size_type count( const Key& key ) const; | (1) | (since C++11) |

template< class K > size_type count( const K& x ) const; | (2) | (since C++20) |

1) Returns the number of elements with key that compares equal to the specified argument

`key`

.
2) Returns the number of elements with key that compares equivalent to the specified argument

`x`

. This overload participates in overload resolution only if `Hash::is_transparent`

and `KeyEqual::is_transparent`

are valid and each denotes a type. This assumes that such `Hash`

is callable with both `K`

and `Key`

type, and that the `KeyEqual`

is transparent, which, together, allows calling this function without constructing an instance of `Key`

.key | - | key value of the elements to count |

x | - | a value of any type that can be transparently compared with a key |

1) Number of elements with key

`key`

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

`x`

.linear in the number of elements with key `key`

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

Feature-test macro | Comment |
---|---|

`__cpp_lib_generic_unordered_lookup` | for overload (2) |

#include <algorithm> #include <iostream> #include <unordered_set> int main() { std::unordered_multiset set{2, 7, 1, 8, 2, 8, 1, 8, 2, 8}; std::cout << "The set is:\n"; for (int e: set) { std::cout << e << ' '; } const auto [min, max] = std::ranges::minmax(set); std::cout << "\nNumbers [" << min << ".." << max << "] frequency:\n"; for (int i{min}; i <= max; ++i) { std::cout << i << ':' << set.count(i) << "; "; } }

Possible output:

The set is: 8 8 8 8 1 1 7 2 2 2 Numbers [1..8] frequency: 1:2; 2:3; 3:0; 4:0; 5:0; 6:0; 7:1; 8:4;

(C++11) | finds element with specific key (public member function) |

(C++20) | checks if the container contains element with specific key (public member function) |

(C++11) | returns range of elements matching a specific key (public member function) |

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