[#BloombergLP-bdlc-FlatHashMap]
= xref:BloombergLP.adoc[BloombergLP]::xref:BloombergLP/bdlc.adoc[bdlc]::FlatHashMap
:relfileprefix: ../../
:mrdocs:

Unordered map of unique keys to mapped values with open‐addressed storage.

== Synopsis

Declared in `<bdlc_flathashmap.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<
    class KEY,
    class VALUE,
    class HASH = xref:BloombergLP/bslh/FibonacciBadHashWrapper.adoc[bslh::FibonacciBadHashWrapper<bsl::hash<KEY>>],
    class EQUAL = xref:bsl/equal_to-0d.adoc[bsl::equal_to<KEY>]>
class FlatHashMap;
----

== Description

This class template implements a value‐semantic container type holding an unordered map of `KEY‐VALUE` pairs having unique keys that provides a mapping from keys of (template parameter) type `KEY` to their associated mapped values of (template parameter) type `VALUE`. The (template parameter) type `HASH` is a functor providing the hash value for `KEY`. The (template parameter) type `EQUAL` is a functor providing the equality function for two `KEY` values. See {Requirements on `KEY`, `HASH`, and `EQUAL`} for more information.

== Type Aliases

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bdlc/FlatHashMap/const_iterator.adoc[`const_iterator`] 
| Iterator over non‐modifiable elements.
| xref:BloombergLP/bdlc/FlatHashMap/const_pointer.adoc[`const_pointer`] 
| Pointer to a non‐modifiable value.
| xref:BloombergLP/bdlc/FlatHashMap/const_reference.adoc[`const_reference`] 
| Reference to a non‐modifiable value.
| xref:BloombergLP/bdlc/FlatHashMap/difference_type.adoc[`difference_type`] 
| Signed type used for iterator distances.
| xref:BloombergLP/bdlc/FlatHashMap/hasher.adoc[`hasher`] 
| Hash functor type.
| xref:BloombergLP/bdlc/FlatHashMap/iterator.adoc[`iterator`] 
| Iterator over modifiable elements.
| xref:BloombergLP/bdlc/FlatHashMap/key_compare.adoc[`key_compare`] 
| Key equality predicate type.
| xref:BloombergLP/bdlc/FlatHashMap/key_type.adoc[`key_type`] 
| Key type of this map.
| xref:BloombergLP/bdlc/FlatHashMap/mapped_type.adoc[`mapped_type`] 
| Mapped value type of this map.
| xref:BloombergLP/bdlc/FlatHashMap/pointer.adoc[`pointer`] 
| Pointer to a modifiable value.
| xref:BloombergLP/bdlc/FlatHashMap/reference.adoc[`reference`] 
| Reference to a modifiable value.
| xref:BloombergLP/bdlc/FlatHashMap/size_type.adoc[`size_type`] 
| Unsigned type used for sizes and capacities.
| xref:BloombergLP/bdlc/FlatHashMap/value_type.adoc[`value_type`] 
| Element type stored by this map (`const KEY`, `VALUE` pair).
|===


