1. CSCI 383 Object-Oriented Programming & Design Lecture 13 Martin van Bommel

2. Fall 2010CSCI 383 Lecture 13 M. van Bommel 2 Orthodox Canonical Class Form Most classes should be defined with (at least) these four important member functions Default constructor ClassName(); Copy constructor ClassName(const ClassName &copy); Assignment operator (overloaded operator) const ClassName &operator=(const ClassName &copy); Destructor ClassName::~ClassName();

3. Fall 2010CSCI 383 Lecture 13 M. van Bommel 3 Operator Overloading Most C++ operators can be overloaded Overloaded operators are implemented using the C++ keyword operator There are some characteristics of the operators that cannot be changed when overloading them New operator names cannot be introduced Built-in operators cannot be overriden Operator precedence cannot be changed Operator associativity cannot be changed The number of operator parameters cannot be changed

4. Fall 2010CSCI 383 Lecture 13 M. van Bommel 4 Operator Overloading Handout #2 Notice that the Timer class contains an overloaded addition operator, which is a binary operator timer1 + timer2 In the addition operator of Timer, the receiver of the message is the left (first) argument of “+” (timer1), and the argument is the second argument of “+” (timer2)

5. Fall 2010CSCI 383 Lecture 13 M. van Bommel 5 Global vs. Method Operators Instead, one could defined the previous overloaded operator as a global function rather than as a method Timer operator+(const Timer& t1, const Timer& t2) { … } Notice the difference between the global and the method definition: the first argument is missing in the method version…or is it? this serves as the first (left) argument of the operator this is a built-in, self-referential identifier; it contains the address of the receiving object In such a case, an operator call such as time1 + time2 is a message sent to the Timer instance time1

6. Fall 2010CSCI 383 Lecture 13 M. van Bommel 6 Operator Overloading Also notice the return value is a copy of a timer return result; Must ensure that copy constructor is included (if necessary) Copy is used for result of “+”, as in c = a + b The Timer class also contains an overloaded assignment operator, which is a binary operator a = b In the assignment operator, the receiver of the message is the left (first) argument of “=”, and the argument is the second argument of “=” This time the return value is a reference to a timer object used in string of assignments c = b = a

7. Fall 2010CSCI 383 Lecture 13 M. van Bommel 7 Global vs. Method Operators Overloaded operators can be called in either operator form or functional form time0 = time1 + time2; // operator form time0 = time1.operator+(time2) // functional form The same rules hold for unary operators. For example, consider the “not” operators (i.e., “!”) int operator!(const Timer& t); // as global function int Timer::operator!() const; // as method Equivalent calls to overloaded operator ! if (!time1) … if (time1.operator!()) …

8. Fall 2010CSCI 383 Lecture 13 M. van Bommel 8 The Subscripting Operator The second example of handout #2 illustrates an implementation of an overloaded subscripting operator (i.e., “[]”)handout #2 Here is an example that uses it IntArray c1(5); for(int i=0; i<5; i++) { c1[i] = i; cout << c1[i] << endl; }

9. Fall 2010CSCI 383 Lecture 13 M. van Bommel 9 The Function Call Operator The second example of handout #2 also illustrates an implementation of an overloaded function call operator (i.e., “()”)handout #2 Here is an example that uses it IntArray c1(5); // … for(int i=0; i<5; i++) cout << c1() << endl; The function call operator can be defined with zero or more parameters in its parameter list

10. Fall 2010CSCI 383 Lecture 13 M. van Bommel 10 Conversion Operators A conversion operator converts an instance of a class to an instance of a built-in class (type) Timer::operator int() { return hours*3600 + minutes*60 + int(seconds); } Conversion operators have no return type specified explicitly since the name of the operator specifies the return type. They also always have a void parameter list

11. Fall 2010CSCI 383 Lecture 13 M. van Bommel 11 Overloaded Operator Q&A Q: What if one wants to use objects as conditions? For example: if (time1) … if (!time1) … A: Define these operators as the methods: operator int(); int operator!();

12. Fall 2010CSCI 383 Lecture 13 M. van Bommel 12 Overloaded Operator Q&A Q: What if one wants to overload both prefix and infix ++? For example, ++time time++ A: In the C++ standard, the two are distinguished as illustrated in the following examples

13. Fall 2010CSCI 383 Lecture 13 M. van Bommel 13 Prefix Overloaded Method Timer Timer::operator++() { if (++seconds >= 60.0) { minutes++; seconds -= 60.0; } if (minutes >= 60) { hours++; minutes -= 60; } return *this; }

14. Fall 2010CSCI 383 Lecture 13 M. van Bommel 14 Postfix Overloaded Method Timer Timer::operator++(int i) { Timer temp = *this; if (++seconds >= 60.0) { minutes++; seconds -= 60.0; } if (minutes >= 60) { hours++; minutes -= 60; } return temp; } WARNING! Since no actual value is sent to the int parameter, do not use the parameter in the method’s code!