Unordered associative containers (C++): Difference between revisions

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Revision as of 21:44, 23 September 2011

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{{DISPLAYTITLE:unordered_map (C++)}}

{{C++ Standard library}}

'''<code>unordered_map</code>''' is a class template representing a [[hash table]] in the [[C++ Technical Report 1]] (TR1)<ref>{{Citation |title=A Proposal to Add Hash Tables to the Standard Library (revision 4) |author=WG21 |date=9 April 2003 |url=http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1456.html |id=n1456}}</ref> and the upcoming [[C++0x11]] standard<ref name="n3126">{{citation|url=http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3126.pdf |title=Working Draft, Standard for Programming Language C++ |author=WG21 |date=2010-08-21 |id=n3126}}</ref>. It is similar to the <code>[[hash map (C++ class)|hash_map]]</code> class template of the original [[Standard Template Library|STL]]<ref>{{cite web |url= http://www.sgi.com/tech/stl/hash_map.html |title=hash_map<Key, Data, HashFcn, EqualKey, Alloc> |publisher=[[Silicon Graphics|SGI]] |accessdate=26 January 2011}}</ref> which was also included in several implementations of the C++ Standard Library (e.g. in the [[GNU Compiler Collection|GCC's]] [[libstdc++]]<ref>{{cite web |url=http://gcc.gnu.org/onlinedocs/libstdc++/libstdc++-html-USERS-4.1/class____gnu__cxx_1_1hash__map.html |title=libstdc++: hash_map Class Template Reference |accessdate=26 January 2011}}</ref> or with [[Visual C++|MSVC]]).

It is similar to the <code>[[map (C++ container)|map]]</code> class in the [[C++ Standard Library|C++ standard library]] but has different constraints. As its name implies, unlike the <code>map</code> class, the elements of an <code>unordered_map</code> are not [[well ordering|ordered]]. This is due to the use of hashing to store objects. <code>unordered_map</code> can still be [[iterator|iterated]] through like a regular map.