Operator overloading

Mrs. Pranali P. Chaudhari
Operator Overloading

Contents
 Operator Overloading
 Overloading Unary and Binary Operator
 Overloading using friend function
 Type casting andType conversion

Quiz 1
 What is Operator Overloading?
 Operator overloading is a specific case of
polymorphism in which some or all operators like +,
= or == are treated as polymorphic functions and as
such have different behaviours depending on the types
of its arguments.

Operator Overloading
 Operator overloading is a specific case of polymorphism in which
some or all of operators like +, =, or == have different
implementations depending on the types of their arguments.
 Operator overloading gives us the opportunity to redefine the
C++ language.
 C++ permits us to add two variables of user defined types with
the same syntax that is applied to the basic data types.

Restrictions on Operator Overloading
 Overloading restrictions
 Precedence of an operator cannot be changed
 Associatively 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

Restrictions on Operator Overloading
 Following operators can’t be overloaded:
 Class member access operators (., .*)
 Scope resolution operator (::)
 Size operator (sizeof )
 Conditional operator (?:)
 All other operators can be overload

Defining Operator Overloading
 General syntax for operator overloading is:
return type classname :: operator op(arglist)
{
Function body
}
For e.g.:
vector operator +(vector);
 vector is a data type of class.

Operator Overloading Process
 The process of overloading involves the following steps:
 Create a class that defines the data type that is to be used in the
overloading operation.
 Declare the operator function operator op() in the public part
of the class.
 Define the operator function to implement the required
operations.

Overloading Unary Operators
 Consider unary minus operator (changes the sign of the
operand)
 The unary minus when applied to an object should change
the sign of each of its data items.

class space
{
int x, y, z;
public:
void getdata( int a, int b, int c);
void display(void);
void operator - (); // overload unary minus operator
};

void space :: getdata(int a, int b, int c)
{
x = a;
y = b;
z = c;
}
void space :: display(void)
{
cout << x ;
cout << y ;
cout << z ;
}

void space :: operator - ()
{
x = -x;
y = -y;
z = -z;
}
int main()
{
space S;
S.getdata(10, 20, 30);
S.display();
-S; // activates operator-() function
S.display();
return 0;
}

Practice Statement 1
 Create a class date with day, month and year as its
members. Accept the date from the user and display it.
Overload the increment and decrement operators for
displaying the next and previous date for the given date.

Overloading Binary Operators
 As a unary operator is overloaded we can also overload a binary
operator.
 For e.g: A binary operator + can be overloaded to add two objects
rather than adding two variables.
 Using operator overloading a functional notation,
C = sum(A, B);
Can be replaced by,
C = A + B;

class complex
{
float x;
float y;
public:
complex(){}
complex(float real, float imag)
{
x = real;
y = imag;
}
complex operator + (complex);
void display(void);
};

complex complex :: operator + (complex c)
{
complex temp;
temp.x = x + c.x;
temp.y = y + c.y;
return (temp);
}
void complex :: display(void)
{
cout << x << “ + j “ << y ;
}

int main()
{
complex C1, C2, C3;
C1 = complex(2.5, 3.5);
C2 = complex(1.6, 2.7);
C3 = C1 + C2; // invokes operator+() function
cout << “ C1 = “; C1.display();
return 0;
}

Overloading binary operators using
Friends
 Friend functions may be used in place of member functions
for overloading a binary operator.
 A friend function requires two arguments to be explicitly
passed to it, while a member function requires only one.
 For e.g:
complex operator + (complex);
friend complex operator + (complex, complex);

Friends
 The operator function definition would also be modified as:
complex complex :: operator+(complex c)
{
complex temp;
temp.x = x + c.x;
temp.y = y + c.y;
return(temp);
}
complex operator + (complex a, complex b)
{
return complex((a.x + b.x), (a.y + b.y));
}

Friends
 The statement,
C3 = C1 + C2;
Is equivalent to
C3 = operator+(C1, C2);

