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Parameterized by an ordering relation `Compare`
Parameterized by an ordering relation `Compare`
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Unordered Associative Containers
Unordered Associative Containers
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Associative Containers
Associative Containers
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Comparison with Associative Containers
Comparison with Associative Containers
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:idprefix: comparison_
:idprefix: comparison_
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so as to not exceed the maximum load factor. + Because the maximum load factor is defined as the number of elements divided by the total number of available buckets, this function is logically equivalent to: + ``` x.rehash(std::ceil(n / x.max_load_factor())) ``` + See the xref:reference/unordered_map.adoc#unordered_map_rehash[reference for more details] on the `rehash` function.
so as to not exceed the maximum load factor. + Because the maximum load factor is defined as the number of elements divided by the total number of available buckets, this function is logically equivalent to: + ``` x.rehash(std::ceil(n / x.max_load_factor())) ``` + See the xref:reference/unordered_map.adoc#unordered_map_rehash[reference for more details] on the `rehash` function.
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`reserve(n)` reserves space for at least `n` elements, allocating enough buckets
`reserve(n)` reserves space for at least `n` elements, allocating enough buckets
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``` x.reserve(n); ```
``` x.reserve(n); ```
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In a similar manner to using `reserve` for ``vector``s, it can be a good idea to call `reserve` before inserting a large number of elements. This will get the expensive rehashing out of the way and let you store iterators, safe in the knowledge that they won't be invalidated. If you are inserting `n` elements into container `x`, you could first call:
In a similar manner to using `reserve` for ``vector``s, it can be a good idea to call `reserve` before inserting a large number of elements. This will get the expensive rehashing out of the way and let you store iterators, safe in the knowledge that they won't be invalidated. If you are inserting `n` elements into container `x`, you could first call:
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As for pointers and references, they are never invalidated for node-based containers (`boost::unordered_[multi]set`, `boost::unordered_[multi]map`, `boost::unordered_node_set`, `boost::unordered_node_map`), but they will be when rehashing occurs for `boost::unordered_flat_set` and `boost::unordered_flat_map`: this is because these containers store elements directly into their holding buckets, so when allocating a new bucket array the elements must be transferred by means of move construction.
As for pointers and references, they are never invalidated for node-based containers (`boost::unordered_[multi]set`, `boost::unordered_[multi]map`, `boost::unordered_node_set`, `boost::unordered_node_map`), but they will be when rehashing occurs for `boost::unordered_flat_set` and `boost::unordered_flat_map`: this is because these containers store elements directly into their holding buckets, so when allocating a new bucket array the elements must be transferred by means of move construction.
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It is not specified how member functions other than `rehash` and `reserve` affect the bucket count, although `insert` can only invalidate iterators when the insertion causes the container's load to be greater than the maximum allowed. For most implementations this means that `insert` will only change the number of buckets when this happens. Iterators can be invalidated by calls to `insert`, `rehash` and `reserve`.
It is not specified how member functions other than `rehash` and `reserve` affect the bucket count, although `insert` can only invalidate iterators when the insertion causes the container's load to be greater than the maximum allowed. For most implementations this means that `insert` will only change the number of buckets when this happens. Iterators can be invalidated by calls to `insert`, `rehash` and `reserve`.
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Iterator Invalidation
Iterator Invalidation
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There are other differences, which are listed in the xref:regular.adoc#comparison[Comparison with Associative Containers] section.
There are other differences, which are listed in the xref:regular.adoc#comparison[Comparison with Associative Containers] section.
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can be in any order. For example, it might be:
can be in any order. For example, it might be:
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But since the elements aren't ordered, the output of:
But since the elements aren't ordered, the output of:
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For readers without previous experience with hash containers but familiar with normal associative containers (`std::set`, `std::map`, `std::multiset` and `std::multimap`), Boost.Unordered containers are used in a similar manner:
For readers without previous experience with hash containers but familiar with normal associative containers (`std::set`, `std::map`, `std::multiset` and `std::multimap`), Boost.Unordered containers are used in a similar manner:
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Boost.Unordered closed-addressing containers (`boost::unordered_set`, `boost::unordered_map`, `boost::unordered_multiset` and `boost::unordered_multimap`) are fully conformant with the C++ specification for unordered associative containers, so for those who know how to use `std::unordered_set`, `std::unordered_map`, etc., their homonyms in Boost.Unordered are drop-in replacements. The interface of open-addressing containers (`boost::unordered_node_set`, `boost::unordered_node_map`, `boost::unordered_flat_set` and `boost::unordered_flat_map`) is very similar, but they present some minor differences listed in the dedicated xref:compliance.adoc#compliance_open_addressing_containers[standard compliance section].
Boost.Unordered closed-addressing containers (`boost::unordered_set`, `boost::unordered_map`, `boost::unordered_multiset` and `boost::unordered_multimap`) are fully conformant with the C++ specification for unordered associative containers, so for those who know how to use `std::unordered_set`, `std::unordered_map`, etc., their homonyms in Boost.Unordered are drop-in replacements. The interface of open-addressing containers (`boost::unordered_node_set`, `boost::unordered_node_map`, `boost::unordered_flat_set` and `boost::unordered_flat_map`) is very similar, but they present some minor differences listed in the dedicated xref:compliance.adoc#compliance_open_addressing_containers[standard compliance section].
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:idprefix: regular_
:idprefix: regular_
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[#regular] = Regular Containers
[#regular] = Regular Containers
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Parsing of the “doc/modules/ROOT/nav.adoc” file was enforced. 06/05/2026

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