Copyright © 2006 Pearson Addison-Wesley. All rights reserved Learning Objectives  Inheritance  Virtual Function

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Introduction to Inheritance  General form of class is defined  Specialized class is then defined  Inherit properties (data member and function member) of general class  Add new data members  Add new functions or modify inherited functions

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Inheritance Basics  Base class (parent class)  "General" class from which others derive  Derived class (child class)  Automatically has base class’s:  Member variables  Member functions  Can then add additional member functions and variables and redefine existing functions

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Employee Example: Base Class  General concept of employee helpful!  All have names  All have social security numbers  Associated functions for these "basics" are similar among all employees  So "general" class can contain all these "things" about employees

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Employee Example (base class) Class Employee { private: string name; string SSN; public: Employee (string, string); void print_check(); }

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Employee Example (base class)  Look at function print_check(), different checks should be printed for different types of employees  So, let’s write it like this Employee:: void print_check() { cout << "This function is left for derived class"; }

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Employee Example (derived class)  A specific employee could be either a:  Monthly employee (salary is calculated monthly)  Hourly employee (salary is calculated hourly)  Each is a type of employee

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Public interitance  class DerivedClass : public BaseClass { … //public keyword (most commonly used) //specifies "publicly inherited“ from //Employee class //other keywords (protected, private) are //rarely used …… }

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Employee Example: Derived Class  Define Monthly_Employee and Hourly_Employee as the derived class of general (base) class Employee  From base class  Inherit all member variables  Inherit all member functions  Can then  Define new member functions  Redefine inherited functions

Copyright © 2006 Pearson Addison-Wesley. All rights reserved MonthlyEmployee Class class MonthlyEmployee : public Employee { private: double wage; //new data member public: MonthlyEmployee(string, string, double) // constructor void reset_wage(double); // new function member void print_check(); // redefine function member }

Copyright © 2006 Pearson Addison-Wesley. All rights reserved HourlyEmployee Class class HourlyEmployee : public Employee { private: double wageRate; //new data member double hours; //new data member public: HourlyEmployee(string, string, double, double) // constructor void reset_wage(double); // new function member void print_check(); // redefine function member }

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Constructors in Derived Classes  Base class constructors are NOT inherited in derived classes!  But they can be invoked within derived class constructor  Base class constructor must initialize all base class member variables  Derived needs to initialize all new data members

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Derived Class Constructor Example  MonthlyEmployee constructor: MonthlyEmployee::MonthlyEmployee(string Name, string Number, double wage): Employee(Name, Number) { this->wage = wage; }

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Derived Class Constructor Example  HourlyEmployee constructor: HourlyEmployee::HourlyEmployee(string Name, string Number, double wageRate, double hours): Employee(Name, Number) { this->wageRate = wageRate; this->hours = hours; }

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Function Redefinition In Derived Class MonthlyEmployee:: void print_check() { cout << wage; } HoulyEmployee:: void print_check() { cout << wageRate*hours; }

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Redefining vs. Overloading  Very different!  Redefining in derived class:  SAME parameter list  Essentially "re-writes" same function  Overloading:  Defined "new" function that takes different parameters  Overloaded functions must have different signatures

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Accessing Redefined Base Function  When function is redefined in derived class, base class’s definition is not "lost"  Can specify it’s use: Employee JaneE; HourlyEmployee SallyH; JaneE.printCheck(); //calls Employee’s printCheck function SallyH.printCheck(); //calls HourlyEmployee printCheck function SallyH.Employee::printCheck(); //Calls Employee’s printCheck function!  Not typical here, but useful sometimes

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Destructors in Derived Classes  Base class destructor handles data defined in base class  Derived class destructors handles derived class variables  When derived class destructor is invoked:  Automatically calls base class destructor!  So no need for explicit call

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Destructor Calling Order  Consider: class B derives from class A class C derives from class B  When object of class C goes out of scope:  Class C destructor called 1 st  Then class B destructor called  Finally class A destructor is called  Opposite of how constructors are called  Class A constructor is called  Class B constructor is called  Class C constructor is called

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Base Class Member Qualifier class DerivedClass : public BaseClass { private: public: protected: }

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Base Class Private Data  Derived class "inherits" private member variables  But still cannot directly access them  Not even through derived class member functions!  Private member variables can ONLY be accessed "by name" in member functions of the class they’re defined in

Copyright © 2006 Pearson Addison-Wesley. All rights reserved The protected: Qualifier  New classification of class members  Allows access "by name" in derived class  In other classes, these members act like private

Class member types Member (dada/function) type publicprotectedprivate Accessible by functions in Same class Yes Derived classes Yes No Other classes YesNo Copyright © 2006 Pearson Addison-Wesley. All rights reserved

Copyright © 2006 Pearson Addison-Wesley. All rights reserved "Is a" vs. "Has a" Relationships  Inheritance  Considered an "Is a" class relationship  e.g., a car is a vehicle  e.g., a computer is a machine  e.g., a dog is an animal  A class contains objects of another class as it’s member data  Considered a "Has a" class relationship  e.g., a car has an engine  e.g., a computer has a CPU  e.g., a dog has four legs

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Multiple Inheritance  Derived class can have more than one base class!  Syntax just includes all base classes separated by commas: class derived: public base1, base2 {…}  Dangerous undertaking!  Some believe this should never be used

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Virtual Function Basics  Polymorphism  Associating many meanings to one function  Virtual functions provide this capability  Polymorphism, virtual function, and dynamic (late) binding talk about the same thing

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Figures Example  Classes for several kinds of figures  Rectangle, Circle, Oval, Square, etc.  Each is an object of different class  Rectangle data: height, width, center point  Circle data: center point, radius  All derive from one parent-class: Figure  Each class needs different draw function Rectangle r; Circle c; r.draw(); //Calls Rectangle class’s draw c.draw(); //Calls Circle class’s draw

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Figures Example (continued)  How about this one? void function(Figure f) { f.draw(); }  We would like different draw functions being called for different type of f  Standard non-virtual function can not do this  Virtual functions are the answer

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Virtual: How?  Virtual function virtual return_type function_name(parameters)  Involves late binding (dynamic binding)  Tells compiler to "wait" until function is used in program  Decide which definition to use based on calling object at runtime

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Overriding  Virtual function definition changed in a derived class  We say it’s been "overridden"  Similar to “redefined” standard (non-virtual) functions  So, in derived class:  Virtual functions changed: overridden  Non-virtual functions changed: redefined  Seem same for the programmer, but treated differently by the compiler: early binding or late binding

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Pure Virtual Functions  Base class might not have "meaningful“ definition; It’s purpose solely for others to derive from  Recall class Figure  All real figures are objects of derived classes  Rectangles, circles, triangles, etc.  Class Figure has no idea how to draw!  Make it a pure virtual function: virtual void draw() = 0;  Pure virtual function  No definitions in based class  Must be overridden in derived classes

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Abstract Class  A class with one or more pure virtual functions is called an abstract class  An abstract class can only be used as a base class to derive other classes  We can not create objects of an abstract class, because it is not a complete class definition.