cpp-lazy-stream/include/stream/streams/forward.hpp

#pragma once

#include "traits.hpp"

namespace stream { namespace detail {
  template <typename> class stream_base;
  template <typename> class iterator_impl;
}}

namespace stream {
  template <typename> class iterator;
  template <typename, typename> struct map_t;
  template <typename> struct filter_t;
  template <typename, typename> struct flatmap_t;
  template <typename T>
  using stream = detail::stream_base<T>; // Should I override instead?
}