Inheritance

1. Inheritance

2. Inheritance • Many objects have a hierarchical relationship • Examples: zoo, car/vehicle, card game, airline reservation system • Inheritance allows software design to take advantage of relationships, supporting reuse • Supports the IS-A relationship • what’s the HAS-A relationship?

3. Terminology • Base class/Parent class/Superclass • defines generic functionality • Derived class/Child class/Subclass • extends or specializes base class • inherits members of parent • may implement new members • may override members of parent

4. Syntax class Student : public Person {…} class Derived : public Base{…} • Derived class may override or reimplement a function implemented by base class • Public inheritance - public members remain public class Person { class Student:public Person{ … … void print(); void print(); } }

5. Function Invocation • Person – print, getName • Student – print, changeMajor Person p(…); Student s(…); s.getName(); p.print(); s.print(); p.changeMajor(); s.changeMajor();

6. Function Invocation • Person – print, getName • Student – print, changeMajor Person p(…); Student s(…); s.getName(); //Person::getName p.print(); //Person::print s.print(); //Student::print p.changeMajor(); //ERROR!!!!!!!! s.changeMajor(); //Student::changeMajor

7. More Syntax - Partial Overriding void Person::print() {…} void Student::print() {Person::print(); //Superclass::function(); … }

8. Protected • private members of parent not accessible to child class • protected members accessible only to derived classes • examples class classname {private: protected: public: }

9. Child Class Constructors • Subclass must create superclass • invoke superclass constructor from subclass constructor • use initializer list Student::Student(string newname, string newmajor) :Person(newname), major(newmajor) {…} Student::Student(string newname, string newmajor) :Person(newname) {major = newmajor;}

10. Static Binding Person* p = new Person(…); Student* s = new Student(…); p->print(); //calls Person::print() p = s; //OKAY p->print(); //calls Person::print() p->changeMajor(); //ERROR • Function called depends on declared type

11. Example Magazine Book Book library … LibraryItem *library[10]; library[0] = new Magazine; library[1] = new Book; … library[9] = new Book;

12. Dynamic Binding • May want to determine which function to call based on object contents • Use virtual functions class Person { virtual void print(); } class Student : public Person {virtual void print(); }

13. Dyanmic Binding Person* p = new Person(…); Student* s = new Student(…); p->print(); //calls Person::print() p = s; //OKAY p->print(); //calls Student::print() p->changeMajor(); //ERROR

14. Polymorphism • Many forms • A variable is polymorphic if it points to an object with 1 or more virtual functions

15. Casting Person* p; p = new Student(…); Student* s = dynamic_cast<Student*>(p); • Create new pointer of subclass type to point to object • Pointer (s in this case) will be NULL if cast was NOT successful • Can also use C-style cast

16. Abstract Classes • May want to defined base class which cannot be instantiated • Examples – bank account, car • Declare one or more functions as pure virtual virtual void print() = 0; virtual void func_name(…) = 0;

17. Summary • Statically declared variables • Static binding always applies (virtual functions don’t matter) • Superclass variable can hold object of subclass type • Cannot call derived class functions if reference to object is of base class • Dynamically declared variables • Dynamic binding applies • virtual functions called will depend on object type • Superclass variable can hold object of subclass type • Cannot call non-virtual derived class functions on variable of type base class • After dynamic cast, can call subclass functions

18. Example Class1 Class2:public Class1 1. f1 3. virtual f2 2. virtual f2 4. f3 Class1 c1; c2.f1(); Class2 c2; c1.f3(); c1.f1(); c1 = c2; c1.f2(); c1.f3(); c2.f2(); c2.f3();

19. Example Class1 Class2:public Class1 1. f1 3. virtual f2 2. virtual f2 4. f3 Class1 c1; c2.f1(); //1 Class2 c2; c1.f3(); //error c1.f1(); //1 c1 = c2; c1.f2(); //2 c1.f3(); //error c2.f2(); //3 c2.f3(); //4

20. Example Class1 Class2:public Class1 1. f1 3. virtual f2 2. virtual f2 4. f3 Class1 *c1_ptr = new Class2(); Class2 *c2_ptr = new Class2(); c1_ptr->f3(); c1_ptr->f2(); c2_ptr = dynamic_cast<Class2*>c1_ptr; c2_ptr->f3();

21. Example Class1 Class2:public Class1 1. f1 3. virtual f2 2. virtual f2 4. f3 Class1 *c1_ptr = new Class2(); Class2 *c2_ptr = new Class2(); c1_ptr->f3(); //error c1_ptr->f2(); //3 c2_ptr = dynamic_cast<Class2*>c1_ptr; c2_ptr->f3(); //4