NAME¶

std::unordered_map - std::unordered_map

Synopsis¶

Defined in header <unordered_map>
template<

class Key,
class T,
class Hash = std::hash<Key>, (1) (since C++11)
class KeyEqual = std::equal_to<Key>,
class Allocator = std::allocator<std::pair<const Key, T>>

> class unordered_map;
namespace pmr {

template<
class Key,
class T,
class Hash = std::hash<Key>,
class KeyEqual = std::equal_to<Key> (2) (since C++17)
> using unordered_map =
std::unordered_map<Key, T, Hash, KeyEqual,
std::pmr::polymorphic_allocator<std::pair<const Key,
T>>>;

}

std::unordered_map is an associative container that contains key-value pairs with
unique keys. Search, insertion, and removal of elements have average constant-time
complexity.

Internally, the elements are not sorted in any particular order, but organized into
buckets. Which bucket an element is placed into depends entirely on the hash of its
key. Keys with the same hash code appear in the same bucket. This allows fast access
to individual elements, since once the hash is computed, it refers to the exact
bucket the element is placed into.

Two keys are considered equivalent if the map's key equality predicate returns true
when passed those keys. If two keys are equivalent, the hash function must return
the same value for both keys.

std::unordered_map meets the requirements of Container, AllocatorAwareContainer,
UnorderedAssociativeContainer.

Notes¶

* The swap functions do not invalidate any of the iterators inside the container,
but they do invalidate the iterator marking the end of the swap region.
* References and pointers to either key or data stored in the container are only
invalidated by erasing that element, even when the corresponding iterator is
invalidated.

Template parameters¶

This section is incomplete
Reason: Add descriptions of the template parameters.

Member types¶

Member type Definition
key_type Key
mapped_type T
value_type std::pair<const Key, T>
size_type Unsigned integer type (usually std::size_t)
difference_type Signed integer type (usually std::ptrdiff_t)
hasher Hash
key_equal KeyEqual
allocator_type Allocator
reference value_type&
const_reference const value_type&
pointer std::allocator_traits<Allocator>::pointer
const_pointer std::allocator_traits<Allocator>::const_pointer
iterator LegacyForwardIterator to value_type
const_iterator LegacyForwardIterator to const value_type
An iterator type whose category, value, difference, pointer
and
local_iterator reference types are the same as iterator. This iterator
can be used to iterate through a single bucket but not
across buckets
An iterator type whose category, value, difference, pointer
and
const_local_iterator reference types are the same as const_iterator. This
iterator
can be used to iterate through a single bucket but not
across buckets
node_type (since C++17) a specialization of node handle representing a container
node
type describing the result of inserting a node_type, a
specialization of

template<class Iter, class NodeType>
struct /*unspecified*/
insert_return_type {
(since C++17) Iter position;
bool inserted;
NodeType node;
};
instantiated with template arguments iterator and
node_type.

Member functions¶

constructor constructs the unordered_map
(public member function)
destructor destructs the unordered_map
(public member function)
operator= assigns values to the container
(public member function)
get_allocator returns the associated allocator
(public member function)

Iterators¶

begin returns an iterator to the beginning
cbegin (public member function)
end returns an iterator to the end
cend (public member function)

Capacity¶

empty checks whether the container is empty
(public member function)
size returns the number of elements
(public member function)
max_size returns the maximum possible number of elements
(public member function)

Modifiers¶

clear clears the contents
(public member function)
inserts elements
insert or nodes
(since C++17)
(public member function)
insert_range inserts a range of elements
(C++23) (public member function)
insert_or_assign inserts an element or assigns to the current element if the key
(C++17) already exists
(public member function)
emplace constructs element in-place
(public member function)
emplace_hint constructs elements in-place using a hint
(public member function)
try_emplace inserts in-place if the key does not exist, does nothing if the
(C++17) key exists
(public member function)
erase erases elements
(public member function)
swap swaps the contents
(public member function)
extract extracts nodes from the container
(C++17) (public member function)
merge splices nodes from another container
(C++17) (public member function)

Lookup¶

at access specified element with bounds checking
(public member function)
operator[] access or insert specified element
(public member function)
count returns the number of elements matching specific key
(public member function)
find finds element with specific key
(public member function)
contains checks if the container contains element with specific key
(C++20) (public member function)
equal_range returns range of elements matching a specific key
(public member function)

