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## Class template unordered_set_base_hook

boost::intrusive::unordered_set_base_hook

## Synopsis

    template<class... Options>
    class unordered_set_base_hook {
    public:
      // construct/copy/destruct
      unordered_set_base_hook();
      unordered_set_base_hook(const unordered_set_base_hook &);
      unordered_set_base_hook& operator=(const unordered_set_base_hook &);
      ~unordered_set_base_hook();

      // private member functions
      void swap_nodes(unordered_set_base_hook &) ;
      bool is_linked() const;
      void unlink() ;
    };

## Description

Derive a class from unordered_set_base_hook in order to store objects in in an unordered_set/unordered_multi_set. unordered_set_base_hook holds the data necessary to maintain the unordered_set/unordered_multi_set and provides an appropriate value_traits class for unordered_set/unordered_multi_set.

The hook admits the following options: `tag<>`, `void_pointer<>`, `link_mode<>`, `store_hash<>` and `optimize_multikey<>`.

`tag<>` defines a tag to identify the node. The same tag value can be used in different classes, but if a class is derived from more than one `list_base_hook`, then each `list_base_hook` needs its unique tag.

`void_pointer<>` is the pointer type that will be used internally in the hook and the the container configured to use this hook.

`link_mode<>` will specify the linking mode of the hook (`normal_link`, `auto_unlink` or `safe_link`).

`store_hash<>` will tell the hook to store the hash of the value to speed up rehashings.

`optimize_multikey<>` will tell the hook to store a link to form a group with other value with the same value to speed up searches and insertions in unordered_multisets with a great number of with equivalent keys.

### `unordered_set_base_hook` public construct/copy/destruct

1.     unordered_set_base_hook();

   Effects: If link_mode is `auto_unlink` or `safe_link` initializes the node to an unlinked state.

   Throws: Nothing.

2.     unordered_set_base_hook(const unordered_set_base_hook &);

   Effects: If link_mode is `auto_unlink` or `safe_link` initializes the node to an unlinked state. The argument is ignored.

   Throws: Nothing.

   Rationale: Providing a copy-constructor makes classes using the hook STL-compliant without forcing the user to do some additional work. `swap` can be used to emulate move-semantics.

3.     unordered_set_base_hook& operator=(const unordered_set_base_hook &);

   Effects: Empty function. The argument is ignored.

   Throws: Nothing.

   Rationale: Providing an assignment operator makes classes using the hook STL-compliant without forcing the user to do some additional work. `swap` can be used to emulate move-semantics.

4.     ~unordered_set_base_hook();

   Effects: If link_mode is `normal_link`, the destructor does nothing (ie. no code is generated). If link_mode is `safe_link` and the object is stored in an unordered_set an assertion is raised. If link_mode is `auto_unlink` and `is_linked()` is true, the node is unlinked.

   Throws: Nothing.

### `unordered_set_base_hook` private member functions

1.     void swap_nodes(unordered_set_base_hook & other) ;

   Effects: Swapping two nodes swaps the position of the elements related to those nodes in one or two containers. That is, if the node this is part of the element e1, the node x is part of the element e2 and both elements are included in the containers s1 and s2, then after the swap-operation e1 is in s2 at the position of e2 and e2 is in s1 at the position of e1. If one element is not in a container, then after the swap-operation the other element is not in a container. Iterators to e1 and e2 related to those nodes are invalidated.

   Complexity: Constant

   Throws: Nothing.

2.     bool is_linked() const;

   Precondition: link_mode must be `safe_link` or `auto_unlink`.

   Returns: true, if the node belongs to a container, false otherwise. This function can be used to test whether `unordered_set::iterator_to` will return a valid iterator.

   Complexity: Constant

3.     void unlink() ;

   Effects: Removes the node if it's inserted in a container. This function is only allowed if link_mode is `auto_unlink`.

   Throws: Nothing.