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102 lines
4.0 KiB
Plaintext
102 lines
4.0 KiB
Plaintext
[/ Copyright 2006-2008 Daniel James.
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/ Distributed under the Boost Software License, Version 1.0. (See accompanying
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/ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) ]
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[def __hash-table__ [@http://en.wikipedia.org/wiki/Hash_table
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hash table]]
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[def __hash-function__ [@http://en.wikipedia.org/wiki/Hash_function
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hash function]]
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[section:intro Introduction]
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For accessing data based on key lookup, the C++ standard library offers `std::set`,
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`std::map`, `std::multiset` and `std::multimap`. These are generally
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implemented using balanced binary trees so that lookup time has
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logarithmic complexity. That is generally okay, but in many cases a
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__hash-table__ can perform better, as accessing data has constant complexity,
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on average. The worst case complexity is linear, but that occurs rarely and
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with some care, can be avoided.
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Also, the existing containers require a 'less than' comparison object
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to order their elements. For some data types this is impossible to implement
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or isn't practical. In contrast, a hash table only needs an equality function
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and a hash function for the key.
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With this in mind, unordered associative containers were added to the C++
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standard. This is an implementation of the containers described in C++11,
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with some [link unordered.compliance deviations from the standard] in
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order to work with non-C++11 compilers and libraries.
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`unordered_set` and `unordered_multiset` are defined in the header
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<[headerref boost/unordered_set.hpp]>
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namespace boost {
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template <
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class Key,
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class Hash = ``[classref boost::hash]``<Key>,
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class Pred = std::equal_to<Key>,
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class Alloc = std::allocator<Key> >
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class ``[classref boost::unordered_set unordered_set]``;
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template<
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class Key,
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class Hash = ``[classref boost::hash]``<Key>,
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class Pred = std::equal_to<Key>,
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class Alloc = std::allocator<Key> >
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class ``[classref boost::unordered_multiset unordered_multiset]``;
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}
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`unordered_map` and `unordered_multimap` are defined in the header
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<[headerref boost/unordered_map.hpp]>
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namespace boost {
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template <
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class Key, class Mapped,
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class Hash = ``[classref boost::hash]``<Key>,
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class Pred = std::equal_to<Key>,
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class Alloc = std::allocator<std::pair<Key const, Mapped> > >
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class ``[classref boost::unordered_map unordered_map]``;
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template<
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class Key, class Mapped,
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class Hash = ``[classref boost::hash]``<Key>,
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class Pred = std::equal_to<Key>,
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class Alloc = std::allocator<std::pair<Key const, Mapped> > >
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class ``[classref boost::unordered_multimap unordered_multimap]``;
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}
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When using Boost.TR1, these classes are included from `<unordered_set>` and
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`<unordered_map>`, with the classes added to the `std::tr1` namespace.
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The containers are used in a similar manner to the normal associative
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containers:
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[import src_code/intro.cpp]
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[intro_example1_2]
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But since the elements aren't ordered, the output of:
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[intro_example1_3]
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can be in any order. For example, it might be:
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two,2
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one,1
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three,3
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To store an object in an unordered associative container requires both a
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key equality function and a hash function. The default function objects in
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the standard containers support a few basic types including integer types,
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floating point types, pointer types, and the standard strings. Since
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Boost.Unordered uses [classref boost::hash] it also supports some other types,
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including standard containers. To use any types not supported by these methods
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you have to [link hash.custom extend Boost.Hash to support the type] or use
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your own custom equality predicates and hash functions. See the
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[link unordered.hash_equality Equality Predicates and Hash Functions] section
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for more details.
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There are other differences, which are listed in the
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[link unordered.comparison Comparison with Associative Containers] section.
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[endsect]
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