ECE 264 Object-Oriented Software Development Instructor: Dr. Honggang Wang Spring 2013 Lecture 18: More on inheritance and Polymorphism

Lecture outline Announcements / reminders  Monday Session(April 15) re-scheduled to April 17, Wednesday; Due April 19, Friday, by 5:00 pm  Project Demonstration: April 25 and April 30 Today  Review inheritance basics  Inheritance examples  Polymorphism 2/24/2016 ECE 264: Lecture 18 2

Reviewing inheritance Can take existing code base and  Produce specialized version Example: Square is a more specific Rectangle  Add extensions Example: Manager class we discussed last time adds extra data/functionality to Employee Existing class: base class New class: derived class  Reuses data/functions from base class Inherits (almost) everything  Access permissions for base class members set in base class Any class can be a base class  can have hierarchy Base class can have multiple derived classes 2/24/2016 ECE 264: Lecture 18 3

4 Constructors and Inheritance Default constructor for a base class is called automatically in the derived class constructor  Ex: Manager() calls Employee()  Will actually traverse inheritance hierarchy, starting at lowest class If a derived class needs the parameterized constructor of a base class, it must explicitly invoke it in an initialization list Manager::Manager(string theName, float thePayRate, bool isSalaried): Employee(theName, thePayRate) { salaried = isSalaried; } 2/24/2016

Inheritance: Manager methods How would you write the Manager pay function? Here’s where we encounter access, reuse issues Would like to:  Return payRate if salaried  Call Employee function if not salaried 2/24/2016 ECE 264: Lecture 18 5

Protected data Problem: Manager pay function can’t access private data in Employee class  Solution: third type of access specifier: protected Protected data in a class can be directly accessed by  Functions within that class  Functions within derived classes of that class Still effectively private data to outside world 2/24/2016 ECE 264: Lecture 18 6

Solution: Manager pay function float Manager::pay(float hrsWorked) { if (salaried) return payRate; else return Employee::pay(hrsWorked); } If Employee data declared protected, not private, can directly access it in Manager function Explicitly call Employee version of pay function 2/24/2016 ECE 264: Lecture 18 7

Inheritance syntax class BClass { protected: int var1; private: int var2; public: BClass(); BClass(int v1, int v2); int sum(); }; class DClass : public BClass { private: int var3; public: DClass(); Dclass(int v1, int v2, int v3); int sum3(); }; DClass inherits from BClass  Has same data members, functions  Can add additional data  May not have access protected data are accessible to derived classes  Still private to outside world 2/24/2016 ECE 264: Lecture 18 8

Inheritance example class BClass { protected: int var1; private: int var2; public: BClass(); BClass(int v1, int v2); int sum(); }; class DClass : public BClass { private: int var3; public: DClass(); Dclass(int v1, int v2, int v3); int sum3(); }; What statements in the program below cause errors? int main() { BClass b1(2,3); DClass d1(3,4,5); int a = b1.sum(); int b = d1.sum(); int c = d1.var1; int d = d1.sum3(); int e = b1.sum3(); return 0; } 2/24/2016 ECE 264: Lecture 18 9

Polymorphism Polymorphism: Code/operations behave differently in different contexts  One example: operator overloading  Inheritance-based polymorphism in form of virtual functions Add virtual to function declaration in.h file Why use virtual functions?  One benefit of inheritance: code reuse  Another benefit: Can write generic code that works for (hopefully) many specific cases Take advantage of fact that derived class “is an” object of base class type (with extra/more specific functions) Virtual functions enable this second benefit 2/24/2016 ECE 264: Lecture 18 10

Virtual functions For example, a base class Animal could have a virtual function eat.  Derived class Fish would implement eat() differently than derived class Wolf  but you can invoke eat() on any class instance referred to as Animal, and get the eat() behavior of the specific derived class  This allows a programmer to process a list of objects of class Animal, telling each in turn to eat (by calling eat()). 2/24/2016 ECE 264: Lecture 18 11

Static binding All methods are, by default, non-virtual methods. Binding of method call is determined by static type of calling object. Example: Employee e1(“John Smith”, 20); Manager m1(“Bob Jones”, 1500, true); e1 = m1; e1.pay(40);//Calls pay() defined in //Employee ECE 264: Lecture /24/2016

Object pointers & inheritance Dynamic binding: determine type of object at runtime With inheritance, pointer to base class supports objects of:  Base class type  Derived class type Pointers also support dynamic binding!  Allows us to write general code using base class pointers 2/24/2016 ECE 264: Lecture 18 13

