Person { public: string getName(void) { return m_name; } ... private: string m_name; int m_age; }; Polymorphism The idea behind  polymorphism  is that once I define a function that accepts a (reference or pointer to a)  Person void SayHi( Person &p) { cout << “Hello “ << p.getName(); } main() {      Person  p (“Eric”,18);     SayHi( p ); } float GPA = 1.6; Student s (“David”,19,  GPA ); SayHi( s ); class Student : public Person { public: // new stuff: int getGPA(); private: // new stuff: float m_gpa; Not only can I pass  Person   variables  to that class… But I can also pass  any variable  that  was derived from a Person!

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Lecture 7 - Polymorphism - Monday,January30th Polymorphism...

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