A "key" type using rank metadata for order statistics.
The class is composed of the logical key and the rank of the element (the number of elements in its subtree).
| Parameter | Description | Default Value | 
| Key | Key type. | - | 
| Allocator | Allocator type. The allocator is only used for definitions, e.g., size_type, pointer, and reference. | std::allocator<char> | 
| Type | Definition | Description | 
| allocator | Allocator | Allocator type. | 
| Type | Definition | Description | 
| key_type | Key | Key type. | 
| const_key_reference | typename Allocator::template rebind< Key >::other::const_reference | Const key reference type. | 
| key_reference | typename Allocator::template rebind< Key >::other::reference | Key reference type. | 
| Type | Definition | Description | 
| size_type | typename allocator::size_type | Size type. | 
| Method | Description | 
| inline explicit order_statistics_key (const_key_reference r_key = Key()) | Constructor which takes a logical key. | 
| Method | Description | 
| inline operator key_reference () | Conversion to logical key. | 
| inline operator key_type () const | Conversion to logical key. |