forked from boostorg/unordered
6aa582a90e
https://svn.boost.org/svn/boost/branches/unordered/trunk ........ r43840 | danieljames | 2008-03-24 17:25:07 +0000 (Mon, 24 Mar 2008) | 1 line Fix a g++ warning. ........ r43844 | danieljames | 2008-03-24 17:56:28 +0000 (Mon, 24 Mar 2008) | 1 line It's a new-ish year. ........ r43885 | danieljames | 2008-03-27 20:36:10 +0000 (Thu, 27 Mar 2008) | 1 line The release script doesn't need to copy images and css - because that's now done in the jamfiles. Also tweak the shell script a tad bit. ........ r43890 | danieljames | 2008-03-27 23:01:40 +0000 (Thu, 27 Mar 2008) | 1 line Starting to add a docbook bibliography. ........ r43894 | danieljames | 2008-03-27 23:24:18 +0000 (Thu, 27 Mar 2008) | 1 line Redeclare 'data' in iterator_base to help compilers which have trouble with accessing the nested typedef. ........ [SVN r43895]
76 lines
2.6 KiB
Plaintext
76 lines
2.6 KiB
Plaintext
[/ Copyright 2006-2008 Daniel James.
|
|
/ Distributed under the Boost Software License, Version 1.0. (See accompanying
|
|
/ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) ]
|
|
|
|
[section:hash_equality Equality Predicates and Hash Functions]
|
|
|
|
While the associative containers use an ordering relation to specify how the
|
|
elements are stored, the unordered associative containers use an equality
|
|
predicate and a hash function. For example, [classref boost::unordered_map]
|
|
is declared as:
|
|
|
|
template <
|
|
class Key, class Mapped,
|
|
class Hash = ``[classref boost::hash]``<Key>,
|
|
class Pred = std::equal_to<Key>,
|
|
class Alloc = std::allocator<Key> >
|
|
class ``[classref boost::unordered_map unordered_map]``;
|
|
|
|
The hash function comes first as you might want to change the hash function
|
|
but not the equality predicate. For example, if you wanted to use the
|
|
[@http://www.isthe.com/chongo/tech/comp/fnv/ FNV-1 hash] you could write:
|
|
|
|
[import src_code/dictionary.cpp]
|
|
[case_sensitive_dictionary_fnv]
|
|
|
|
An example implementation of FNV-1, and some other hash functions are supplied
|
|
in the examples directory.
|
|
|
|
If you wish to use a different equality function,
|
|
you will also need to use a matching hash function. For
|
|
example, to implement a case insensitive dictionary you need to define a
|
|
case insensitive equality predicate and hash function:
|
|
|
|
[case_insensitive_functions]
|
|
|
|
Which you can then use in a case insensitive dictionary:
|
|
|
|
[case_insensitive_dictionary]
|
|
|
|
This is a simplified version of the example at
|
|
[@../../libs/unordered/examples/case_insensitive.hpp /libs/unordered/examples/case_insensitive.hpp]
|
|
which supports other locales and string types.
|
|
|
|
[h2 Custom Types]
|
|
|
|
Similarly, a custom hash function can be used for custom types:
|
|
|
|
[import src_code/point1.cpp]
|
|
[point_example1]
|
|
|
|
Since the default hash function is [link hash Boost.Hash],
|
|
we can [link hash.custom extend it to support the type]
|
|
so that the hash function doesn't need to be explicitly given:
|
|
|
|
[import src_code/point2.cpp]
|
|
[point_example2]
|
|
|
|
See the [link hash.custom Boost.Hash documentation] for more detail on how to
|
|
do this. Remember that it relies on extensions to the draft standard - so it
|
|
won't work on other implementations of the unordered associative containers.
|
|
|
|
[table Methods for accessing the hash and equality functions.
|
|
[[Method] [Description]]
|
|
|
|
[
|
|
[``hasher hash_function() const``]
|
|
[Returns the container's hash function.]
|
|
]
|
|
[
|
|
[``key_equal key_eq() const``]
|
|
[Returns the container's key equality function.]
|
|
]
|
|
]
|
|
|
|
[endsect]
|