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lecture7

# lecture7 - Monday,April24 Polymorphism VirtualFunctions...

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Monday, April 24 th 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 . 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); ... private: string m_sName; int m_nAge; }; Polymorphism Any time you have a function that takes a  (reference to) a   superclass  as an  argument... void SayHi( Person &p ) { cout << “Hello “ << p.getName(); } main() { } Person c; SayHi( c ); You may also pass in a  subclass  to the function. Student s; SayHi( s ); class Student : public Person { public: // new stuff: int getStudentID(); private: // new stuff: int m_nStudentID; };   Person  Student

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Polymorphism Why is this?   Well a Student  IS a  Person.   Everything a Person can do, it  can do.   void SayHi( Person &p ) { cout << “Hello “ << p.getName(); } main() { } Person c; SayHi( c ); Student s; SayHi( s ); So if I can ask for a  Person’s  name with  getName , I can ask for a  Student’s   name with  getName  too!
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