Static vs. dynamic binding Methods default to non-virtual  type of object determines method called With virtual method, can use dynamic binding if using pointers or references Example: Employee e1(“John Smith”, 20; Manager m1(“Bob Jones”, 1500, true); Employee *ePtr; e1 = m1; ePtr = &m1; e1.pay(40);// Calls pay() defined in // Employee ePtr->pay(40);// Calls pay() defined in // Manager ECE 264: Lecture /24/2016

References and virtual methods Remember: passing arguments by reference essentially passes pointer  Can use virtual methods on references Example: float payAnyone(Employee &e, float h) { return e.pay(h); } int main() { Employee e1(“John Smith”, 20; Manager m1(“Bob Jones”, 1500, true); payAnyone(e1, 40);// Calls pay() defined in // Employee payAnyone(m1, 40);// Calls pay() defined in // Manager return 0; } 2/24/2016 ECE 264: Lecture 18 15

Virtual functions and member functions Calling one member function inside another implicitly uses pointers (and therefore dynamic binding)  If f1 is a function of class C Calling f1 in another class C function is equivalent to: this->f1(); Example: assume both Employee & Manager have virtual function printPay(float hoursWorked) void Employee::print(float hoursWorked) { cout << “Name: “ << name << endl; cout << “Pay rate: “ << payRate << endl; printPay(hoursWorked);// Uses dynamic // binding; will call // Manager version in // Manager objects } 2/24/2016 ECE 264: Lecture 18 16

Destructors and virtual functions If a class has virtual functions, the destructor should be virtual Often use pointers with dynamic allocation: Employee *ePtr; char eType = ‘e’; while (eType != ‘x’) { cin >> eType; if (eType == ‘e’) ePtr = new Employee(“John Smith”, 20); else if (eType == ‘m’) ePtr = new Manager(“Bob Jones”, 1500, true); if (ePtr != NULL) ePtr->print(40); delete ePtr;// Needs virtual destructors to ensure // either ~Employee() or ~Manager() is // called correctly ePtr = NULL; } 2/24/2016 ECE 264: Lecture 18 17

Example (functions in red are virtual) If we have: Employee e1(“Bob”,25); Manager m1(“Jim”, 40, true); Employee *ePtr; Manager *mPtr; e1 = m1; ePtr = &m1; mPtr = &m1; 2/24/2016 ECE 264: Lecture Employee # name : string # payRate : float + Employee(string theName, float thePayRate) + getName() : string + getPayRate() : float + print(float hoursWorked) + pay(float hoursWorked) : float + printPay(float hoursWorked) Manager # salaried : bool + Manager(string theName, float thePayRate, bool isSalaried) + isSalaried() : bool + pay(float hoursWorked) : float + printPay(float hoursWorked) Which statements are legal? Which function version gets called (for virtual functions)? a. e1.isSalaried(); b. m1.getName(); c. e1.pay(40); d. ePtr->pay(40); e. mPtr->print(40); f. e1.printPay(40);

Example solution If we have: Employee e1(“Bob”,25); Manager m1(“Jim”, 40, true); Employee* ePtr; Manager *mPtr; e1 = m1; ePtr = &m1; mPtr = &m1; 2/24/2016 ECE 264: Lecture Employee # string name # float payRate + Employee(string theName, float thePayRate) + string getName() + float getPayRate() + void print(float hoursWorked) + float pay(float hoursWorked) + void printPay(float hoursWorked) Manager # bool salaried + Manager(string theName, float thePayRate, bool isSalaried) + bool isSalaried() + float pay(float hoursWorked) + void printPay(float hoursWorked) Which statements are legal? Which function version gets called (for virtual functions)? a. e1.isSalaried();  ILLEGAL b. m1.getName();  calls Manager.getName() c. e1.pay(40);  calls Employee.pay(40) d. ePtr->pay(40);  calls Manager.pay(40) e. mPtr->print(40);  calls Employee.print(40), which calls Manager.printPay(40) f. e1.printPay(40);  calls Employee.printPay(40)

Exam 2 Average Score: 85 STD: 8 Solutions will be uploaded today 2/24/2016 ECE 264: Lecture 18 20

Final notes Next time  Abstract Classes Acknowledgements: this lecture borrows heavily from lecture slides provided with the following texts: Deitel & Deitel, C++ How to Program, 8 th ed. Etter & Ingber, Engineering Problem Solving with C++, 2 nd ed. 2/24/2016 ECE 264: Lecture 18 21