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95 lines
3.0 KiB
Plaintext
95 lines
3.0 KiB
Plaintext
[def __tr1__
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[@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2009.pdf
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C++ Standard Library Technical Report]]
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[def __draft__
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[@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2009.pdf
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Working Draft of the C++ Standard]]
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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 keys, 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 lookup time has
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logarithmic complexity. Which 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 practicle. For a hash table you need an equality function
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and a hash function for the key.
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So the __tr1__ introduced the unordered associative containers, which are
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implemented using hash tables, and they have now been added to the __draft__.
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There are four containers to match the existing
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associate containers. In the header <[headerref boost/unordered_set.hpp]>:
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template <
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class Key,
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class Hash = 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 = 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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and in <[headerref boost/unordered_map.hpp]>:
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template <
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class Key, class T,
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class Hash = 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_map unordered_map]``;
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template<
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class Key, class T,
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class Hash = 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_multimap unordered_multimap]``;
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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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#include <``[headerref boost/unordered_map.hpp]``>
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#include <cassert>
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int main()
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{
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boost::unordered_map<std::string, int> x;
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x["one"] = 1;
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x["two"] = 2;
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x["three"] = 3;
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assert(x["one"] == 1);
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assert(x["missing"] == 0);
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}
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But since the elements aren't ordered, the output of:
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BOOST_FOREACH(map::value_type i, x) {
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std::cout<<i.first<<","<<i.second<<"\n";
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}
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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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missing,0
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There are other differences, which will be detailed later.
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[endsect]
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