Scott Meyers' suggestion to aviod const/nonconst method duplication

nonconst_call.cpp

template<typename Result, typename M, typename A>
Result& nonconst_call(const M& m, A& argument)
{
  return const_cast<Result&>(static_cast<const M&>(m)(argument));
}

Value const& Container::operator () (int index) const
{
  return data[index];
}

Value& Container::operator () (int index) 
{
  return nonconst_call<Value>(*this, index);
}