Const node iterator.
This is an &qout;iterator to an iterator&qout; - it iterates over nodes, and de-referencing it returns one of the tree's iterators
| Type | Definition | Description | 
| iterator_category | trivial_iterator_tag | Category. This tag identifies that the iterator has none of the STL's iterators' movement abilities. | 
| difference_type | void | Difference type. | 
Note that a node iterator's value type is actually a tree iterator.
| Type | Definition | Description | 
| value_type | container_base::const_iterator | Iterator's value type. | 
| reference | container_base::const_iterator | Iterator's reference type. | 
| const_reference | container_base::const_iterator | Iterator's const reference type. | 
These are only defined if basic_tree::Node_Update is not null_tree_node_update
| Type | Definition | Description | 
| metadata_type | typename basic_tree::Node_Update::metadata_type | Metadata type. | 
| const_metadata_reference | 
typename Allocator::template rebind<
    metadata_type>::other::const_reference
 | Const metadata reference type. | 
| Method | Description | 
| inline const_node_iterator () | Default constructor. | 
| Method | Description | 
| inline const_reference operator* () const | Access. | 
These are only defined if basic_tree::Node_Update is not null_tree_node_update
| Method | Description | 
| inline const_metadata_reference get_metadata () const | Metadata access. | 
| Method | Description | 
| 
inline const_node_iterator
  get_l_child
  () const
 | Returns the const node iterator associated with the left node. | 
| 
inline const_node_iterator
  get_r_child
  () const
 | Returns the const node iterator associated with the right node. | 
| Method | Description | 
| 
inline bool
  operator==
  (const const_node_iterator &other) const
 | Compares to a different iterator object. | 
| 
inline bool
  operator!=
  (const const_node_iterator &other) const
 | Compares (negatively) to a different iterator object. |