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Variants

std:: default_sentinel_t, std:: default_sentinel

From cppreference.net
C++
Iterator library
Iterator concepts
Iterator primitives
Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
(C++20)
(C++20)
(C++20)
Utilities
(C++20)
Iterator adaptors
(C++14)
(C++11)
(C++11)
default_sentinel_t default_sentinel
(C++20) (C++20)
(C++20) (C++20)
Range access
(C++11) (C++14)
(C++14) (C++14)
(C++11) (C++14)
(C++14) (C++14)
(C++17) (C++20)
(C++17)
(C++17)
Defined in header <iterator>
struct default_sentinel_t { } ;
(1) (since C++20)
inline constexpr default_sentinel_t default_sentinel { } ;
(2) (since C++20)
1) default_sentinel_t is an empty class type used to denote the end of a range. It can be used together with iterator types that know the bound of their range (e.g., std::counted_iterator ).
2) default_sentinel is a constant of type default_sentinel_t .

Example

Run this code 
#include <print>
#include <regex>
#include <string>
int main()
{
    const std::string s = "Quick brown fox.";
    const std::regex words_regex("[^\\s]+");
    const std::ranges::subrange words(
        std::sregex_iterator(s.begin(), s.end(), words_regex), std::default_sentinel);
    std::println("Found {} words:", std::ranges::distance(words));
    for (const std::smatch& match : words)
        std::println("{}", match.str());
}

Output: 

Found 3 words:
Quick
brown
fox.

See also

input iterator that reads from std::basic_istream
(class template)
input iterator that reads from std::basic_streambuf
(class template)
(C++20)
iterator adaptor that tracks the distance to the end of the range
(class template)