Reference Parameters

void reorder(int a, int b)
{ if (a<=b)
    return;
  int t = a;
  a = b;
  b = t; }

void main()
{ int x = 34, y = 23;
  cout << "x, y are " << x << ", " << y << "\n";

  // I want (x, y) to keep the same two values, but arranged so that x <= y

  reorder(x, y);

  cout << "x, y are " << x << ", " << y << "\n";

  // nothing happens!
}

// In reorder, a and b are just ordinary temporary variables.
// They are initially set up with the values of x and y, but
// have no connection after that. Reorder does correctly swap
// around a and b, but that is no use. x and y are unaffected.

// The solution is this

void reorder(int & a, int & b)
{ if (a<=b)
    return;
  int t = a;
  a = b;
  b = t; }

// The only change is in the first line. Ampersands '&' have appeared
// before the names a and b. This means that a and b are reference
// parameters, and not just temporary independent variables.

// now, when
  reorder(x, y);
// is used, a is just another name for x, and b is just another name
// for y. When a and b are swapped around, it is really x and y that
// are getting swapped around. So this version works.



void add_ten_percent_to(double x)
{ x = x + x*0.1; }

void main()
{ double pay = 155.23;
  cout << "pay = " << pay << "\n";
  add_ten_percent_to(pay);
  cout << "after adding 10%, pay = " << pay << "\n"; }

// This one doesn't work, and for the same reason. It prints

  pay = 155.23
  after adding 10%, pay = 155.23

// The correct version is

void add_ten_percent_to(double & x)
{ x = x + x*0.1; }