String Manipulations using friends
class string
{
char *p;
int len;
public:
string() { len = 0; p = 0;}
string( const char * s );
string ( const string & s );
~ string () { delete p; }
friend string operator+(const string & s , const string & t );
friend int operator<=(const string & s, const string & t );
friend void show( const string s);
};

string :: string ( const char * s)
{
len = strlen(s);
p = new char[len + 1];
strcpy(p, s);
}
string :: string (const string & s)
{
len = s. len;
p = new char[len+1];
strcpy(p, s.p);
}

String operator+(const string &s, const string &t)
{
string temp;
temp.len = s.len + t.len;
temp.p = new char[temp.len+1];
strcpy(temp.p, s.p);
strcat(temp.p, t.p);
return(temp);
}

int operator<=(const string &s , const string &t)
{
int m = strlen(s.p);
int n = strlen(t.p);
if( m <= n) return(1);
else return (0);
}
void show (const string s)
{
cout << s.p;
}

int main()
{
string s1 = “New”;
string s2 = “York”;
string s3 = “Delhi”;
string t1, t2, t3;
t1 = s1;
t2 = s2;
t3 = s1 + s3;
show(t1); show(t2); show(t3);

if ( t1 <= t3)
{
show(t1);
cout << “smaller than”;
show(t3);
}
else
{
show(t3);
cout << “smaller than”;
show(t1);
}
return 0;
}

Overloading Stream Insertion and
Extraction Operators
 It is possible to overload the stream insertion (<<) and
extraction operators (>>) to work with classes.
 This has advantages, in that
 It makes programs more readable
 It makes programs more extensible
 It makes input and output more consistent

istream & operator >> (istream &din, vector &b)
{
for (int i = 0; i < size; i++)
{
din>>b.v[i];
}
return(din);
}

ostream & operator << (ostream &dout, rational &b)
{
dout<<“(“;
for (int i = 0; i < size; i++)
{
din>>b.v[i];
}
dout << “)”;
return(dout);
}

Practice Statement 2
 Create a class rational to accept a rational number.
Perform all the arithmetic operations on rational
numbers by overloading all arithmetic operators ( +, - ,
* , / ). Also overload >> and << operators for taking
rational numbers as input and display them.

Type Conversion
 Type conversion is a process of converting one data type to
another.
 Type conversion is done automatically for the built-in
datatypes by the compiler.
 Example:
int m;
float x = 3.14159;
m = x;
 The above statements convert x to an integer before
assigning it to m.
 For user defined data type the compiler has to be provided
with the type conversion function.

Type Conversion
 There are three situations that arise when performing data
type conversion between incompatible types:
 Conversion from basic type to class type
 Conversion from class type to basic type
 Conversion from one class type to another class type

Basic type to Class type
 The conversion from basic type to class type is easily accomplish
by the use of constructors.
 Constructors are used to initialize the objects and we can use
them to build a class type object from an basic type.
 For example we can create a vector object from an int type array.
 The constructors perform a defacto type conversion from the
argument’s type to the constructor’s class type.
 The constructors used for the type conversion take a single
argument whose type is to be converted.

Basic type to class type (Example)
class time
{
int hrs;
int mins;
public:
time (int t) // constructor
{
hrs = t/60; // t in minutes
mins = t%60;
}
};
The following conversion statements can be used in a function:
time t1; // object t1 created
int duration = 85;
t1 = duration; // int to class type

Class type to Basic type
 C++ allows an overloaded casting operator that could be
used to convert a class type data to a basic type.
 General syntax:
Operator typename()
{
...........
...........
}
This function converts a class type data to typename.
 For example, the operator double() converts a class
object to type double.

Class type to Basic type (Example)
Vector :: operator double()
{
double sum = 0;
for(int i =0; i<size; i++)
{
sum = sum + v[i] * v[i];
return sqrt(sum);
}
}
 This function converts a vector to the corresponding scalar
magnitude.
 The operator double() can be used as
double length = double(v1); OR
double length = v1;

Class type to Basic type
 The casting operator function should satisfy the following
conditions:
 It must be a class member.
 It must not specify a return type.
 It must not have any arguments.

