Operator Overloading

1
Operator Overloading

• Operator Overloading allows a programmer to
  define new uses of the existing C/C operator
  symbols.
• useful for defining common operations for user
  defined types.
• should be used to perform the same or similar
  function on class objects as the function built
  in for the basic types.

2
Operator Overloading

• An operator is overloaded by writing a function
  definition that has a function name of operator
  followed by the symbol for the operator to be
  overloaded.
• operator/() will define a function that uses the
  binary division operator.
• operator!() will define a function that uses the
  unary negation operator.

3
Operator Functions

• Operator functions may be defined as either
  member functions or as non-member functions.
• Non-member functions are frequently made friends
  for performance reasons.
• A member operator function uses the target object
  as its first operand.

4
Operator Functions

• The operator overloading functions for
  overloading (), , -gt or the assignment
  operators must be declared as class member
  functions.
• All other operators may be declared as non-member
  functions.

5
Operator Overloading

• The handout lists all the operators that may be
  overloaded as well as the operators that can not
  be overloaded.
• There are 5 operators that cannot be overloaded.
• . . ? sizeof

6
Operator Overloading

• Overloading an operator cannot change
• the operator precedence
• the operator associativity
• the arity of the operator
• the definition of the operator on built-in types

7
Operator Overloading

• Each individual operator must be defined
  separately.
• For example
• overloading operator() and operator-() does not
  overload operator-().
• overloading operator() does not overload
  operator!().

8
Overloading Unary Operators

• Unary operators can be overloaded as
• member functions with no arguments
• static, friend, or global functions with one
  argument where the argument must be either a
  class or a reference to a class

9
Overloading Unary Operators

• The preference for overloading unary operators is
  to make the operator functions class members
  instead of non-member friend functions.

10
Overloading Binary Operators

• Binary operators can be overloaded as
• member functions with one argument
• static, friend, or global functions with two
  arguments where at least one of the arguments
  must be either a class or a reference to a class

11
Member Functions

• When an operator function is implemented as a
  member function
• The leftmost operand becomes an instance of the
  class of which the function is a member.

12
Examples

• class IntegerMatrix IntegerMatrix
  operator( const Matrix rightOp
  ) IntegerMatrix operator( int scalar
  )

13
Examples

• IntegerMatrix m1(..), m2(..)
• int i(..)
• m1 m1 m2
• m2 m2 i

14
Examples

• class StringArray String operator(
  int index )
• Define the indexing operator to work on
  StringArrays indexed with integers.

15
Examples

• String Stringoperator( int index ) ..
• To use
• StringArray stones()
• cout ltlt 5th element ltlt stones4
• int i( 0 )
• stones i air

16
Examples

• class String String operator()( int
  start, int end ) const
• Define the function call operator to create a
  substring of the given string.

17
Examples

• String Stringoperator()( int start, int end )
  const ..
• To use
• String junk( grandiose )
• cout ltlt junk( 2, 4 ) ltlt endl
• Prints and

18
Operator Overloading

• There are two operators that have default
  definitions for user defined objects.
• The assignment operator ()
• The assignment operator will do a member wise
  assignment of the data of the class.
• The address operator ()
• The address operator simply returns the address
  of the object in memory.

19
Non-member Functions

• When an operator function is implemented as a
  non-member function
• The left-most operand may be any type, as long as
  at least one operand is a class.
• If the function must access non-public data, then
  the function must either
• be defined as a friend function, or
• use public accessor and mutator functions.

20
Example

• MyType operator( MyType m ) stdcerr ltlt
  You have violated the C style guidelines and
  will be asked to leave the future immediately.
  ltlt stdendlexit( 86 )

21
Complex Numbers

• Complex numbers consist of two real numbers the
  real and the imaginary parts.
• realPart imaginaryPart i where i sqrt(-1)
• A program should be able to input and output
  complex numbers.
• A program should be able to add, subtract, and
  multiply complex numbers.
• A program should also be able to compare complex
  numbers.

22
Type Conversions

• Compilers do not know how to convert between user
  defined types and built-in types.
• Such conversions must be defined by the
  programmer using conversion constructors and
  conversion operators.

23
Type Conversions

• A conversion constructor (type cast operator) is
  a single argument constructor that converts the
  objects of other types or built-in types into
  objects of a particular class.
• Overloading a cast operator must be a non-static
  member function it cannot be defined as an
  external or friend function.

24
Type Conversion

• Overloaded casting operators do not have a return
  type.
• The name of the operator implies its type.

25
Example

• class istream // return true iff stream// is
  OKoperator bool()
• while ( cin )

26
Example

• Class string string( const char cStr )
• String greeting( hello )

27
Uh-oh

• class A A( const B obj )
• class B operator A()

28
Overloading and --

• When and/or -- are overloaded, both
  pre-increment/decrement and post-increment/decreme
  nt should be overloaded.
• Each version must have a distinct signature.

29
Overloading and --

• The pre-increment/decrement member function is
  declared without any parameters.
• The post-increment/decrement member function is
  declared with a dummy parameter of type int.
• Date operator()
• Date operator( int )
• Date d
• d // calls the first one
• d // calls the second one

30
Example

• Date class