Designing function f(f(n)) == -n

Question

A question I got on my last interview:

Design a function f, such that:

f(f(n)) == -n

Where n<

Answer 1

A C++ version, probably bending the rules somewhat but works for all numeric types (floats, ints, doubles) and even class types that overload the unary minus:

template 
struct f_result
{
  T value;
};

template 
f_result  f (T n)
{
  f_result  result = {n};
  return result;
}

template 
T f (f_result  n)
{
  return -n.value;
}

void main (void)
{
  int n = 45;
  cout << "f(f(" << n << ")) = " << f(f(n)) << endl;
  float p = 3.14f;
  cout << "f(f(" << p << ")) = " << f(f(p)) << endl;
}