std::unordered_map equal_range() method

since C++20

// (1) Non const version
std::pair<iterator,iterator> equal_range( const Key& key );

// (2) Const version
std::pair<const_iterator,const_iterator> equal_range( const Key& key ) const;

// (3) Non const version
template< class K >
std::pair<iterator,iterator> equal_range( const K& x );

// (4) Const version
template< class K >
std::pair<const_iterator,const_iterator> equal_range( const K& x ) const;

since C++11

// (1) Non const version
std::pair<iterator,iterator> equal_range( const Key& key );

// (2) Const version
std::pair<const_iterator,const_iterator> equal_range( const Key& key ) const;

• (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.

Parameters

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

Return value

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.

Complexity

Average case, linear in the number of elements with the key key - O(N), where N is number of elements with the key key.
Worst case, linear in the size of the container - O(size()).

Exceptions

(none)

Notes

Feature testing macro: __cpp_lib_generic_unordered_lookup (for overload (3-4)).

Example

Main.cpp

#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