std::ranges::equal_range

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>
    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*,
              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 = {} );

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().

The function-like entities described on this page are niebloids, that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler extensions.

Parameters

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 * log₂(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

#include <algorithm>
#include <compare>
#include <iostream>
#include <vector>
 
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; }
    friend std::ostream& operator<<(std::ostream& os, S o)
    {
        return os << '{' << o.number << ", '" << o.name << "'}";
    }
};
 
void println(auto rem, auto const& v)
{
    for (std::cout << rem; auto const& e : v)
        std::cout << e << ' ';
    std::cout << '\n';
}
 
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 a = ranges::equal_range(vec, value);
    println("1. ", a);
 
    auto b = ranges::equal_range(vec.begin(), vec.end(), value);
    println("2. ", b);
 
    auto c = ranges::equal_range(vec, 'D', ranges::less {}, &S::name);
    println("3. ", c);
 
    auto d = ranges::equal_range(vec.begin(), vec.end(), 'D', ranges::less {}, &S::name);
    println("4. ", d);
}

1. {2, 'B'} {2, 'C'} {2, 'D'}
2. {2, 'B'} {2, 'C'} {2, 'D'}
3. {2, 'D'} {4, 'D'}
4. {2, 'D'} {4, 'D'}

See also