template <typename T>
class ParamIterator {
 public:
  typedef T value_type;
  typedef const T& reference;
  typedef ptrdiff_t difference_type;

  // ParamIterator assumes ownership of the impl_ pointer.
  ParamIterator(const ParamIterator& other) : impl_(other.impl_->Clone()) {}
  ParamIterator& operator=(const ParamIterator& other) {
    if (this != &other)