Call by Reference

In C++ language, the call by reference method of passing arguments to a function copies the reference of an argument into the formal parameter. Also, inside the function, the exact argument used in the call is accessed by the reference. Therefore, changes made to the parameter affect the passed argument. 

In addition, to pass the value by reference, the argument reference is passed to the functions just like any other value. 

Therefore, you need to declare the function parameters as reference types as in the function swap(). The swap() function exchanges the values of the two integer variables pointed to by its arguments. 

Syntax:

// function definition to swap the values.
void swap(int &x, int &y)
{
int temp;
temp = x; /* save the value at address x */
x = y; /* put y into x */
y = temp; /* put x into y */
return;
}

Example:

#include <iostream>

// function declaration
void swap(int &x, int &y);

int main ()
{
// local variable declaration:
int a = 100;
int b = 200;

cout << "Before swap, value of a :" << a << endl;
cout << "Before swap, value of b :" << b << endl;

/* calling a function to swap the values using variable reference.*/
swap(a, b);

cout << "After swap, value of a :" << a << endl;
cout << "After swap, value of b :" << b << endl;

return 0;
}