Exit Zen
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= Standard Compliance
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:idprefix: compliance_
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:cpp: C++
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Closed-addressing Containers
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`boost::unordered_[multi]set` and `boost::unordered_[multi]map` provide a conformant implementation for {cpp}11 (or later) compilers of the latest standard revision of {cpp} unordered associative containers, with very minor deviations as noted. The containers are fully https://en.cppreference.com/w/cpp/named_req/AllocatorAwareContainer[AllocatorAware^] and support https://en.cppreference.com/w/cpp/named_req/Allocator#Fancy_pointers[fancy pointers^].
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Deduction Guides
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Deduction guides for https://en.cppreference.com/w/cpp/language/class_template_argument_deduction[class template argument deduction (CTAD)^] are only available on {cpp}17 (or later) compilers.
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Piecewise Pair Emplacement
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In accordance with the standard specification, `boost::unordered_[multi]map::emplace` supports piecewise pair construction:
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Additionally, the same functionality is provided via non-standard `boost::unordered::piecewise_construct` and Boost.Tuple:
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This feature has been retained for backwards compatibility with previous versions of Boost.Unordered: users are encouraged to update their code to use `std::piecewise_construct` and ``std::tuple``s instead.
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Swap
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When swapping, `Pred` and `Hash` are not currently swapped by calling `swap`, their copy constructors are used. As a consequence, when swapping an exception may be thrown from their copy constructor.
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Open-addressing Containers
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The C++ standard does not currently provide any open-addressing container specification to adhere to, so `boost::unordered_flat_set`/`unordered_node_set` and `boost::unordered_flat_map`/`unordered_node_map` take inspiration from `std::unordered_set` and `std::unordered_map`, respectively, and depart from their interface where convenient or as dictated by their internal data structure, which is radically different from that imposed by the standard (closed addressing).
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Open-addressing containers provided by Boost.Unordered only work with reasonably compliant C++11 (or later) compilers. Language-level features such as move semantics and variadic template parameters are then not emulated. The containers are fully https://en.cppreference.com/w/cpp/named_req/AllocatorAwareContainer[AllocatorAware^] and support https://en.cppreference.com/w/cpp/named_req/Allocator#Fancy_pointers[fancy pointers^].
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The main differences with C++ unordered associative containers are:
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In general:
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** `begin()` is not constant-time. ** `erase(iterator)` does not return an iterator to the following element, but a proxy object that converts to that iterator if requested; this avoids a potentially costly iterator increment operation when not needed. ** 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. The maximum load, exposed through the public function `max_load`, may decrease on erasure under high-load conditions.
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Flat containers (`boost::unordered_flat_set` and `boost::unordered_flat_map`):
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