Bucket interface¶

begin(size_type) returns an iterator to the beginning of the specified bucket
cbegin(size_type) (public member function)
end(size_type) returns an iterator to the end of the specified bucket
cend(size_type) (public member function)
bucket_count returns the number of buckets
(public member function)
max_bucket_count returns the maximum number of buckets
(public member function)
bucket_size returns the number of elements in specific bucket
(public member function)
bucket returns the bucket for specific key
(public member function)

Hash policy¶

load_factor returns average number of elements per bucket
(public member function)
max_load_factor manages maximum average number of elements per bucket
(public member function)
reserves at least the specified number of buckets and regenerates
rehash the hash table
(public member function)
reserves space for at least the specified number of elements and
reserve regenerates the hash table
(public member function)

Observers¶

hash_function returns function used to hash the keys
(public member function)
key_eq returns the function used to compare keys for equality
(public member function)

Non-member functions¶

operator==
operator!= compares the values in the unordered_map
(C++11) (function template)
(C++11)(removed in C++20)
std::swap(std::unordered_map) specializes the std::swap algorithm
(C++11) (function template)
erase_if(std::unordered_map) erases all elements satisfying specific criteria
(C++20) (function template)

Deduction guides (since C++17)

Notes¶

Feature-test macro Value Std Feature
__cpp_lib_containers_ranges 202202L (C++23) Ranges construction and insertion for
containers

Example¶

// Run this code

#include <iostream>
#include <string>
#include <unordered_map>

int main()
{
// Create an unordered_map of three strings (that map to strings)
std::unordered_map<std::string, std::string> u =
{
{"RED", "#FF0000"},
{"GREEN", "#00FF00"},
{"BLUE", "#0000FF"}
};

// Helper lambda function to print key-value pairs
auto print_key_value = [](const auto& key, const auto& value)
{
std::cout << "Key:[" << key << "] Value:[" << value << "]\n";
};

std::cout << "Iterate and print key-value pairs of unordered_map, being\n"
"explicit with their types:\n";
for (const std::pair<const std::string, std::string>& n : u)
print_key_value(n.first, n.second);

std::cout << "\nIterate and print key-value pairs using C++17 structured binding:\n";
for (const auto& [key, value] : u)
print_key_value(key, value);

// Add two new entries to the unordered_map
u["BLACK"] = "#000000";
u["WHITE"] = "#FFFFFF";

std::cout << "\nOutput values by key:\n"
"The HEX of color RED is:[" << u["RED"] << "]\n"
"The HEX of color BLACK is:[" << u["BLACK"] << "]\n\n";

std::cout << "Use operator[] with non-existent key to insert a new key-value pair:\n";
print_key_value("new_key", u["new_key"]);

std::cout << "\nIterate and print key-value pairs, using `auto`;\n"
"new_key is now one of the keys in the map:\n";
for (const auto& n : u)
print_key_value(n.first, n.second);
}

Possible output:¶

Iterate and print key-value pairs of unordered_map, being
explicit with their types:
Key:[BLUE] Value:[#0000FF]
Key:[GREEN] Value:[#00FF00]
Key:[RED] Value:[#FF0000]

Iterate and print key-value pairs using C++17 structured binding:
Key:[BLUE] Value:[#0000FF]
Key:[GREEN] Value:[#00FF00]
Key:[RED] Value:[#FF0000]

Output values by key:
The HEX of color RED is:[#FF0000]
The HEX of color BLACK is:[#000000]

Use operator[] with non-existent key to insert a new key-value pair:
Key:[new_key] Value:[]

Iterate and print key-value pairs, using `auto`;
new_key is now one of the keys in the map:
Key:[new_key] Value:[]
Key:[WHITE] Value:[#FFFFFF]
Key:[BLACK] Value:[#000000]
Key:[BLUE] Value:[#0000FF]
Key:[GREEN] Value:[#00FF00]
Key:[RED] Value:[#FF0000]

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
the definitions of reference,
LWG 2050 C++11 const_reference, pointer based on value_type and
and const_pointer were based on std::allocator_traits
allocator_type

See also¶

map collection of key-value pairs, sorted by keys, keys are unique
(class template)

Category:¶

* Todo with reason