One class to Another class type
 Conversions between objects of different classes can be
carried out by either a constructor or a conversion function.
 Consider the statement:
objX = objY;
 The class Y type data is converted to the class X type data and
the converted value is assigned to the objX.
 The class Y is known as source class and class X is known
as destination class.

 When to use constructor and type conversion function?

(Example)
 Consider an example of an inventory of products in store.
 One way of recording the details of the product is to record
their code number, total items in the stock and the cost of
each item.
 Another approach is to just specify the item code and the
value of the item in the stock.
 Example.

Summary
 State whetherTrue or False:
 Using the operator overloading concept, we can change the
meaning of an operator.
 Friend functions cannot be used to overload operators.
 When using an overloaded binary operator, the left operand is
implicitly passed to the member function.
 The overloaded operator must have atleast one operand that is
user-defined type.
 Operator functions never return a value.
 Through operator overloading, a class type data can be converted
to a basic type.
 A constructor can be used to convert a basic type to a class type
data.

Short Answer Questions
 Why is it necessary to overload and operator?
 Operator overloading allows us to provide new implementations
for existing operators. Using operator overloading one can perform
the operation on the user defined data types in the same way as that
of built-in data types.
 Example: Two integer values can be added as c = a + b. With the
use of operator overloading the + operator can also be used to
perform addition of two complex numbers as:
C3 = C1 + C2 where C1, C2 and C3 are the objects of class
complex.

 When is a friend function compulsory? Give example.
 Friend function is used in a situation where we need to use two
different types of operands for a binary operator. One an object and
another a built-in data type.
A = 2 + B
 Here the statement will not be executed by the member functions
as the left hand operand which is responsible for invoking the
member function is not the object of the same class. However the
friend function allows the statement to be executed.

 A friend function cannot be used to overload the
assignment operator = . Explain why?
 In the assignment operator, the right hand operand is source and
lefthand operand is the target of assignment. When we overload
assignment operator as member function then the Lvalue becomes
the object for which the assignment operator is called and the Rvalue
object is passed as parameter. If the Rvalue is constant then by using
the constructor that value can be converted into object and then will
be passed as parameter to the assignment operator. But in case of
friend function both the Lvalue and Rvalue needs to be passed as
parameter. If the Lvalue is constant then it gets converted into the
class object by constructor which results in changing the constant
value which is not allowed/illegal. Hence, to avoid modifying
constant as Lvalue assignment operator was restricted to be
overloaded as friend function.

 Name the operators that cannot be overloaded.
 Class member access operators (., .*)
 Scope resolution operator (::)
 Size operator (sizeof )
 Conditional operator (?:)
 Name the operators that cannot be overloaded
using friend function.
 Assignment operator ( = )
 Function call operator ( )
 Subscripting operator [ ]
 Class member access operator ( -> )

 What is conversion function. How is it created.
Explain its syntax.
 The conversion function is the overloaded casting operator
that is used to convert a class type data to basic type data.
 Syntax:
Operator typename()
{
...........
...........
}
 This function converts a class type data to typename.
 Operator int() converts a class type object to type int.

 A class alpha has a constructor as follows:
Alpha ( int a, double b);
Can we use this constructor to convert types?
 No, we cannot use this constructor to convert types because
when the constructors are used for the type conversion it take a
single argument whose type is to be converted.
 In the given example the alpha constructor is having two
arguments int and double.

 We have two classes X and Y. If a is an object of X
and b is an object of Y and we want to say a = b;
what type of conversion routine should be used and
where?
 To perform the conversion a = b we use the type casting
operator function in the source class. The conversion takes place
in theY class and the result is given to the X class.

References
 Object Oriented Programming with C++ by E.
Balagurusamy.

Operator overloading

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