std::pair<iterator,iterator> equal_range( const Key& key ); | (1) | (since C++11) |

std::pair<const_iterator,const_iterator> equal_range( const Key& key ) const; | (2) | (since C++11) |

template< class K > std::pair<iterator,iterator> equal_range( const K& x ); | (3) | (since C++20) |

template< class K > std::pair<const_iterator,const_iterator> equal_range( const K& x ) const; | (4) | (since C++20) |

1,2) Returns a range containing all elements with key

`key`

in the container. The range is defined by two iterators, the first pointing to the first element of the wanted range and the second pointing past the last element of the range.
3,4) Returns a range containing all elements in the container with key equivalent to

`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 to compare the elements to |

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

`std::pair`

containing a pair of iterators defining the wanted range. If there are no such elements, past-the-end (see `end()`

) iterators are returned as both elements of the pair.

Average case linear in the number of elements with the key `key`

, worst case linear in the size of the container.

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

`__cpp_lib_generic_unordered_lookup` | for overloads (3,4) |

#include <iostream> #include <unordered_map> int main() { std::unordered_map<int,char> map = {{1,'a'},{1,'b'},{1,'d'},{2,'b'}}; auto range = map.equal_range(1); for (auto it = range.first; it != range.second; ++it) { std::cout << it->first << ' ' << it->second << '\n'; } }

Output:

1 a

(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 the number of elements matching specific key (public member function) |

returns range of elements matching a specific key (function template) |

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