Operator overloading

Unit - III

Operator Overloading
Customised behaviour of operators

Unit Introduction
This unit covers operator overloading

Unit Objectives
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After covering this unit you will understand…
Operator overloading
Different types of operator and their
overloading
Operators that cannot be overloaded
Inheritance and overloading
Automatic type conversion

Introduction
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Enabling C++’s operators to work with class
objects
Using traditional operators with user-defined
objects
Requires great care; when overloading is misused,
program difficult to understand
Examples of already overloaded operators
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4

Operator << is both the stream-insertion operator and
the bitwise left-shift operator
+ and -, perform arithmetic on multiple types

Compiler generates the appropriate code based on
the manner in which the operator is used



Overloading an operator
Write function definition as normal
 Function name is keyword operator followed by
the symbol for the operator being overloaded
 operator+ used to overload the addition
operator (+)
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Using operators


To use an operator on a class object it must be
overloaded unless the assignment operator(=)or
the address operator(&)
Assignment operator by default performs memberwise
assignment
 Address operator (&) by default returns the address of an
object


5

Restrictions on Operator Overloading

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Operators that can be overloaded
C++ operators that can be overloaded
+

*

/

%

^

&

|

~

!

=

<

>

+=

-=

*=

/=

%=

^=

&=

|=

<<

>>

>>=

<<=

==

!=

<=

>=

&&

||

++

--

->*

,

->

[]

()

new

delete

new[]



-

delete[]

C++ Operators that cannot be overloaded
.

6

.*

::

?:

sizeof

Restrictions on Operator Overloading
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7

Overloading restrictions
 Precedence of an operator cannot be changed
 Associativity of an operator cannot be changed
 Arity (number of operands) cannot be changed
 Unary operators remain unary, and binary operators
remain binary
 Operators &, *, + and - each have unary and binary
versions
 Unary and binary versions can be overloaded separately
No new operators can be created
 Use only existing operators
No overloading operators for built-in types
 Cannot change how two integers are added
 Produces a syntax error

Operator Functions as Class
Members vs. as friend Functions
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8

Member vs non-member
 In general, operator functions can be member or non-member
functions
 When overloading ( ), [ ], -> or any of the assignment
operators, must use a member function
Operator functions as member functions
 Leftmost operand must be an object (or reference to an object)
of the class
 If left operand of a different type, operator function must be
a non-member function
Operator functions as non-member functions
 Must be friends if needs to access private or protected
members
 Enable the operator to be commutative

Overloading Stream-Insertion and Stream-Extraction
Operators
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Overloaded << and >> operators
Overloaded to perform input/output for userdefined types
 Left operand of types ostream & and istream
&
 Must be a non-member function because left
operand is not an object of the class
 Must be a friend function to access private data
members
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9

Overloading Unary Operators
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Overloading unary operators
Can be overloaded with no arguments or one
argument
 Should usually be implemented as member functions
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Avoid friend functions and classes because they
violate the encapsulation of a class

Example declaration as a member function:
class String {
public:
bool operator!() const;
...
};

10

Overloading Unary Operators
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Example declaration as a non-member function
class String {
friend bool operator!( const
String & )
...
}

11

Overloading Binary Operators
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Overloaded Binary operators
Non-static member function, one argument
 Example:


class Complex {
public:
const Complex operator+ (
const Complex &);
...
};

12

Overloading Binary Operators
Non-member function, two arguments
 Example:


class Complex {
friend Complex operator +(
const Complex &, const Complex &
);
...
};

13

Example: Operator Overloading
class OverloadingExample
{
private:
int m_LocalInt;
public:
OverloadingExample(int j) // default constructor

{
m_LocalInt = j;
}
int operator+ (int j) // overloaded + operator

{
return (m_LocalInt + j);
}
};

Example: Operator Overloading (contd.)
void main()
{
OverloadingExample object1(10);
cout << object1 + 10; // overloaded operator called
}

Types of Operator
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Unary operator

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Binary operator

Unary Operators
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Operators attached to a single operand (-a, +a, -a, a--, ++a, a++)

