13.9) Use of Reference Parameters

In the original version of swap.cpp, function parameters x and y are copies of the variables first and second that were used as arguments in the function call; they do not represent the variables themselves. This is known as call-by-value behaviour.

What happens when we change the function definition to this?

void swap(int& x, int& y)
{
  ...
}

x and y are now references to the original variables that were used as arguments - so any changes made to x and y are in fact changing first and second, giving us precisely the outcome we desire. This is known as call-by-reference behaviour.