ECE 264 Object-Oriented Software Development Instructor: Dr. Honggang Wang Spring 2013 Lecture 17: Operator overloading and inheritance intro

Lecture outline Announcements / reminders  Lab 9 (last assignment) Wednesday Session (April 10) Due April 12, Friday by 5:00 pm Monday Session(April 15) Rescheduled to April 17, Wednesday; Due April 19, Friday, by 5:00 pm  Exam 2 review next week  Focus on your term project (great team!) Today  Review copy constructors  Operator overloading  Introduce inheritance ECE 264: Lecture 17 2

Copy constructors Used to initialize a newly declared variable from an existing variable  Not called for assignments  Example: Point p1(2,3), p3; Point p2 = p1;// calls copy constructor p3 = p2;// uses assignment Default behavior: shallow copy  Directly copy data members  Doesn’t work for arrays and pointers  Perform deep copy—allocate enough space for data, then copy the data (not the pointer) ECE 264: Lecture 17 3

Motivating operator overloading Recall earlier example Point p1(2,3), p3; Point p2 = p1;// calls copy constructor p3 = p2;// uses assignment We handled first case with copy constructors What about second case?  Uses assignment operator  How is that operator defined? ECE 264: Lecture 17 4

Overloading Overloading: giving a function more than one meaning  We’ve seen examples of overloading before Constructors: default vs. parameterized vs. copy  All have same name, but different arguments  We’ve even seen overloaded operators String concatenation: + +=  Works with strings, char*, single characters, etc. Input/output operators: >  Work with any built-in type Can define own operators that allow use of our classes in same manner as built-in type ECE 264: Lecture 17 5

Overloading Operators Definitions for overloaded operators (usually) included in class  Look just like functions  May be binary or unary  Example double operator –(const Point& p2);  Syntax: Keyword operator is used, followed by the actual operator “Argument” is (usually) an object reference of the same type const declarations are optional but often preferred  Only predefined operators may be overloaded. Exceptions:. ::.* ?: sizeof ECE 264: Lecture 17 6

Example: Overloaded Point operator //Distance Formula double Point::operator –(const Point& rhs) { double t1, t2, d; t1 = rhs.xCoord – xCoord; //(x2-x1) t2 = rhs.yCoord – yCoord; //(y2-y1) d = std::sqrt( std::pow(t1,2) + std::pow(t2,2) ); return d; } ECE 264: Lecture 17 7 xCoord and yCoord are provided by the calling object. To access the corresponding data from the object on the right hand side of the operator, use rhs.xCoord and rhs.yCoord

Forms of overloaded operators Member functions  Left hand side of binary operator (x, if operation is x + y) invokes operator Operator requires one argument (y)  Object used with unary operator (e.g., -x) invokes operator  Type of x must match the class Friend functions  Typically used when left hand side of binary operator does not match class For example, cout << x  cout (type ostream ) is on LHS Want operator to have access to data members of class ECE 264: Lecture 17 8

Friend Functions binary operators  friend function requires two arguments unary operator  friend function requires one argument Disadvantage  A friend function is NOT a member function  Friend functions violate a strict interpretation of object oriented principals (implementation is hidden)  Recommended for operator overloading only Particularly useful with stream operators > ECE 264: Lecture 17 9

Example: In class definition (.h file): friend complex operator +(complex c1, complex c2); In class implementatio (.cpp file) n: complex operator +(complex c1, complex c2) { complex temp; temp.real = c1.real + c2.real; temp.imag = c1.imag + c2.imag; return temp; } In client program: complex cA, cB, cC; cC = cA+cB; cC = cB; //this is ok, when + is a friend function ECE 264: Lecture 17 10

Inheritance Animal, mammal, dog, poodle - Inheritance. A poodle is-a dog, a dog is-a mammal, a mammal is- an animal. (refer to image.google.com) Food, dessert, pie, banana-cream - Inheritance. Pie is food. Dessert is food. Notice this is a tree structure - you could have many types of dessert, and many types of pies. ECE 264: Lecture 17 11

Motivating Inheritance Say we have two classes as part of a company’s payroll system  Employee models: general company employee who is paid hourly Pay = (pay rate) * (hours worked)  Manager models: a specific type of employee that may be salaried or hourly Pay = (pay rate) ECE 264: Lecture 17 12

Inheritance example: class diagram Employee - string name - float payRate + Employee() + Employee(string theName, float thePayRate) + getName() : string + getPayRate() : float + pay(float hrsWorked) : float Manager - string name - float payRate - bool salaried + Manager() + Manager(string theName, float thePayRate, bool isSalaried) + getName() : string + getPayRate() : float + isSalaried() : bool + setSalaried(bool sal) + pay(float hrsWorked) : float ECE 264: Lecture Functions/data in red completely redundant Functions in blue share some functionality Would like to reuse code wherever possible

Inheritance The creation of a new class from an existing class is called inheritance Promotes software reusability Terminology:  Base class is existing class  Derived class reuses data and functions from base class Can customize base class functions Can add functions to derived class Can have inheritance hierarchy ECE 264: Lecture 17 14

Inheritance and UML Inheritance models “is a” relationship  Example: a square is a rectangle with equal length sides Contrast with composition (“has a”)  Example: a rectangle has a point of origin ECE 264: Lecture Rectangle Square Point

Inheritance example: base class class Employee { private: string name; float payRate; public: Employee(); Employee(string n, float pr); string getName(); float getPayRate(); float pay(float hrsWorked); }; ECE 264: Lecture 17 16

Inheritance example: derived class class Manager : public Employee { private: bool salaried; public: Manager(); Manager(string theName, float thePayRate, bool isSalaried); bool isSalaried(); void setSalaried(bool sal); float pay(float hrsWorked); }; The notation above indicates that Manager inherits from Employee Only declare data/functions that are not shared ECE 264: Lecture 17 17

ECE 264: Lecture 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

Inheritance: Manager constructors How would you write the two constructors for the Manager class?  Manager(); Manager::Manager() { salaried = false;} Employee default constructor called automatically  Manager(string theName, float thePayRate, bool isSalaried); Manager::Manager(string theName, float thePayRate, bool isSalaried): Employee(theName, thePayRate) { salaried = isSalaried; } Explicitly call Employee parameterized constructor ECE 264: Lecture 17 19

Final notes Next time  Operator overloading examples  Inheritance  Exam review 2 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. ECE 264: Lecture 17 20