std::ranges::equal_range

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*, std::projected<I, Proj>> Comp = ranges::less > constexpr ranges::subrange<I> equal_range(I first, S last, const T& value, Comp comp = {}, Proj proj = {});	(1)	(since C++20)
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 = {});	(2)	(since C++20)

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

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

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