This is related to typecasting of user defined datatypes ie 
Convertion of one class object to other class object.
Ex 
I want to type cast REAL class object to COMPLEX Class object
Both REAL class and COMPLEX Class are user defined.

COMPLEX  objComplex(6,3); //6 is real and 3 is imagnary
REAL objReal(5);
objComplex=objReal; 
//end result of the above statement should be that
objComplex //should have 5 as real part and 0 as imaginary
//There are two solutions to it 
//a)write conversion constuctor 
//b)Overload assignment operator
// (a) for this add this in the COMPLEX Class
COMPLEX :: COMPLEX(REAL r)
{
real=r.value; // value is the only data member of REAL class
imag=0;
}