Is there a standard C++ Hash Container?

Question

I have a variety of textbooks that note that there wasn't a hash container originally incorporated into the STL, but most also say that most standard library implementations do have some form of hash container since it was a known shortfall.

These books aren't necessarily as up to date as possible, and I got a little confused about what was really "standard" when googling, so:

At this point in time, do most standard library implementations provide a fairly consistent hash container, and if so, is it considered a part of the STL?

What's the "standard" header for this container? (I'm guessing it's #include <hash>, but just in case!).

Is there a hash set and hash map separately defined for use?

See also: http://stackoverflow.com/questions/5653097/stl-hash-functions — Mateen Ulhaq, Jan 01 '12 at 19:56

score 10 · Accepted Answer · answered Aug 24 '11 at 13:26

10

The new standard hash map container is called unordered_map. You can include it in your files by #include <unordered_map>. It is part of the now-approved standard C++11.

Before C++11, you had various things like hash_map which were (widely?) supported by some vendors so you could use them, but if you did, your code wasn't really portable because hash_map wasn't part of the standard. hash_map and all the other vendor-specific versions of a hash table structure shouldn't be used now.

answered Aug 24 '11 at 13:26

Seth Carnegie

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don't forget the quasi standards `boost::unordered_map` and co, which were the experimentation ground for the current standard and thus provide an easy transition. – Matthieu M. Aug 24 '11 at 14:22

score 6 · Answer 2 · answered Aug 24 '11 at 13:26

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See C++11 types:

unordered_map
unordered_set
unordered_multimap
unordered_multiset

answered Aug 24 '11 at 13:26

Didier Trosset

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And their boost counterparts for non C++11 compiler users. – RedX Aug 24 '11 at 13:27

score 2 · Answer 3 · answered Aug 24 '11 at 13:26

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Not in the current standard (03), you can use boost::unordered_map. In the new standard there will be, std::unordered_map.

answered Aug 24 '11 at 13:26

Nim

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score 1 · Answer 4 · answered Aug 24 '11 at 13:29

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The new C++11 standard has hash-equivalent containers. They are not in most implementations at this time, but they have been in TR1 extensions to the standard library, which are provided with some implementations.

answered Aug 24 '11 at 13:29

AndrzejJ

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score 1 · Answer 5 · answered Aug 24 '11 at 13:29

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There are some hash containers in the TR1 namespace. See for example a short intro here: http://drdobbs.com/184402066

answered Aug 24 '11 at 13:29

Anton Daneyko

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The tr1 namespace is already old. With the C++0x version of the standard practically here everything has rolled from tr1 into std. – Martin York Aug 24 '11 at 16:00
I thought C++0x is still in the draft, isn't it? – Anton Daneyko Aug 29 '11 at 15:54
Yes the standard is still in draft but all the good compilers have already moved stuff out of tr1 into std (with the appropriate flag). – Martin York Aug 29 '11 at 15:58

Is there a standard C++ Hash Container?

5 Answers5