One of the most difficult topics about C++ functions is
about passing parameters to and from a function

For example it is very important to see the difference
between the way parameters are passed BY VALUE 
(input paramters are normally passed this way - by value)
and how they are passed BY REFERENCE (output parameters
or those which are both input and output, i.e. those
modified by the function).

Here is a classical example of the situation where
one needs to use the REFERENCE passing method.


// This program shows how reference parameters can be used 
// in order to swap two values in the main program by
// using a special function, swap

#include <iostream>
using namespace std;

// function prototypes

void sillySwap(int, int);
void smartSwap(int&, int&);

int main()
{
  int a = 1;
  int b = 5;
// we would like to swap these values in such a way that
// eventually a=5 and b=1
// Let's first output the initial values of a and b
  cout << "Initially: a = " << a << ", b= " << b << endl;
// sillySwap is a swapping by passing actual parameters a and b
// by value
  sillySwap(a,b);
  cout << "After silly swap: a = " << a << ", b = " << b << endl;
// smartSwap passes references (i.e. addresses) to the values 
// of a and b
  smartSwap (a,b);
  cout << "After smart swap: a = " << a << ", b = " << b << endl;
return 0;
} // end of main()

// function definitions

// this function uses passing by value

void sillySwap(int p, int q)
{
  int temp = p;
  p = q;
  q = temp;
  return;
// note that  p and q have indeed swapped their
// values but the values of the actuals in the main 
// function (a and b) have not - they are intact!

} 

// this function uses passing by reference

void smartSwap(int& p,int& q)
{
  int temp = p;
  p = q;
  q = temp;
  return;
// since p and q are references (aliases) to the actuals
// swapping their values is the same as swapping the
// values of the actuals!

}              

=============================================

The result of the run of this program is:

Initially: a = 1, b= 5
After silly swap: a = 1, b = 5
After smart swap: a = 5, b = 1


============================================


I strongly reecommend you to go through other examples
of parameter passing in the Lee and Phillips book
(Sections 9.4 and 11.2)

Best wishes
AY