Overloading Operators

1. 8 Overloading Operators Object-Oriented Programming Using C++ Second Edition

2. 8 Objectives • In this chapter, you will learn: • About the benefits of overloading • About the rules that apply to operator overloading • How to overload math operators • How to overload operators to work with a class object and a primitive object • How to chain multiple mathematical operations in a statement

3. 8 Objectives • In this chapter, you will learn: • How to overload the insertion (<<) operator for output • How to overload the extraction operator (>>) for input • How to overload the prefix and postfix ++ and -- operators • How to overload the = = operator • How to overload the = operator • How to overload the subscript and parentheses operators

4. 8 Understanding the Benefits of Overloading • Having more than one function with the same name is beneficial because you can use one easy-to-understand function name without paying attention to the data types involved • Polymorphism allows the same operation to be carried out differently, depending on the object • Purists find a subtle difference between overloading and polymorphism • Some reserve the term polymorphism (or pure polymorphism) for situations in which one function body is used with a variety of arguments

5. 8 Using the + Operator Polymorphically • Separate actions can result from what seems to be the same operation or command • The + operator has a variety of meanings, which include: • Alone before a value (called unary form), + indicates a positive values, as in the expression +7 • Between two integers (called binary form), + indicates integer addition, as in the expression 5+ 9 • Between two floating-point numbers (also called binary form), + indicates floating-point addition, as in the expression 6.4 + 2.1

6. 8 Overloading Operators—The Rules • Operator overloading is the process by which you apply operators to your own abstract data types • The +, -, *, and / symbols make it easy to work with built-in data types such as int and double • Classes, however, contain a variety of data members • As a result, if you want the compiler to perform arithmetic with two class objects, you must tell the compiler what you mean • Good programming style dictates that you endow the operator with a reasonable meaning

7. 8 Overloading Operators—The Rules • You overload an operator by making it a function; subsequently, you can use it just like any other function • C++ operators are classified as unary or binary, depending on whether they take one or two arguments, respectively

8. 8 Binary Operators that Can Be Overloaded

9. 8 Overloading Operators—The Rules • Associativity refers to the order in which actions within an expression are carried out • You cannot change associativity when you overload operators • You also cannot change the normal precedence of any operator

10. 8 Overloading Operators—The Rules

11. 8 Overloading Math Operators • When you code an expression such as 4 + 7, C++ understands that you intend to carry out binary integer addition because of the context of the + symbol • When you code an expression such as regularSal + bonus, if C++ can recognize regularSal and bonus as declared double variables, then floating-point addition takes place • The name of the operator function that overloads the + symbol is operator+()

12. 8 Overloading Math Operators

13. 8 Overloading Math Operators • The operator+() function in Figure 8-1 can work like any other member function • When you examine the code for the addTwo() and operator+() functions in Figure 8-1, you see that the only difference is the function name • Instead of the awkward sum = clerk.operator+(driver);, the operator+() function allows you to leave off the word operator in the function name and add either of the following statements: sum = clerk + driver; sum = driver + clerk;

14. 8 Overloading Math Operators

15. 8 Paying Attention to the Order of the Operands • You can choose to overload any of the arithmetic operators for any classes you develop • Then you can use the corresponding operator symbol in a natural way with class objects

16. 8 Paying Attention to the Order of the Operands • In the steps outlined on pages 270 to 282 of the textbook, you create a class for a SalesOffice • The class will include an overloaded division operator (operator /) so you can divide one office’s sales by another to determine the ratio of their sales

17. 8 Output of SalesOffice2.cpp

18. 8 Overloading an Operator to Work with a Class Object and a Primitive Type • When you add two objects using the + operator, the objects do not have to be the same type • You can add an integer and a double with an expression such as 5 + 7.84

19. 8 Overloading an Operator to Work with a Class Object and a Primitive Type • You cannot overload operators that work with C++’s built-in data types • You cannot overload the + that works with two doubles to make it do anything but add two doubles • Similarly, you can’t overload operators whose first operand is an object that is a built-in type, even if the second operand is a class object

20. 8 Using Multiple Operations in a Statement • Most modern programming languages allow several operators to be used in the same statement • If you want to sum three values in an older programming language such as assembler or RPG, you first must add two values, producing a temporary total • Then, in a separate statement, you add the third value to that total

21. 8 The Sale Class

22. 8 Using Multiple Operations in a Statement • Because the associativity of addition occurs from left to right, the attempt to execute the addition highlighted in Figure 8-9 follows this sequence: 1. The left-most + operator is encountered, and C++ recognizes a Sale on each side of the + symbol. The overloaded operator+() function is called, and saleAmounts for a Shirt and a Tie are added. A double is returned 2. The next + operator is encountered. A Sale object is found as the operand to the right of the +, but a double value is used as the operand to the left

23. 8 Program that Adds Three Sale Objects

24. 8 Using Multiple Operations in a Statement • When the Sale class operator+() function does not return a double, but instead returns an object of Sale type (as shown in Figure 8-8), the multiple addition works correctly • The sequence of events now occurs as follows: 1. The left-most + operator is encountered, and C++ recognizes a Sale object on each side of the + symbol. The overloaded operator+() function is called, and saleAmounts for a Shirt and a Tie are added 2. The next + operator is encountered. A Sale object now is found on each side of the +—the temporary object returned by the first addition, and the pants object 3. The temporary object is assigned to the total Sale object

