CSCI 383

1. CSCI 383 Object-Oriented Programming & Design Lecture 13 Martin 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(); CSCI 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 CSCI 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) CSCI 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 + time2is a message sent to the Timer instance time1 CSCI 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 CSCI 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!()) … CSCI 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., “[]”) • Here is an example that uses it IntArray c1(5); for(int i=0; i<5; i++) { c1[i] = i; cout << c1[i] << endl; } CSCI 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., “()”) • 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 CSCI 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 CSCI 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!(); CSCI 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 CSCI 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; } CSCI 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! CSCI 383 Lecture 13 M. van Bommel