std::equal_range(3)

NAME
std::equal_range - std::equal_range

Synopsis
   Defined in header <algorithm>
   template< class ForwardIt, class T >
   std::pair<ForwardIt,ForwardIt> (until C++20)
   equal_range( ForwardIt first, ForwardIt last,
   const T& value );
   template< class ForwardIt, class T >
   constexpr std::pair<ForwardIt,ForwardIt> (since C++20)
   equal_range( ForwardIt first, ForwardIt last,
   const T& value );
   template< class ForwardIt, class T, class (1)
   Compare >
   std::pair<ForwardIt,ForwardIt> (until C++20)
   equal_range( ForwardIt first, ForwardIt last,
   const T& value, Compare comp ); (2)
   template< class ForwardIt, class T, class
   Compare >
   constexpr std::pair<ForwardIt,ForwardIt> (since C++20)
   equal_range( ForwardIt first, ForwardIt last,
   const T& value, Compare comp );
   Returns a range containing all elements equivalent to value in the range [first,
   last).
   The range [first, last) must be at least partially ordered with respect to value,
   i.e. it must satisfy all of the following requirements:
     * partitioned with respect to element < value or comp(element, value) (that is,
       all elements for which the expression is true precedes all elements for which
       the expression is false)
     * partitioned with respect to !(value < element) or !comp(value, element)
     * for all elements, if element < value or comp(element, value) is true then
       !(value < element) or !comp(value, element) is also true
   A fully-sorted range meets these criteria.
   The returned range is defined by two iterators, one pointing to the first element
   that is not less than value and another pointing to the first element greater than
   value. The first iterator may be alternatively obtained with std::lower_bound(), the
   second - with std::upper_bound().
   The first version uses operator< to compare the elements, the second version uses
   the given comparison function comp.

Parameters
   first, last - the range of elements to examine
   value - value to compare the elements to
                 binary predicate which returns true if the first argument is less
                 than (i.e. is ordered before) the second.
                 The signature of the predicate function should be equivalent to the
                 following:
                 bool pred(const Type1 &a, const Type2 &b);
   comp - While the signature does not need to have const &, the function must
                 not modify the objects passed to it and must be able to accept all
                 values of type (possibly const) Type1 and Type2 regardless of value
                 category (thus, Type1 & is not allowed
                 , nor is Type1 unless for Type1 a move is equivalent to a copy
                 (since C++11)).
                 The types Type1 and Type2 must be such that an object of type T can be
                 implicitly converted to both Type1 and Type2, and an object of type
                 ForwardIt can be dereferenced and then implicitly converted to both
                 Type1 and Type2.

Type requirements
   -
   ForwardIt must meet the requirements of LegacyForwardIterator.
   -
   Compare must meet the requirements of BinaryPredicate. it is not required to satisfy
   Compare

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 value and the second pointing to
   the first element greater than value.
   If there are no elements not less than value, last is returned as the first element.
   Similarly if there are no elements greater than value, last is returned as the
   second element

Complexity
   The number of comparisons performed is logarithmic in the distance between first and
   last (At most 2 * log
   2(last - first) + O(1) comparisons). However, for non-LegacyRandomAccessIterators,
   the number of iterator increments is linear. Notably, std::set and std::multiset
   iterators are not random access, and so their member functions std::set::equal_range
   (resp. std::multiset::equal_range) should be preferred.

Possible implementation

First version
   template<class ForwardIt, class T>
   std::pair<ForwardIt,ForwardIt>
       equal_range(ForwardIt first, ForwardIt last,
                   const T& value)
   {
       return std::make_pair(std::lower_bound(first, last, value),
                        std::upper_bound(first, last, value));
   }

Second version
   template<class ForwardIt, class T, class Compare>
   std::pair<ForwardIt,ForwardIt>
       equal_range(ForwardIt first, ForwardIt last,
                   const T& value, Compare comp)
   {
       return std::make_pair(std::lower_bound(first, last, value, comp),
                        std::upper_bound(first, last, value, comp));
   }

Example
// Run this code
 #include <algorithm>
 #include <vector>
 #include <iostream>
 struct S
 {
     int number;
     char name;
     // note: name is ignored by this comparison operator
     bool operator< ( const S& s ) const { return number < s.number; }
 };
 int main()
 {
     // note: not ordered, only partitioned w.r.t. S defined below
     const std::vector<S> vec = { {1,'A'}, {2,'B'}, {2,'C'}, {2,'D'}, {4,'G'}, {3,'F'} };
     const S value = {2, '?'};
     std::cout << "Compare using S::operator<(): ";
     const auto p = std::equal_range(vec.begin(), vec.end(), value);
     for ( auto i = p.first; i != p.second; ++i )
         std::cout << i->name << ' ';
     std::cout << "\n" "Using heterogeneous comparison: ";
     struct Comp
     {
         bool operator() ( const S& s, int i ) const { return s.number < i; }
         bool operator() ( int i, const S& s ) const { return i < s.number; }
     };
     const auto p2 = std::equal_range(vec.begin(),vec.end(), 2, Comp{});
     for ( auto i = p2.first; i != p2.second; ++i )
         std::cout << i->name << ' ';
 }

Output:
 Compare using S::operator<(): B C D
 Using heterogeneous comparison: B C D
  Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
   LWG 270 C++98 Compare was required to be a only a partitioning is needed;
                      strict weak ordering heterogeneous comparisons permitted

See also
                       returns an iterator to the first element not less than the given
   lower_bound value
                       (function template)
                       returns an iterator to the first element greater than a certain
   upper_bound value
                       (function template)
   binary_search determines if an element exists in a partially-ordered range
                       (function template)
   partition divides a range of elements into two groups
                       (function template)
   equal determines if two sets of elements are the same
                       (function template)
   equal_range returns range of elements matching a specific key
                       (public member function of std::set<Key,Compare,Allocator>)
   equal_range returns range of elements matching a specific key
                       (public member function of std::multiset<Key,Compare,Allocator>)
   ranges::equal_range returns range of elements matching a specific key
   (C++20) (niebloid)