25. 8 Using Multiple Operations in a Statement • The results of the execution of the program in Figure 8-9 are shown in Figure 8-10 • C++ forces you to use the built-in precedence rules for your class operators • If you want to be able to add either a double or a Sale object to a Sale object, then simply write both versions of the overloaded operator for the class

26. 8 Overloading Output • The << operator also is overloaded by C++ • It is both a bitwise left-shift operator and an output operator; it is called the insertion operator when used for output • The << operator acts as an output operator only when cout (or another output stream object) appears on the left side • When you use cout in a program, you must include #include<iostream.h> • The preceding function, called operator<<(), returns a reference to ostream

27. 8 Overloading Output • It accepts two arguments: a reference to ostream (locally named out in this example) and an integer (locally named n in this example) • C++ overloads the << operator to work with the built-in data types; you also may overload the << operator to work with your own classes • To overload << operator so it can work with a Sale object, you must add the overloaded operator <<() function to the Sale class

28. 8 Overloading Output • The operator <<() function is a friend to the ostream class

29. 8 Overloading Output • In the steps on page 288 of the textbook, you overload the insertion operator to work with the SalesOffice class you created earlier in this chapter

30. 8 Overloading Input • If the << operator can be overloaded for output, it makes sense that the >> operator also can be overloaded for input • The advantage of overloading operators such as >> is that the resulting programs look cleaner and are easier to read • You can create an extraction operator, or operator>>() function, that uses istream (which is defined in iostream.h, along with ostream) by using a prototype as follows: friend istream& operator>>(istream &in, Sale &Sale);

31. 8 Overloaded Operator>>() Function for the Sale Class

32. 8 Overloading Input • You could improve the operator>>() function shown in Figure 8-13 by adding code that verifies valid receipt numbers and sale amounts • In the steps referred to on pages 289 and 290 of the textbook, you add an overloaded operator>>() function for the SalesOffice class

33. 8 Overloading ++ and - - • With C++, you use ++ to increment variables, and - - to decrement variables • When a prefix operator such as ++ is used in an expression, the mathematical operation takes place before the expression is evaluated • When the postfix operator is used, the expression is evaluated before the mathematical operation takes place • Within the operator ++() function in the Inventory class, you can write the statement that increases numSold in several different ways

34. 8 Using the Prefix and Postfix ++ Operators with an Integer

35. 8 The Inventory Class

36. 8 Overloading ++ and - - • The statements numSold++;, numSold = numSold +1;, and numSold += 1; all would work

37. 8 Using Postfix Increment and Decrement Operators • A problem arises if you want to use a postfix ++ operator as well as a prefix ++ operator with a class • When you overload any C++ function, you must supply different argument lists; for the postfix ++ operator, you use an integer argument • The Inventory class postfix operator ++() function prototype is: Inventory& operator++(int);

38. 8 Overloading the = = Operator • Writing an operator = =() function should be an easy task • You simply decide what will constitute equality in class members • When you create your own classes, you choose whether equivalency means that every data field must be equivalent, or only specific data members • The operator = =() function may return either an integer or a boolean variable representing true or false

39. 8 Overloading the = = Operator • A variable of type bool can hold one of two values: true or false • Some older C++ compilers do not support the bool type; with those compilers you would use the first version of operator = =() that returns an integer

40. 8 Overloading the = Operator • The = operator can be overloaded for use with your own classes • Unlike other operators, if you don’t define the = operator, C++ provides a definition for you • If you want the = operator to do something other than assign each member, then you must create a customer operator=()function • In addition, if the class contains data fields that are pointers, you should create a custom function

41. 8 The Movie Class

42. 8 Program that Uses the Movie Class

43. 8 Overloading the = Operator • Figure 8-21 shows a program that declares two Movie objects • This sample program uses unconventional blocking to illustrate the potential error situation • You can avoid the preceding problem by overloading the = operator so the two Movie title pointers don’t point to the same memory

44. 8 Overloading the = Operator

45. 8 Overloading [ ] and ( ) • The subscript operator, operator[ ], is declared like any other function, but called in a manner similar to accessing an array element • You can include any instructions you want within an operator [ ] function • Typically, you use this function to perform a task that both requires an argument and does not quite fit into another operator’s usual meaning • Consider a Book class such as the one shown in Figure 8-25

46. 8 The Book Class

47. 8 Overloading [ ] and ( )

48. 8 Using the Parentheses Operator • You can use the parentheses operator to make multiple assignments within a class • To overload the parentheses operator to assign both an author and a price to a member of the Book class, you can create the function

49. 8 Using the Parentheses Operator

50. 8 Summary • The built-in + operator is polymorphic in C++; it can take one or two arguments and have a different meaning in each case • Operator overloading is the process by which you apply operators to your own abstract data types • The name of the operator function that overloads the + symbol is operator+() • The syntax involved in using the + operator alone is simpler, more natural, and easier to remember than using an ordinary member function