Operator overloading allows operators like + and << to be used with user-defined types like classes. It is done by defining corresponding member functions for the operators. For example, to overload + to add two Sale objects, you would define a member function called operator+(). This allows sale1 + sale2 syntax. Similarly, << can be overloaded to allow output to cout. Overloaded operators must follow certain rules regarding arguments and precedence. Postfix operators require an additional integer argument to distinguish them from prefix operators. Overloaded operators allow user-friendly syntax when working with custom data types.

1. 08 c++ Operator Overloading.ppt — Presentation Transcript
1. Overloading Operators
2. 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 Some reserve the term polymorphism (or pure polymorphism) for situations in which one
function body is used with a variety of arguments
3. 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
4. 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
5. 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
6. Binary Operators that Can Be Overloaded
7. 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
8. Overloading Operators— The Rules
9. 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+()
10. Overloading Math Operators Ex8-1
11. 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

2. 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;
12. Overloading Math Operators
13. 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
14. 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 Ex8-3
15. 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
16. 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
17. The Sale Class Ex8-4
18. 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
19. Program that Adds Three Sale Objects Ex8-4
20. 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
21. 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

3. 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
22. 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
23. Overloading Output It accepts two arguments: a reference to ostream (locally named out in this
example) and an integer (locally named i 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
24. Overloading Output The operator <<() function is a friend to the class of the object it wants to print
out, e.g. Sale here.
25. 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);
26. Overloaded Operator>>() Function for the Sale Class
27. Overloading Input Ex8-6
28. 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
29. Using the Prefix and Postfix ++ Operators with an Integer
30. The Inventory Class
31. Overloading ++ and - - The statements numSold++;, numSold = numSold +1;, and numSold += 1; all
would work 8 Ex8-8
32. 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); Ex8-8

4. 33. 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 8
34. 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 EX8-9
35. 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 EX8-9
36. 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
37. The Book Class Ex8-10
38. Overloading [ ] and ( )
39. 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
40. Using the Parentheses Operator