std::multiset::equal_range(3)

NAME
std::multiset::equal_range - std::multiset::equal_range

Synopsis
   std::pair<iterator,iterator> equal_range( const Key& key ); (1)
   std::pair<const_iterator,const_iterator> equal_range( const Key& (2)
   key ) const;
   template< class K > (3) (since C++14)
   std::pair<iterator,iterator> equal_range( const K& x );
   template< class K >
   std::pair<const_iterator,const_iterator> equal_range( const K& x ) (4) (since C++14)
   const;
   Returns a range containing all elements with the given key in the container. The
   range is defined by two iterators, one pointing to the first element that is not
   less than key and another pointing to the first element greater than key.
   Alternatively, the first iterator may be obtained with lower_bound(), and the second
   with upper_bound().
   1,2) Compares the keys to key.
   3,4) Compares the keys to the value x. This overload participates in overload
   resolution only if the qualified-id Compare::is_transparent is valid and denotes a
   type. It allows calling this function without constructing an instance of Key.
    This section is incomplete
    Reason: explain better

Parameters
   key - key value to compare the elements to
   x - alternative value that can be compared to Key

Return value
   std::pair containing a pair of iterators defining the wanted range: the first
   pointing to the first element that is not less than key and the second pointing to
   the first element greater than key.
   If there are no elements not less than key, past-the-end (see end()) iterator is
   returned as the first element. Similarly if there are no elements greater than key,
   past-the-end iterator is returned as the second element.
   Since emplace and unhinted insert always insert at the upper bound,
   the order of equivalent elements in the equal range is the order of (since C++11)
   insertion unless hinted insert or emplace_hint was used to insert an
   element at a different position.

Complexity
   Logarithmic in the size of the container.

Notes
   Feature-test macro: __cpp_lib_generic_associative_lookup (for overloads (3,4))

Example
// Run this code
 #include <set>
 #include <iostream>
 template <typename I>
 void print_equal_range(I first, I lb, I ub, I last) {
     for (I i{first}; i != lb; ++i)
         std::cout << *i << " ";
     std::cout << "[ ";
     for (I i{lb}; i != ub; ++i)
         std::cout << *i << " ";
     std::cout << ") ";
     for (I i{ub}; i != last; ++i)
         std::cout << *i << " ";
     std::cout << '\n';
 }
 int main()
 {
     std::multiset<int> c{4, 3, 2, 1, 3, 3};
     std::cout << "c = ";
     print_equal_range(begin(c), begin(c), end(c), end(c));
     for (int key{}; key != 6; ++key) {
         std::cout << "key = " << key << "; equal range = ";
         const auto [lb, ub] = c.equal_range(key);
         print_equal_range(begin(c), lb, ub, end(c));
     }
 }

Output:
 c = [ 1 2 3 3 3 4 )
 key = 0; equal range = [ ) 1 2 3 3 3 4
 key = 1; equal range = [ 1 ) 2 3 3 3 4
 key = 2; equal range = 1 [ 2 ) 3 3 3 4
 key = 3; equal range = 1 2 [ 3 3 3 ) 4
 key = 4; equal range = 1 2 3 3 3 [ 4 )
 key = 5; equal range = 1 2 3 3 3 4 [ )

See also
   find finds element with specific key
               (public member function)
   contains checks if the container contains element with specific key
   (C++20) (public member function)
   count returns the number of elements matching specific key
               (public member function)
   upper_bound returns an iterator to the first element greater than the given key
               (public member function)
   lower_bound returns an iterator to the first element not less than the given key
               (public member function)
   equal_range returns range of elements matching a specific key
               (function template)