Operator Overloading

1
Operator Overloading

• What is operator overloading?
• Most predefined operators (arithmetic,
  logic, etc.) in C can be
• overloaded when applied to objects of a class by
  providing an operator
• function. Examples of those that can not be
  overloaded are , . , and ?
• Operators keep their predefined
  precedence and associatively
• class Xpublic
• X operator -() // unary -ve operator invoked by
  X a -a
• X operator () // prefix increment invoked by
  X a a
• X operator (int) // postfix increment, X a
  a
• X operator -(X) // binary -, X b a - b
• X operator (X) // add operator, a b
• // The return type X is needed to cascade
  operators
• // global functions can also be used to overload
  operators on objects

2
Operator Overloading
A simple example class Complex double real,
imag public Complex(double r0.0, double
I0.0)real r imag I Complex operator
()real 1.0 imag 1.0 return
this Complex operator (Complex
c) Complex temp temp.real real
c.real temp.imag imag c.imag return
temp // this invokes the default copy
constructor // can also be implemented in one
line as return Complex(real c.real , imag
c.imag) main() Complex c1(1.5,2.0),
c2(1.2,2.2), c3 c3 c1 c2 //why the
functions must return a Complex data type
3
Operator Overloading
What happens when the assignment exists as in
operators , -, assume we will add the
following member functions to class
Complex Complex operator (Complex) Complex
operator -(Complex) Complex operator
(Complex) Complex Complexoperator
(Complex a) real a.real imag
a.imag return this Complex Complexoperator
(Complex a) real reala.real - imag
a.imag // there is an error here imag real
a.imag imag a.real return this
4
Operator Overloading
The indexing operator for an array class
Vector float p int size public Vector(i
nt n) p new floatn sizen Vector()
delete p float operator (int I)return
pI Vector operator (const Vector a)
Vector temp(size) for(int I0 Iltsize I)
temp.pI pI a.pI return temp
// this will destroy the resulting Vector why? //
a default copy constructor will be used to copy
the attributes p and size // only // To remedy
this, a special copy constructor must be defined
as follows, Vector(Vector c)p new
floatc.size, size c.size for(int I0
Iltsize I) pI c.pI // this will form
a new dynamic object for the result
5
Operator Overloading
The assignment operator Although the assignment
operator is supported by a default copy
operator, the following example will produce a
Null pointer assignment fun() Vector x1(20),
x2(10) x2 x1 // now x2.p and x1.p have the
same address, size 20 // the destructor
function will be called once for x1 and once for
x2 // upon function exit, the same array will be
deleted twice The following assignment operator
function is needed to remedy this Vector
operator (const Vector a) if(this ! a)//
check that this is not xx delete p // clean
up p new float size a.size for(int
I 0IltsizeI) pI a.pI return
this // every class should contain a copy
constructor and an assignment operator
6
Operator Overloading
Non-member operator functions used to define
operations between different types of
objects e.g. Vector operator (const Matrix
a, const Vector b) Vector temp for(int I
0 I lt a.no_rows I) for(int j 0 j lt
a.no_cols j) tempI aIj
bI return temp The above function should
be declared as friend function in both class
matrix and class vector as follows, class
Matrixfriend Vector operator (const
Matrix,const Vector) class Vectorfriend
Vector operator (const Matrix,const Vector)
7
Operator Overloading
Overloading the stream input/output operators gtgt
and ltlt this is needed to apply these operators
to the instances of objects e.g., Complex
x(0.0,0.0) cin gtgt x cout ltlt x // this
needs the following functions which input or
outputs complex numbers as (flt.pt ,
flt.pt.) istream operator gtgt (istream input,
const Complex a) double re0,im0 char c
input gtgt c // input a character in c if(c
( ) input gtgt re gtgt c // input real part
followed by , if ( c , ) input gtgt im gtgt
c // input imag part followed by ) if ( c
) ) a Complex(re,im) return
input cout ltlt error in input
format ostream operator ltlt (ostream output,
const Complex a) return output ltlt
\n ltlt ( ltlt a.real ltlt , ltlt a.imag ltlt ) ltlt
\n // the above functions must be declared as
friends in class Complex