C++ overloading binary operators using friends

C++ Overloading binary operators using friends

As stated earlier, in c++ friend functions may be used in the place of member functions for overloading a binary operator, so the only difference being that a friend function requires 2(two arguments) to be explicitly passed to it, while a member function requires only one.

The program of complex number discussed in the previous section can be modified using a friend operator function as follows:

1). Replace the member declaration by the friend function declaration.

friend complex operator+(complex,complex);

2) Redefine the operator function as follows:

Complex operator+(complex a, complex b)
{
return complex (( a.x + b.x), (a.y+b.y));
}

In this case, the statement
C3 = C1+ C2;
Is equivalent to
C3 = operator+(C1 , C2);

In the most cases, we get the same results by the use of either a member function or a friend function. Why then an alternative is made available? There are certain conditions where we would like to use a friend function rather than a member function. For instance, consider the situation where you need to use two different types of operands for a binary operator, say, one an object and another a built in type data as shown in below,

A= B +2; (or A = B * 2;)
Where in the above example A and B are objects of the same class. it will work for a member function but the statement
A = 2 + B; (or A = 2 * B)

will not work. it is because the left-hand operand which is responsible for invoking the member function should be an object of the same class. However friend function allows both approaches. How?

it may be re-called that an object need to be used to invoke a friend function but can be passed as an argument. Thus, we can use a friend function or(method) with a built in type data as the left hand operand and an object as the right hand operand. Program no. 7.3 illustrates this, using the scalar multiplication of a vector. It also shows how to overload the input and output operators >> and<<.

[OVERLOADING OPERATORS USING FRIENDS] #include<iostream.h > const size = 3; class vector { int v[size]; public: vector(); // constructs null vector vector( int *x); // constructs vector from array friend vector operator *( int a, vector b); //friend 1 friend vector operator *(vector b, int a); // friend 2 friend istream & operator >> (istream &, vector &); friend ostream & operator << (ostream &,vector &); }; vector : : vector() { for (int i=0; i < size; i++) v[i] =0; } vector : : vector(int * x) { for (int i =0; i< size; i++) v[i] = x[i]; } vector operator *(int a, vector b) { vector c; for (int i=0; i< size; i++) c.v[i] = a*b.v[i]; return c; } vector operator *(vector b, int a) { vector c; for (int i=0; i< size; i++) c.v[i] = b.v[i], *a; return c; } istream & operator >> (istream &din, vector &b) { for(int i=0; i< size; i++) din >> b.v[i]; return(din); } ostream & operator << (ostream &dout, vector &b) { dout << "(" << b.v[0]; for(int i=1; i < size; i++) dout << ", " << b.v[i]; dout << ")"; return(dout); } int x[size] = {2,4,6}; int main() { vector m; // invokes constructor 1 vector n= x; // invokes constructor 2 std::cout<< "Enter elements of vector m" << "\n"; std::cin>> m; // invokes operator>>() function std::cout<< "\n"; std::cout << "m= " << m << "\n"; // invokes operator <<() vector p, q; p = 2 * m; // invokes friend 1 q = n * 2; // invokes friend 2 std::cout<< "\n"; std::cout<< "p = " << p << "\n"; // invokes operator<<() std::cout<< "q = " << q << "\n"; return 0; }