std::ranges::equal_range(3)

NAME
std::ranges::equal_range - std::ranges::equal_range

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::forward_iterator I, std::sentinel_for<I> S,
   class T, class Proj = std::identity,
   std::indirect_strict_weak_order<
   const T*, (1) (since C++20)
   std::projected<I, Proj>> Comp = ranges::less >
   constexpr ranges::subrange<I>
   equal_range(I first, S last, const T& value, Comp comp = {}, Proj
   proj = {});
   template< ranges::forward_range R, class T, class Proj =
   std::identity,
   std::indirect_strict_weak_order<
   const T*, (2) (since C++20)
   std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less >
   constexpr ranges::borrowed_subrange_t<R>
   equal_range(R&& r, const T& value, Comp comp = {}, Proj proj =
   {});
   1) Returns a view 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 view is constructed from 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::ranges::lower_bound(), the second - with std::ranges::upper_bound().
   2) Same as (1), but uses r as the source range, as if using the range
   ranges::begin(r) as first and ranges::end(r) as last.
   The function-like entities described on this page are niebloids, that is:
     * Explicit template argument lists may not be specified when calling any of them.
     * None of them is visible to argument-dependent lookup.
     * When one of them is found by normal unqualified lookup for the name to the left
       of the function-call operator, it inhibits argument-dependent lookup.
   In practice, they may be implemented as function objects, or with special compiler
   extensions.

Parameters
   first, last - the range of elements to examine
   r - the range of the elements to examine
   value - value to compare the elements to
   comp - if the first argument is less than (i.e. is ordered before) the second
   proj - projection to apply to the elements

Return value
   std::ranges::subrange 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, the last iterator (iterator that is
   equal to last or ranges::end(r)) is returned as the first element. Similarly if
   there are no elements greater than value, the last iterator 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 an iterator that does not model
   random_access_iterator, the number of iterator increments is linear.

Possible implementation
 struct equal_range_fn {
     template< std::forward_iterator I, std::sentinel_for<I> S,
               class T, class Proj = std::identity,
               std::indirect_strict_weak_order<
                   const T*,
                   std::projected<I, Proj>> Comp = ranges::less >
     constexpr ranges::subrange<I>
     operator()(I first, S last, const T& value, Comp comp = {}, Proj proj = {}) const
     {
         return ranges::subrange(
             ranges::lower_bound(first, last, value, std::ref(comp), std::ref(proj)),
             ranges::upper_bound(first, last, value, std::ref(comp), std::ref(proj)));
     }
     template< ranges::forward_range R, class T, class Proj = std::identity,
               std::indirect_strict_weak_order<
                   const T*,
                   std::projected<std::ranges::iterator_t<R>, Proj>> Comp = ranges::less >
     constexpr ranges::borrowed_subrange_t<R>
     operator()(R&& r, const T& value, Comp comp = {}, Proj proj = {}) const
     {
         return (*this)(ranges::begin(r), ranges::end(r), value,
                        std::ref(comp), std::ref(proj));
     }
 };
 inline constexpr equal_range_fn equal_range;

Example
// Run this code
 #include <algorithm>
 #include <compare>
 #include <vector>
 #include <iostream>
 struct S
 {
     int number;
     char name;
     // note: name is ignored by these comparison operators
     friend bool operator== ( const S s1, const S s2 ) { return s1.number == s2.number; }
     friend auto operator<=> ( const S s1, const S s2 ) { return s1.number <=> s2.number; }
 };
 int main()
 {
     // note: not ordered, only partitioned w.r.t. S defined below
     std::vector<S> vec = { {1,'A'}, {2,'B'}, {2,'C'}, {2,'D'}, {4, 'D'}, {4,'G'}, {3,'F'} };
     const S value = {2, '?'};
     namespace ranges = std::ranges;
     {
         auto p = ranges::equal_range(vec, value);
         std::cout << "1. ";
         for ( auto i : p )
             std::cout << i.name << ' ';
     }
     {
         auto p = ranges::equal_range(vec.begin(), vec.end(), value);
         std::cout << "\n2. ";
         for ( auto i = p.begin(); i != p.end(); ++i )
             std::cout << i->name << ' ';
     }
     {
         auto p = ranges::equal_range(vec, 'D', ranges::less{}, &S::name);
         std::cout << "\n3. ";
         for ( auto i : p )
             std::cout << i.name << ' ';
     }
     {
         auto p = ranges::equal_range(vec.begin(), vec.end(), 'D', ranges::less{}, &S::name);
         std::cout << "\n4. ";
         for ( auto i = p.begin(); i != p.end(); ++i )
             std::cout << i->name << ' ';
     }
 }

Output:
 1. B C D
 2. B C D
 3. D D
 4. D D

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