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namespace std _GLIBCXX_VISIBILITY (default)
 

Detailed Description

Function Documentation

namespace std _GLIBCXX_VISIBILITY ( default  )

Primary class template _Hashtable.

Template Parameters
_ValueCopyConstructible type.
_KeyCopyConstructible type.
_AllocAn allocator type ([lib.allocator.requirements]) whose _Alloc::value_type is _Value. As a conforming extension, we allow for _Alloc::value_type != _Value.
_ExtractKeyFunction object that takes an object of type _Value and returns a value of type _Key.
_EqualFunction object that takes two objects of type k and returns a bool-like value that is true if the two objects are considered equal.
_H1The hash function. A unary function object with argument type _Key and result type size_t. Return values should be distributed over the entire range [0, numeric_limits<size_t>:max()].
_H2The range-hashing function (in the terminology of Tavori and Dreizin). A binary function object whose argument types and result type are all size_t. Given arguments r and N, the return value is in the range [0, N).
_HashThe ranged hash function (Tavori and Dreizin). A binary function whose argument types are _Key and size_t and whose result type is size_t. Given arguments k and N, the return value is in the range [0, N). Default: hash(k, N) = h2(h1(k), N). If _Hash is anything other than the default, _H1 and _H2 are ignored.
_RehashPolicyPolicy class with three members, all of which govern the bucket count. _M_next_bkt(n) returns a bucket count no smaller than n. _M_bkt_for_elements(n) returns a bucket count appropriate for an element count of n. _M_need_rehash(n_bkt, n_elt, n_ins) determines whether, if the current bucket count is n_bkt and the current element count is n_elt, we need to increase the bucket count. If so, returns make_pair(true, n), where n is the new bucket count. If not, returns make_pair(false, <anything>)
_TraitsCompile-time class with three boolean std::integral_constant members: __cache_hash_code, __constant_iterators, __unique_keys.

Each _Hashtable data structure has:

  • _Bucket[] _M_buckets
  • _Hash_node_base _M_before_begin
  • size_type _M_bucket_count
  • size_type _M_element_count

with _Bucket being _Hash_node* and _Hash_node containing:

  • _Hash_node* _M_next
  • Tp _M_value
  • size_t _M_hash_code if cache_hash_code is true

In terms of Standard containers the hashtable is like the aggregation of:

  • std::forward_list<_Node> containing the elements
  • std::vector<std::forward_list<_Node>::iterator> representing the buckets

The non-empty buckets contain the node before the first node in the bucket. This design makes it possible to implement something like a std::forward_list::insert_after on container insertion and std::forward_list::erase_after on container erase calls. _M_before_begin is equivalent to std::forward_list::before_begin. Empty buckets contain nullptr. Note that one of the non-empty buckets contains &_M_before_begin which is not a dereferenceable node so the node pointer in a bucket shall never be dereferenced, only its next node can be.

Walking through a bucket's nodes requires a check on the hash code to see if each node is still in the bucket. Such a design assumes a quite efficient hash functor and is one of the reasons it is highly advisable to set __cache_hash_code to true.

The container iterators are simply built from nodes. This way incrementing the iterator is perfectly efficient independent of how many empty buckets there are in the container.

On insert we compute the element's hash code and use it to find the bucket index. If the element must be inserted in an empty bucket we add it at the beginning of the singly linked list and make the bucket point to _M_before_begin. The bucket that used to point to _M_before_begin, if any, is updated to point to its new before begin node.

On erase, the simple iterator design requires using the hash functor to get the index of the bucket to update. For this reason, when __cache_hash_code is set to false the hash functor must not throw and this is enforced by a static assertion.

Functionality is implemented by decomposition into base classes, where the derived _Hashtable class is used in _Map_base, _Insert, _Rehash_base, and _Equality base classes to access the "this" pointer. _Hashtable_base is used in the base classes as a non-recursive, fully-completed-type so that detailed nested type information, such as iterator type and node type, can be used. This is similar to the "Curiously Recurring Template Pattern" (CRTP) technique, but uses a reconstructed, not explicitly passed, template pattern.

Base class templates are:

  • __detail::_Hashtable_base
  • __detail::_Map_base
  • __detail::_Insert
  • __detail::_Rehash_base
  • __detail::_Equality

References __catch, __throw_exception_again, __try, __gnu_parallel::max(), and std::__exception_ptr::swap().

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