1 | |
---|
2 | [/ Copyright 2005-2006 Daniel James. |
---|
3 | / Distributed under the Boost Software License, Version 1.0. (See accompanying |
---|
4 | / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) ] |
---|
5 | |
---|
6 | [def __multi-index-short__ [@../../libs/multi_index/doc/index.html |
---|
7 | Boost.MultiIndex]] |
---|
8 | |
---|
9 | [section:tutorial Tutorial] |
---|
10 | |
---|
11 | When using a hash index with __multi-index-short__, you don't need to do |
---|
12 | anything to use [classref boost::hash] as it uses it by default. |
---|
13 | To find out how to use a user-defined type, read the |
---|
14 | [link hash.custom section on extending boost::hash for a custom data type]. |
---|
15 | |
---|
16 | If your standard library supplies its own implementation of the unordered |
---|
17 | associative containers and you wish to use |
---|
18 | [classref boost::hash], just use an extra template parameter: |
---|
19 | |
---|
20 | std::unordered_multiset<int, ``[classref boost::hash]``<int> > |
---|
21 | set_of_ints; |
---|
22 | |
---|
23 | std::unordered_set<std::pair<int, int>, ``[classref boost::hash]``<std::pair<int, int> > |
---|
24 | set_of_pairs; |
---|
25 | |
---|
26 | std::unordered_map<int, std::string, ``[classref boost::hash]``<int> > map_int_to_string; |
---|
27 | |
---|
28 | To use [classref boost::hash] directly, create an instance and call it as a function: |
---|
29 | |
---|
30 | #include <``[headerref boost/functional/hash.hpp]``> |
---|
31 | |
---|
32 | int main() |
---|
33 | { |
---|
34 | ``[classref boost::hash]``<std::string> string_hash; |
---|
35 | |
---|
36 | std::size_t h = string_hash("Hash me"); |
---|
37 | } |
---|
38 | |
---|
39 | For an example of generic use, here is a function to generate a vector |
---|
40 | containing the hashes of the elements of a container: |
---|
41 | |
---|
42 | template <class Container> |
---|
43 | std::vector<std::size_t> get_hashes(Container const& x) |
---|
44 | { |
---|
45 | std::vector<std::size_t> hashes; |
---|
46 | std::transform(x.begin(), x.end(), std::insert_iterator(hashes), |
---|
47 | ``[classref boost::hash]``<typename Container::value_type>()); |
---|
48 | |
---|
49 | return hashes; |
---|
50 | } |
---|
51 | |
---|
52 | [endsect] |
---|
53 | |
---|
54 | [section:custom Extending boost::hash for a custom data type] |
---|
55 | |
---|
56 | [classref boost::hash] is implemented by calling the function |
---|
57 | [funcref boost::hash_value hash_value]. |
---|
58 | The namespace isn't specified so that it can detect overloads via argument |
---|
59 | dependant lookup. So if there is a free function `hash_value` in the same |
---|
60 | namespace as a custom type, it will get called. |
---|
61 | |
---|
62 | If you have a structure `library::book`, where each `book` is uniquely |
---|
63 | defined by it's member `id`: |
---|
64 | |
---|
65 | namespace library |
---|
66 | { |
---|
67 | struct book |
---|
68 | { |
---|
69 | int id; |
---|
70 | std::string author; |
---|
71 | std::string title; |
---|
72 | |
---|
73 | // .... |
---|
74 | }; |
---|
75 | |
---|
76 | bool operator==(book const& a, book const& b) |
---|
77 | { |
---|
78 | return a.id == b.id; |
---|
79 | } |
---|
80 | } |
---|
81 | |
---|
82 | Then all you would need to do is write the function `library::hash_value`: |
---|
83 | |
---|
84 | namespace library |
---|
85 | { |
---|
86 | std::size_t hash_value(book const& b) |
---|
87 | { |
---|
88 | ``[classref boost::hash]``<int> hasher; |
---|
89 | return hasher(b.id); |
---|
90 | } |
---|
91 | } |
---|
92 | |
---|
93 | And you can now use [classref boost::hash] with book: |
---|
94 | |
---|
95 | library::book knife(3458, "Zane Grey", "The Hash Knife Outfit"); |
---|
96 | library::book dandelion(1354, "Paul J. Shanley", |
---|
97 | "Hash & Dandelion Greens"); |
---|
98 | |
---|
99 | ``[classref boost::hash]``<library::book> book_hasher; |
---|
100 | std::size_t knife_hash_value = book_hasher(knife); |
---|
101 | |
---|
102 | // If std::unordered_set is available: |
---|
103 | std::unordered_set<library::book, ``[classref boost::hash]``<library::book> > books; |
---|
