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87 lines
3.2 KiB
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87 lines
3.2 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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[section:hash_equality Equality Predicates and Hash Functions]
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While the associative containers use an ordering relation to specify how the
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elements are stored, the unordered associative containers use an equality
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predicate and a hash function. For example, [classref boost::unordered_map]
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is declared as:
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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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The hash function comes first as you might want to change the hash function
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but not the equality predicate. For example, if you wanted to use the
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[@http://www.isthe.com/chongo/tech/comp/fnv/ FNV-1 hash] you could write:
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[import src_code/dictionary.cpp]
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[case_sensitive_dictionary_fnv]
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There is an [@boost:/libs/unordered/examples/fnv1.hpp implementation
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of FNV-1] in the examples directory.
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If you wish to use a different equality function,
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you will also need to use a matching hash function. For
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example, to implement a case insensitive dictionary you need to define a
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case insensitive equality predicate and hash function:
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[case_insensitive_functions]
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Which you can then use in a case insensitive dictionary:
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[case_insensitive_dictionary]
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This is a simplified version of the example at
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[@boost:/libs/unordered/examples/case_insensitive.hpp /libs/unordered/examples/case_insensitive.hpp]
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which supports other locales and string types.
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[caution
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Be careful when using the equality (`==`) operator with custom equality
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predicates, especially if you're using a function pointer. If you compare two
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containers with different equality predicates then the result is undefined.
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For most stateless function objects this is impossible - since you can only
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compare objects with the same equality predicate you know the equality
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predicates must be equal. But if you're using function pointers or a stateful
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equality predicate (e.g. boost::function) then you can get into trouble.
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]
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[h2 Custom Types]
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Similarly, a custom hash function can be used for custom types:
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[import src_code/point1.cpp]
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[point_example1]
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Since the default hash function is [link hash Boost.Hash],
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we can [link hash.custom extend it to support the type]
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so that the hash function doesn't need to be explicitly given:
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[import src_code/point2.cpp]
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[point_example2]
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See the [link hash.custom Boost.Hash documentation] for more detail on how to
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do this. Remember that it relies on extensions to the draft standard - so it
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won't work for other implementations of the unordered associative containers,
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you'll need to explicitly use Boost.Hash.
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[table:access_methods Methods for accessing the hash and equality functions.
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[[Method] [Description]]
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[
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[`hasher hash_function() const`]
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[Returns the container's hash function.]
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]
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[
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[`key_equal key_eq() const`]
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[Returns the container's key equality function.]
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]
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]
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[endsect]
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