Example: Unary Operators
class UnaryExample
{
private:
int m_LocalInt;
public:
UnaryExample(int j)

{
m_LocalInt = j;
}
int operator++ ()

{
return (m_LocalInt++);
}
};

Example: Unary Operators (contd.)
void main()
{
UnaryExample object1(10);
cout << object1++; // overloaded operator results in value
// 11
}

Unary Overloaded Operators -- Member Functions
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Invocation in Two Ways -- Object@ (Direct) or Object.operator@() (As a
Function)
class number{
int n;
public:
number(int x = 0):n(x){};
number operator-(){return number (-n);}
};
main()
{
number a(1), b(2), c, d;
//Invocation of "-" Operator -- direct
d = -b; //d.n = -2
//Invocation of "-" Operator -- Function
c = a.operator-(); //c.n = -1
}
20

Binary Overloaded Operators -- Member Functions
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Invocation in Two Ways -- ObjectA @ ObjectB (direct) or
ObjectA.operator@(ObjectB) (As a Function)
class number{
int n;
public:
number(int x = 0):n(x){};
number operator+(number ip)
{return number (ip.n + n);}
};
main()
{
number a(1), b(2), c, d;
//Invocation of "+" Operator -- direct
d = a + b; //d.n = 3
//Invocation of "+" Operator -- Function
c = d.operator+(b); //c.n = d.n + b.n = 5
}
21

Binary Operators
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Operators attached to two operands (a-b, a+b,
a*b, a/b, a%b, a>b, a>=b, a<b, a<=b, a==b)

Example: Binary Operators
class BinaryExample
{
private:
int m_LocalInt;
public:
BinaryExample(int j)

{
m_LocalInt = j;
}
int operator+ (BinaryExample& rhsObj)

{
return (m_LocalInt + rhsObj.m_LocalInt);
}
};

Example: Binary Operators (contd.)
void main()
{
BinaryExample object1(10), object2(20);
cout << object1 + object2; // overloaded operator called
}

Non-Overloadable Operators
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Operators that can not be overloaded due to
safety reasons:
Member Selection ‘.’ operator
 Member dereference ‘.*’ operator
 Exponential ‘**’ operator
 User-defined operators
 Operator precedence rules
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Operator Overloading and Inheritance
An operator is overloaded in super class but
not overloaded in derived class is called nonmember operator in derived class
In above, if operator is also overloaded in
derived class it is called member-operator
= ( ) [ ] –> –>* operators must be member
operators
Other operators can be non-member
operators

Automatic Type Conversion
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Automatic type conversion by the C++
compiler from the type that doesn’t fit, to the
type it wants
Two types of conversion:
Constructor conversion
 Operator conversion
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Constructor Conversion
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Constructor having a single argument of another
type, results in automatic type conversion by the
compiler
Prevention of constructor type conversion by
use of explicit keyword

Example: Constructor Conversion
class One
{
public:
One() {}
};
class Two
{
public:
Two(const One&) {}
};
void f(Two) {}
void main()
{
One one;
f(one); // Wants a Two, has a One
}

Operator Conversion
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Create a member function that takes the current
type
Converts it to the desired type using the
operator keyword followed by the type you
want to convert to
Return type is the name of the operator
overloaded
Reflexivity - global overloading instead of
member overloading; for code saving

Example: Operator Conversion
class Three
{
int m_Data;
public:
Three(int ii = 0, int = 0) : m_Data(ii) {}
};
class Four
{
int m_Data;
public:
Four(int x) : m_Data(x) {}
operator Three() const
{
return Three(m_Data);
}
};
void g(Three) {}

Example: Operator Conversion (contd.)
void main()
{
Four four(1);
g(four);
g(1); // Calls Three(1,0)
}

Type Conversion Pitfalls
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Compiler performs automatic type conversion
independently, therefore it may have the
following pitfalls:
Ambiguity with two classes of same type
 Automatic conversion to more than one type - fanout
 Adds hidden activities (copy-constructor etc)
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Unit Summary
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In this unit you have covered …
Different types of operator
Inheritance and overloading
Automatic type conversion

Operator overloading

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