104 | books.insert(knife); |
---|
105 | books.insert(library::book(2443, "Lindgren, Torgny", "Hash")); |
---|
106 | books.insert(library::book(1953, "Snyder, Bernadette M.", |
---|
107 | "Heavenly Hash: A Tasty Mix of a Mother's Meditations")); |
---|
108 | |
---|
109 | assert(books.find(knife) != books.end()); |
---|
110 | assert(books.find(dandelion) == books.end()); |
---|
111 | |
---|
112 | The full example can be found in: |
---|
113 | [@../../libs/functional/hash/examples/books.cpp /libs/functional/hash/examples/books.hpp] |
---|
114 | and |
---|
115 | [@../../libs/functional/hash/examples/books.cpp /libs/functional/hash/examples/books.cpp]. |
---|
116 | |
---|
117 | [tip |
---|
118 | When writing a hash function, first look at how the equality function works. |
---|
119 | Objects that are equal must generate the same hash value. |
---|
120 | When objects are not equal they should generate different hash values. |
---|
121 | In this object equality was based just on the id so the hash function |
---|
122 | only hash the id. If it was based on the objects name and author |
---|
123 | then the hash function should take them into account |
---|
124 | (how to do this is discussed in the next section). |
---|
125 | ] |
---|
126 | |
---|
127 | [endsect] |
---|
128 | |
---|
129 | [section:combine Combining hash values] |
---|
130 | |
---|
131 | Say you have a point class, representing a two dimensional location: |
---|
132 | |
---|
133 | class point |
---|
134 | { |
---|
135 | int x; |
---|
136 | int y; |
---|
137 | public: |
---|
138 | point() : x(0), y(0) {} |
---|
139 | point(int x, int y) : x(x), y(y) {} |
---|
140 | |
---|
141 | bool operator==(point const& other) const |
---|
142 | { |
---|
143 | return x == other.x && y == other.y; |
---|
144 | } |
---|
145 | }; |
---|
146 | |
---|
147 | and you wish to use it as the key for an `unordered_map`. You need to |
---|
148 | customise the hash for this structure. To do this we need to combine |
---|
149 | the hash values for `x` and `y`. The function |
---|
150 | [funcref boost::hash_combine] is supplied for this purpose: |
---|
151 | |
---|
152 | class point |
---|
153 | { |
---|
154 | ... |
---|
155 | |
---|
156 | friend std::size_t hash_value(point const& p) |
---|
157 | { |
---|
158 | std::size_t seed = 0; |
---|
159 | ``[funcref boost::hash_combine]``(seed, p.x); |
---|
160 | ``[funcref boost::hash_combine]``(seed, p.y); |
---|
161 | |
---|
162 | return seed; |
---|
163 | } |
---|
164 | |
---|
165 | ... |
---|
166 | }; |
---|
167 | |
---|
168 | Calls to hash_combine incrementally build the hash from the different members |
---|
169 | of point, it can be repeatedly called for any number of elements. It calls |
---|
170 | [funcref boost::hash_value hash_value] on the supplied element, and combines it with the seed. |
---|
171 | |
---|
172 | Full code for this example is at |
---|
173 | [@../../libs/functional/hash/examples/point.cpp /libs/functional/hash/examples/point.cpp]. |
---|
174 | |
---|
175 | [note |
---|
176 | When using [funcref boost::hash_combine] the order of the |
---|
177 | calls matters. |
---|
178 | ''' |
---|
179 | <programlisting> |
---|
180 | std::size_t seed = 0; |
---|
181 | boost::hash_combine(seed, 1); |
---|
182 | boost::hash_combine(seed, 2); |
---|
183 | </programlisting> |
---|
184 | results in a different seed to: |
---|
185 | <programlisting> |
---|
186 | std::size_t seed = 0; |
---|
187 | boost::hash_combine(seed, 2); |
---|
188 | boost::hash_combine(seed, 1); |
---|
189 | </programlisting> |
---|
190 | ''' |
---|
191 | If you are calculating a hash value for data where the order of the data |
---|
192 | doesn't matter in comparisons (e.g. a set) you will have to ensure that the |
---|
193 | data is always supplied in the same order. |
---|
194 | ] |
---|
195 | |
---|
196 | To calculate the hash of an iterator range you can use [funcref boost::hash_range]: |
---|
197 | |
---|
198 | std::vector<std::string> some_strings; |
---|
199 | std::size_t hash = ``[funcref boost::hash_range]``(some_strings.begin(), some_strings.end()); |
---|
200 | |
---|
201 | [endsect] |
---|
202 | |
---|