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## Class template avl_set_base_hook

boost::intrusive::avl_set_base_hook

## Synopsis

    // In header: <boost/intrusive/avl_set_hook.hpp>

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

      // public member functions
      void swap_nodes(avl_set_base_hook &);
      bool is_linked() const;
      void unlink();
    };

## Description

Derive a class from avl_set_base_hook in order to store objects in in an avl_set/avl_multiset. avl_set_base_hook holds the data necessary to maintain the avl_set/avl_multiset and provides an appropriate value_traits class for avl_set/avl_multiset.

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

`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`).

`optimize_size<>` will tell the hook to optimize the hook for size instead of speed.

### `avl_set_base_hook` public construct/copy/destruct

1.     avl_set_base_hook();

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

   Throws: Nothing.

2.     avl_set_base_hook(const avl_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.     avl_set_base_hook& operator=(const avl_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.     ~avl_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 a set an assertion is raised. If `link_mode` is `auto_unlink` and `is_linked()` is true, the node is unlinked.

   Throws: Nothing.

### `avl_set_base_hook` public member functions

1.     void swap_nodes(avl_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 `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.