std::unordered_map equal_range() method
- od C++20
- od 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;
// (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;
// (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 ifHash::is_transparent
andKeyEqual::is_transparent
are valid and each denotes a type. This assumes that suchHash
is callable with bothK
andKey
type, and that theKeyEqual
is transparent, which, together, allows calling this function without constructing an instance ofKey
.
Parameters
key
- key value of the elements to countx
- 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
#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';
}
}
1 a
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