[boost] Why is iterator_traits<unordered_map<K, M>>::pointer a detail::ptr_node type rather than value_type *?

The iterator type looks like: template <typename NodePointer, typename Value> struct iterator : public boost::iterator< std::forward_iterator_tag, Value, std::ptrdiff_t, NodePointer, Value&> The NodePointer argument here (which is boost::unordered::detail::ptr_node<...>) becomes the iterator_traits<>::pointer type. But iterator's operator->() returns Value *: Value* operator->() const { return &node_->value(); } Why shouldn't iterator_traits<>::pointer be Value* here? This detail::ptr_node<> type seems intended to be completely internal. Thanks, Alex