Exit Zen
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xref:reference/unordered_multimap.adoc[`unordered_multimap`]
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Custom Types
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Storing an object in an unordered associative container requires both a key equality function and a hash function. The default function objects in the standard containers support a few basic types including integer types, floating point types, pointer types, and the standard strings. Since Boost.Unordered uses link:../../../../container_hash/index.html[boost::hash^] it also supports some other types, including standard containers. To use any types not supported by these methods you have to extend Boost.Hash to support the type or use your own custom equality predicates and hash functions. See the xref:hash_equality.adoc#hash_equality[Equality Predicates and Hash Functions], section for more details.
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The main differences with C++ unordered associative containers are:
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An link:../../../benchmark/string_stats.cpp[example^] is provided that displays container statistics for `boost::hash<std::string>`, an implementation of the https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function#FNV-1a_hash[FNV-1a hash^] and two ill-behaved custom hash functions that have been incorrectly marked as avalanching:
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Associative Containers
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Added GDB pretty-printers for all containers and iterators. For a container with an allocator that uses fancy pointers, these only work if the proper pretty-printer is written for the fancy pointer type itself.
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Always, when using pretty-printers, you must enable pretty-printing like below. This is typically a one-time setup.
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In the absence of iterators, `visit_all` is provided as an alternative way to process all the elements in the container:
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A binary function object that induces an equivalence relation on values of type `Key`. It takes two arguments of type `Key` and returns a value of type `bool`.
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image::benchmarks-set/gcc/running insertion.xlsx.practice norehash non-unique 5.png[width=250,link=_images/benchmarks-set/gcc/running insertion.xlsx.practice norehash non-unique 5.png,window=_blank]
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So closed addressing is used.
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The elements are organized into buckets. Keys with the same hash code are stored in the same bucket.
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An upper bound on the number of buckets.
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An allocator whose value type is the same as the container's value type.
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A binary function object that induces an equivalence relation on values of type `Key`. It takes two arguments of type `Key` and returns a value of type `bool`.
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Open-addressing Containers
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xref:reference/header_unordered_flat_set_fwd.adoc[+++<code style="color: inherit;">+++<boost/unordered/unordered_flat_set_fwd.hpp>+++</code>+++ Synopsis]
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A binary function object that induces an equivalence relation on values of type `Key`. It takes two arguments of type `Key` and returns a value of type `bool`.
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_Pred_
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