std::equal_range(3)

NAME
std::equal_range - std::equal_range

Synopsis
   Defined in header <algorithm>
   template< class ForwardIt, class T >
                        (constexpr since
   std::pair<ForwardIt, ForwardIt> C++20)
                        (until C++26)
       equal_range( ForwardIt first,
   ForwardIt last, const T& value );
   template< class ForwardIt, class T =
   typename std::iterator_traits
    <ForwardIt>::value_type > (since C++26)
   constexpr std::pair<ForwardIt, ForwardIt>
       equal_range( ForwardIt first,
   ForwardIt last, const T& value );
   template< class ForwardIt, class T, class
   Compare >
                        (1)
   std::pair<ForwardIt, ForwardIt> (constexpr since
       equal_range( ForwardIt first, C++20)
   ForwardIt last, (until C++26)
                    const T& value, Compare
   comp );
   template< class ForwardIt, class T =
   typename std::iterator_traits (2)
    <ForwardIt>::value_type,
             class Compare > (since C++26)
   constexpr std::pair<ForwardIt, ForwardIt>
       equal_range( ForwardIt first,
   ForwardIt last,
                    const T& value, Compare
   comp );
   Returns a range containing all elements equivalent to value in the partitioned range
   [first, last).
   1) The equivalence is checked using operator<:
   Returns the results of std::lower_bound(first, last, value) and
   std::upper_bound(first, last, value).
   If any of the following conditions is satisfied, the behavior is
   undefined: (until C++20)
     * For any element elem of [first, last), bool(elem < value) does not
       imply !bool(value < elem).
     * The elements elem of [first, last) are not partitioned with
       respect to expressions bool(elem < value) and !bool(value < elem).
   Equivalent to std::equal_range(first, last, value, std::less{}). (since C++20)
   2) The equivalence is checked using comp:
   Returns the results of std::lower_bound(first, last, value, comp) and
   std::upper_bound(first, last, value, comp).
   If any of the following conditions is satisfied, the behavior is undefined:
     * For any element elem of [first, last), bool(comp(elem, value)) does not imply
       !bool(comp(value, elem)).
     * The elements elem of [first, last) are not partitioned with respect to
       expressions bool(comp(elem, value)) and !bool(comp(value, elem)).

Parameters
   first, last - the partitioned range of elements to examine
   value - value to compare the elements to
                 binary predicate which returns true if the first argument 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
   A std::pair containing a pair of iterators, where
     * first is an iterator to the first element of the range [first, last) not ordered
       before value (or last if no such element is found), and
     * second is an iterator to the first element of the range [first, last) ordered
       after value (or last if no such element is found).

Complexity
   Given \(\scriptsize N\)N as std::distance(first, last):
   1) At most \(\scriptsize 2\log_{2}(N)+O(1)\)2log
   2(N)+O(1) comparisons with value using
   operator<
   (until C++20)
   std::less{}
   (since C++20).
   2) At most \(\scriptsize 2\log_{2}(N)+O(1)\)2log
   2(N)+O(1) applications of the comparator comp.
   However, if ForwardIt is not a LegacyRandomAccessIterator, the number of iterator
   increments is linear in \(\scriptsize N\)N. 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.

Notes
   Although std::equal_range only requires [first, last) to be partitioned, this
   algorithm is usually used in the case where [first, last) is sorted, so that the
   binary search is valid for any value.
   On top of the requirements of std::lower_bound and std::upper_bound,
   std::equal_range also requires operator< or comp to be asymmetric (i.e., a < b and b
   < a always have different results).
   Therefore, the intermediate results of binary search can be shared by
   std::lower_bound and std::upper_bound. For example, the result of the
   std::lower_bound call can be used as the argument of first in the std::upper_bound
   call.
             Feature-test macro Value Std Feature
   __cpp_lib_algorithm_default_value_type 202403 (C++26) List-initialization for
                        algorithms (1,2)

Possible implementation
                        equal_range (1)
   template<class ForwardIt,
            class T = typename std::iterator_traits<ForwardIt>::value_type>
   constexpr std::pair<ForwardIt, ForwardIt>
       equal_range(ForwardIt first, ForwardIt last, const T& value)
   {
       return std::equal_range(first, last, value, std::less{});
   }
                        equal_range (2)
   template<class ForwardIt,
            class T = typename std::iterator_traits<ForwardIt>::value_type,
            class Compare>
   constexpr 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 <complex>
 #include <iostream>
 #include <vector>
 struct S
 {
     int number;
     char name;
     // note: name is ignored by this comparison operator
     bool operator<(const S& s) const { return number < s.number; }
 };
 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; }
 };
 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 it = p.first; it != p.second; ++it)
         std::cout << it->name << ' ';
     std::cout << '\n';
     std::cout << "Using heterogeneous comparison: ";
     const auto p2 = std::equal_range(vec.begin(), vec.end(), 2, Comp{});
     for (auto it = p2.first; it != p2.second; ++it)
         std::cout << it->name << ' ';
     std::cout << '\n';
     using CD = std::complex<double>;
     std::vector<CD> nums{{1, 0}, {2, 2}, {2, 1}, {3, 0}, {3, 1}};
     auto cmpz = [](CD x, CD y) { return x.real() < y.real(); };
     #ifdef __cpp_lib_algorithm_default_value_type
         auto p3 = std::equal_range(nums.cbegin(), nums.cend(), {2, 0}, cmpz);
     #else
         auto p3 = std::equal_range(nums.cbegin(), nums.cend(), CD{2, 0}, cmpz);
     #endif
     for (auto it = p3.first; it != p3.second; ++it)
         std::cout << *it << ' ';
     std::cout << '\n';
 }

Output:
 Compare using S::operator<(): B C D
 Using heterogeneous comparison: B C D
 (2,2) (2, 1)
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
                      Compare was required to satisfy only a partitioning is required;
   LWG 270 C++98 Compare and T was required heterogeneous comparisons
                      to be LessThanComparable (strict permitted
                      weak ordering required)
                      at most \(\scriptsize
                      2\log_{2}(N)+1\)2log corrected to \(\scriptsize
   LWG 384 C++98 2(N)+1 comparisons 2\log_{2}(N)+O(1)\)2log
                      were allowed, which is not 2(N)+O(1)
                      implementable^[1]
    1. ↑ Applying equal_range to a single-element range requires 2 comparisons, but at
       most 1 comparison is allowed by the complexity requirement.

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)
                       returns range of elements matching a specific key
   equal_range (public member function of std::set<Key,Compare,Allocator>)
                       returns range of elements matching a specific key
   equal_range (public member function of std::multiset<Key,Compare,Allocator>)
   ranges::equal_range returns range of elements matching a specific key
   (C++20) (niebloid)