== Member Functions

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bdlc/FlatHashMap/2constructor-056.adoc[`FlatHashMap`]         [.small]#[constructor]#
| Constructors
| xref:BloombergLP/bdlc/FlatHashMap/2destructor.adoc[`~FlatHashMap`] [.small]#[destructor]#
| Destroy this object and each of its elements.
| xref:BloombergLP/bdlc/FlatHashMap/operator_assign-04.adoc[`operator=`] 
| Assignment operators
| xref:BloombergLP/bdlc/FlatHashMap/allocator.adoc[`allocator`] 
| Return the allocator used by this flat hash map to supply memory.
| xref:BloombergLP/bdlc/FlatHashMap/at-09.adoc[`at`] 
| `at` overloads
| xref:BloombergLP/bdlc/FlatHashMap/begin-06.adoc[`begin`] 
| `begin` overloads
| xref:BloombergLP/bdlc/FlatHashMap/capacity.adoc[`capacity`] 
| Return the number of elements this map could hold if the load factor were 1.
| xref:BloombergLP/bdlc/FlatHashMap/cbegin.adoc[`cbegin`] 
| Return a `const_iterator` to the first element in the sequence of elements maintained by this map, or the `end` iterator if this map is empty.
| xref:BloombergLP/bdlc/FlatHashMap/cend.adoc[`cend`] 
| Return a `const_iterator` to the past‐the‐end element in the sequence of elements maintained by this map.
| xref:BloombergLP/bdlc/FlatHashMap/clear.adoc[`clear`] 
| Remove all elements from this map.
| xref:BloombergLP/bdlc/FlatHashMap/contains-09.adoc[`contains`] 
| Return whether this map contains an element with `key`.
| xref:BloombergLP/bdlc/FlatHashMap/count-0d.adoc[`count`] 
| Return the number of elements with the specified `key`.
| xref:BloombergLP/bdlc/FlatHashMap/emplace.adoc[`emplace`] 
| Emplace a newly constructed element into this map.
| xref:BloombergLP/bdlc/FlatHashMap/emplace_hint.adoc[`emplace_hint`] 
| Emplace a newly constructed element using a placement hint.
| xref:BloombergLP/bdlc/FlatHashMap/empty.adoc[`empty`] 
| Return `true` if this map contains no elements, and `false` otherwise.
| xref:BloombergLP/bdlc/FlatHashMap/end-03.adoc[`end`] 
| `end` overloads
| xref:BloombergLP/bdlc/FlatHashMap/equal_range-02.adoc[`equal_range`] 
| `equal_range` overloads
| xref:BloombergLP/bdlc/FlatHashMap/erase-04b.adoc[`erase`] 
| `erase` overloads
| xref:BloombergLP/bdlc/FlatHashMap/find-04.adoc[`find`] 
| Return an iterator to the element with the specified `key`.
| xref:BloombergLP/bdlc/FlatHashMap/hash_function.adoc[`hash_function`] 
| Return (a copy of) the unary hash functor used by this map to generate a hash value (of type `bsl::size_t`) for a `KEY` object.
| xref:BloombergLP/bdlc/FlatHashMap/insert-08.adoc[`insert`] 
| `insert` overloads
| xref:BloombergLP/bdlc/FlatHashMap/insert_or_assign-0f.adoc[`insert_or_assign`] 
| `insert_or_assign` overloads
| xref:BloombergLP/bdlc/FlatHashMap/key_eq.adoc[`key_eq`] 
| Return (a copy of) the binary key‐equality functor that returns `true` if the value of two `KEY` objects are equivalent, and `false` otherwise.
| xref:BloombergLP/bdlc/FlatHashMap/load_factor.adoc[`load_factor`] 
| Return the current ratio between the number of elements in this container and its capacity.
| xref:BloombergLP/bdlc/FlatHashMap/max_load_factor.adoc[`max_load_factor`] 
| Return the maximum load factor allowed for this map.
| xref:BloombergLP/bdlc/FlatHashMap/operator_subs-03.adoc[`operator[]`] 
| Return a modifiable reference to the mapped value for `key`.
| xref:BloombergLP/bdlc/FlatHashMap/print.adoc[`print`] 
| Format this map to the specified output `stream`.
| xref:BloombergLP/bdlc/FlatHashMap/rehash.adoc[`rehash`] 
| Rehash this map to at least the specified `minimumCapacity`.
| xref:BloombergLP/bdlc/FlatHashMap/reserve.adoc[`reserve`] 
| Reserve capacity for at least the specified `numEntries`.
| xref:BloombergLP/bdlc/FlatHashMap/reset.adoc[`reset`] 
| Remove all elements from this map and release all memory from this map, returning the map to the default constructed state.
| xref:BloombergLP/bdlc/FlatHashMap/size.adoc[`size`] 
| Return the number of elements in this map.
| xref:BloombergLP/bdlc/FlatHashMap/swap.adoc[`swap`] 
| Exchange the value of this map with that of `other`.
| xref:BloombergLP/bdlc/FlatHashMap/try_emplace-03.adoc[`try_emplace`] 
| `try_emplace` overloads
|===


== Friends

[cols="1,4"]
|===
| Name| Description
| `xref:BloombergLP/bdlc/swap-05.adoc[BloombergLP::bdlc::swap]`
| Exchange the contents of the two maps.
| `xref:BloombergLP/bdlc/operator_not_eq-06.adoc[BloombergLP::bdlc::operator!=]`
| Return whether the maps differ in key/value contents.
| `xref:BloombergLP/bdlc/operator_eq-04c.adoc[BloombergLP::bdlc::operator==]`
| Return whether the maps have the same key/value contents.
|===

== Non-Member Functions

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bdlc/operator_not_eq-0b1.adoc[`operator!=`]
| Return whether `lhs` and `rhs` do not have the same value.
| xref:BloombergLP/bdlc/operator_eq-0d9.adoc[`operator==`]
| Return whether `lhs` and `rhs` have the same value.
| xref:BloombergLP/bdlc/swap-061.adoc[`swap`]
| Exchange the value of `a` with that of `b`.
|===


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