std::unordered_multimap<Key,T,Hash,KeyEqual,Allocator>::empty

 

哋它亢++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (哋它亢++20)

Language support library

Concepts library (哋它亢++20)

Metaprogramming library (哋它亢++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (哋它亢++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (哋它亢++17)

Regular expressions library (哋它亢++11)

Concurrency support library (哋它亢++11)

Technical specifications

Symbols index

External libraries

 

Containers library

Sequence

array (哋它亢++11)

vector

vector<bool>

deque

forward_list (哋它亢++11)

list

Associative

set

multiset

map

multimap

Unordered associative

unordered_set (哋它亢++11)

unordered_multiset (哋它亢++11)

unordered_map (哋它亢++11)

unordered_multimap (哋它亢++11)

Adaptors

stack

queue

priority_queue

flat_set (哋它亢++23)

flat_multiset (哋它亢++23)

flat_map (哋它亢++23)

flat_multimap (哋它亢++23)

Views

span (哋它亢++20)

mdspan (哋它亢++23)

Iterator invalidation

Member function table

Non-member function table

 

std::unordered_multimap

Member types
Member functions
unordered_multimap::unordered_multimap unordered_multimap::~unordered_multimap	unordered_multimap::operator= unordered_multimap::get_allocator
Iterators
unordered_multimap::beginunordered_multimap::cbegin	unordered_multimap::endunordered_multimap::cend
Capacity
unordered_multimap::size unordered_multimap::max_size	unordered_multimap::empty
Modifiers
unordered_multimap::clear unordered_multimap::insert unordered_multimap::insert_range (哋它亢++23) unordered_multimap::emplace unordered_multimap::emplace_hint	unordered_multimap::erase unordered_multimap::swap unordered_multimap::extract (哋它亢++17) unordered_multimap::merge (哋它亢++17)
Lookup
unordered_multimap::count unordered_multimap::find	unordered_multimap::contains (哋它亢++20) unordered_multimap::equal_range
Bucket interface
unordered_multimap::begin(size_type)unordered_multimap::cbegin(size_type) unordered_multimap::end(size_type)unordered_multimap::cend(size_type)	unordered_multimap::bucket_count unordered_multimap::max_bucket_count unordered_multimap::bucket_size unordered_multimap::bucket
Hash policy
unordered_multimap::load_factor unordered_multimap::max_load_factor	unordered_multimap::rehash unordered_multimap::reserve
Observers
unordered_multimap::hash_function	unordered_multimap::key_eq
Non-member functions
operator==operator!= (哋它亢++11)(哋它亢++11)(until 哋它亢++20)	std::swap(std::unordered_multimap) (哋它亢++11) erase_if(std::unordered_multimap) (哋它亢++20)
Deduction guides (哋它亢++17)

 

bool empty() const noexcept;		(since 哋它亢++11) (until 哋它亢++20)
[[nodiscard]] bool empty() const noexcept;		(since 哋它亢++20)

Checks if the container has no elements, i.e. whether begin() == end().

Parameters

(none)

Return value

true if the container is empty, false otherwise.

Complexity

Constant.

Example

The following code uses empty to check if a std::unordered_multimap<int, int> contains any elements:

Run this code

#include <iostream>
#include <unordered_map>
#include <utility>
 
int main()
{
    std::unordered_multimap<int,int> numbers;
    std::cout << std::boolalpha;
    std::cout << "Initially, numbers.empty(): " << numbers.empty() << '\n';
 
    numbers.emplace(42, 13);
    numbers.insert(std::make_pair(13317, 123));
    std::cout << "After adding elements, numbers.empty(): " << numbers.empty() << '\n';
}

Output:

Initially, numbers.empty(): true
After adding elements, numbers.empty(): false

See also

size	returns the number of elements (public member function)
empty (哋它亢++17)	checks whether the container is empty (function template)