Copyright 2005, The Ohio State University 1 Inheritance What is Inheritance? Overriding vs. Overloading Access Control –Inheritance Types Virtual Functions Abstract Classes

Copyright 2005, The Ohio State University 2 What is Inheritance ? A method to form new classes using the ones that are already defined Defines an hierarchy Defines a is-a relationship –Student is-a Person –Apple is-a Fruit –Circle is-a Shape Also known as Generalization –Person is an abstraction/generalization of Student –Fruit is a generalization of Apple Derived Class vs. Base Class

Copyright 2005, The Ohio State University 3 Derived Classes Also known as Sub classes Derived class inherits attributes and behavior from Base class => Inheritance Manager is-a Employee class Employee { string name; Date hiring_date; int emp_no; … } class Manager : public Employee { // name, hiring_date, emp_no are inherited int level; Employee *group; … } Employee Manager Base class Derived class Inheritance Type

Copyright 2005, The Ohio State University 4 Examples Rectangle Triangle Polygon Point Circle3D-Point Sphere Mammal Pet Cat

Copyright 2005, The Ohio State University 5 Derived class is generally bigger than Base class –Adds extra attributes and behavior => Specialization –Ex: class Bank_Account - account #, owner, balance class Saving_Account : public Bank_Account - interest rate, interest earned Manager can be used wherever Employee can be used void foo (Manager mm, Employee ee) { Employee * pe = &mm; // ok: every Manager is an Employee Manager * pm = ⅇ // error: Employee need not be a Manager pm->level = 2; // not possible: pm does not know about level pm = static_cast (pe); // ok: pe points to a Manager pm->level = 2; // ok ! } Base Class pointer can point to a Derived Class but not vice versa !!

Copyright 2005, The Ohio State University 6 Derived Classes can choose to modify the behavior of inherited methods  Overriding In case of Specialization, modification can be necessary class Bank_Account { private: string address; int account_no; float balance; public: string name; void print () const; … } class Savings_Acc : public Bank_Account { private: float interest_rate, interest_earned; Date balance_date; public: void print () const { Bank_Account::print ( ); cout << interest_rate … } … } Overriding

Copyright 2005, The Ohio State University 7 Overriding vs. Overloading Overriding operates across scopes Overloading operates within a scope class Point { int x, y; public: void update (int xx, int yy); void update (int xx); void update (int yy); … } class Circle : public Point { int radius; public: update ( int xx, int yy, int rr ); … } Overriding Overloading

Copyright 2005, The Ohio State University 8 Access Control on members Access control of inherited members in derived classes are governed by: 1.Access control of members 2.Inheritance Type Access control of members –Derived class can access public and protected members of base class –protected: Acts as public to derived classes and private to others

Copyright 2005, The Ohio State University 9 Inheritance Types privateprotectedpublic private--- protectedprivateprotected publicprivateprotectedpublic Type of Inheritance Inheritance type defines the access control in derived class No derived class can access private of A In B_priv, public and protected parts of A are private In B_prot, public and protected parts of A are protected In B_public, public parts of A are public and protected parts of A are protected A B_priv Access control in A Access control in B B_prot B_public

Copyright 2005, The Ohio State University 10 Virtual Functions (1) class Employee { // data members public: void print() const; … } class Manager : public Employee { // data members public: void print() const; … } void global_print ( Employee *pe) { pe->print(); } Which print() is invoked??? Given a pointer Base_class *, to which derived type does the object pointed to belong?

Copyright 2005, The Ohio State University 11 Virtual Functions (2) Intelligent way! - Type Fields class Employee { char type; Employee() : type(‘E’) { … } // data members public: void print() const; … } class Manager : public Employee { Manager() { type = ‘M’; } // data members public: void print() const; … } void global_print ( Employee *pe) { if (pe->type == ‘E’) // Print Employee else // Print Manager }

Copyright 2005, The Ohio State University 12 Virtual Functions (3) Type Fields is smart method but it suffers from following: –Cumbersome –Difficult to manage –Difficult to extend Elegant way of doing it is by using Virtual Functions If a function is defined in base and derived classes and if it is virtual, then given a pointer we can unambiguously (at run time) determine which function needs to be invoked Since ambiguity is resolved at run time, it is known as late Binding or Dynamic Binding

Copyright 2005, The Ohio State University 13 Virtual Functions (4) class Foo { public: void f() { cout << "Foo::f()"; } virtual void g() { cout << "Foo::g()"; } class Bar : public foo { public: void f() { cout << “Bar::f()"; } virtual void g() { cout << “Bar::g()"; } Foo foo; Bar bar; Foo *baz = &bar ; Bar *quux = &bar ; foo.f(); // "Foo::f()" foo.g(); // "Foo::g()" bar.f(); // "Bar::f()" bar.g(); // "Bar::g()" // So far everything we would expect... baz->f(); // "Foo::f()" baz->g(); // "Bar::g()" quux->f(); // "Bar::f()" quux->g(); // "Bar::g()"

Copyright 2005, The Ohio State University 14 Abstract Classes (1) Not all classes are concrete They might represent some abstract concept Rectangle Triangle Shape Class shape can define abstract methods like draw and rotate which should be overridden in derived classes class Shape {// intelligent but inelegant public: virtual void draw () { cout << “error: draw() not defined here.”; } virtual void rotate (int degrees) { cout << “error: rotate() not defined here.”; } }

Copyright 2005, The Ohio State University 15 Abstract Classes (2) class Shape {// elegant public: virtual void draw () = 0; // pure virtual function virtual void rotate (int degrees) = 0; // pure virtual function } Class with one or more pure virtual functions is abstract Abstract class can only be used as an interface or base class –i.e., can not instantiate an object of abstract class –Shape s; // silly: shapeless shape We can instantiate only a concrete class

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