Exit Zen
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:idprefix: concurrent_flat_map_
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xref:regular.adoc[]
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```cpp template < class Key, class Mapped, class Hash = boost::hash<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<std::pair<Key const, Mapped> > > class unordered_map; ```
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image::buckets.png[]
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xref:concurrent.adoc[]
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`boost::hash_is_avalanching` in `<boost/container_hash/hash_is_avalanching.hpp>`. Use that header directly instead. `<boost/unordered/hash_traits.hpp>` will be removed in the future.
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image::bucket-groups.png[align=center]
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xref:hash_quality.adoc[]
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The first thing a new user of Boost.Unordered concurrent containers will notice is that these classes _do not provide iterators_ (which makes them technically not https://en.cppreference.com/w/cpp/named_req/Container[Containers^] in the C++ standard sense). The reason for this is that iterators are inherently thread-unsafe. Consider this hypothetical code:
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- `begin()` is not constant-time. - There is no API for bucket handling (except `bucket_count`). - The maximum load factor of the container is managed internally and can't be set by the user.
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- `value_type` must be move-constructible. - Pointer stability is not kept under rehashing. - `begin()` is not constant-time. - There is no API for bucket handling (except `bucket_count`) or node extraction/insertion. - The maximum load factor of the container is managed internally and can't be set by the user.
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There is an link:../../../examples/fnv1.hpp[implementation of FNV-1a^] in the examples directory.
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^.^h|*Closed addressing* ^m| boost::unordered_set + boost::unordered_map + boost::unordered_multiset + boost::unordered_multimap ^|
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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:
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The visualizations mirror those for the standard unordered containers. A container has a maximum of 100 elements displayed at once. Each set element has its item name listed as `[i]`, where `i` is the index in the display, starting at `0`. Each map element has its item name listed as `[\{key-display}]` by default. For example, if the first element is the pair `("abc", 1)`, the item name will be `["abc"]`. This behaviour can be overridden by using the view "ShowElementsByIndex", which switches the map display behaviour to name the elements by index. This same view name is used in the standard unordered containers.
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