std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::count

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Containers library

array (C++11)

vector

deque

forward_list (C++11)

list

set

multiset

map

multimap

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

stack

queue

priority_queue

span (C++20)

 

std::unordered_map

Member functions

unordered_map::unordered_map

unordered_map::~unordered_map

unordered_map::operator=

unordered_map::get_allocator

Iterators

unordered_map::beginunordered_map::cbegin

unordered_map::endunordered_map::cend

Capacity

unordered_map::empty

unordered_map::size

unordered_map::max_size

Modifiers

unordered_map::clear

unordered_map::insert

unordered_map::insert_or_assign (C++17)

unordered_map::emplace

unordered_map::emplace_hint

unordered_map::try_emplace (C++17)

unordered_map::erase

unordered_map::swap

unordered_map::extract (C++17)

unordered_map::merge (C++17)

Lookup

unordered_map::at

unordered_map::operator[]

unordered_map::count

unordered_map::find

unordered_map::contains (C++20)

unordered_map::equal_range

Bucket interface

unordered_map::begin(size_type)unordered_map::cbegin(size_type)

unordered_map::end(size_type)unordered_map::cend(size_type)

unordered_map::bucket_count

unordered_map::max_bucket_count

unordered_map::bucket_size

unordered_map::bucket

Hash policy

unordered_map::load_factor

unordered_map::max_load_factor

unordered_map::rehash

unordered_map::reserve

Observers

unordered_map::hash_function

unordered_map::key_eq

Deduction guides(C++17)

 

size_type count( const Key& key ) const;	(1)	(since C++11)
size_type count( const Key& key, std::size_t hash ) const;	(1)	(since C++20)
template< class K > size_type count( const K& x ) const;	(2)	(since C++20)
template< class K > size_type count( const K& x, std::size_t hash ) const;	(2)	(since C++20)

1) Returns the number of elements with key that compares equal to the specified argument key, which is either 1 or 0 since this container does not allow duplicates.

2) Returns the number of elements with key that compares equivalent to the specified argument x. This overload only participates in overload resolution if the qualified-id Hash::transparent_key_equal is valid and denotes a type. This assumes that such Hash is callable with both K and Key type, and that its key_equal is transparent, which, together, allows calling this function without constructing an instance of Key.

If present, the hash parameter may be used to pass a pre-computed hash value as an optimization. The behavior is undefined if invoking hash_function() on the key passed do not produce the same value as hash.

(since C++20)

Parameters

key	-	key value of the elements to count
x	-	a value of any type that can be transparently compared with a key
hash	-	the hash value of the key

Return value

1) Number of elements with key key, that is either 1 or 0.

2) Number of elements with key that compares equivalent to x.

Complexity

Constant on average, worst case linear in the size of the container.

See also

find	finds element with specific key (public member function)
equal_range	returns range of elements matching a specific key (public member function)