Exit Zen
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`Key` must be https://en.cppreference.com/w/cpp/named_req/MoveInsertable[MoveInsertable^] into the container
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`std::pair<const Key, T>` must be https://en.cppreference.com/w/cpp/named_req/EmplaceConstructible[EmplaceConstructible^] into the table from any `std::pair` object convertible to it, and it also must be https://en.cppreference.com/w/cpp/named_req/Erasable[Erasable^] from the table.
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_Key_
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_Pred_
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_Key_
3/100 · 5
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_Hash_
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Provides statistics on insertion, successful and unsuccessful lookups performed by a container. If the supplied hash function has good quality, then:
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xref:reference/header_unordered_map_fwd.adoc[`<boost/unordered/unordered_map_fwd.hpp>`]
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Which you can then use in a case insensitive dictionary: ```cpp boost::unordered_map<std::string, int, ihash, iequal_to> idictionary; ```
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(more than 2 times faster in typical scenarios) while slightly diverging from the standard interface to accommodate the implementation. There are two variants: **flat** (the fastest) and **node-based**, which provide pointer stability under rehashing at the expense of being slower.
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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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Average probe lengths should be close to 1.0.
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`reserve(n)` reserves space for at least `n` elements, allocating enough buckets
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allow for visitation of an element right after insertion (by contrast, `insert_or_visit(x, f)` only visits the element if insertion did _not_ take place).
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The container visualizations also work if you are using fancy pointers in your allocator, such as `boost::interprocess::offset_ptr`. While this is rare, Boost.Unordered has natvis customization points to support any type of fancy pointer. `boost::interprocess::offset_ptr` has support already defined in the Boost.Interprocess library, and you can add support to your own type by following the instructions contained in a comment near the end of the file link:https://github.com/boostorg/unordered/blob/develop/extra/boost_unordered.natvis[/extra/boost_unordered.natvis].
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Note that in this last example the visitation function could actually _modify_ the element: as a general rule, operations on a concurrent map `m` will grant visitation functions const/non-const access to the element depending on whether `m` is const/non-const. Const access can be always be explicitly requested by using `cvisit` overloads (for instance, `insert_or_cvisit`) and may result in higher parallelization. For concurrent sets, on the other hand, visitation is always const access.
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and https://en.cppreference.com/w/cpp/named_req/Erasable[Erasable^] from the container.
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image::benchmarks-set/gcc/running insertion.xlsx.practice non-unique 5.png[width=250,link=_images/benchmarks-set/gcc/running insertion.xlsx.practice non-unique 5.png,window=_blank]
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There are also the restrictions on when iterators can be invalidated. Since
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A binary function object that implements an equivalence relation on values of type `Key`. 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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