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std::stop_token::stop_possible

From cppreference.com
< cpp‎ | thread‎ | stop token
 
 
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[[nodiscard]] bool stop_possible() const noexcept;
(since C++20)

Checks if the stop_token object has associated stop-state, and that state either has already had a stop requested or it has associated std::stop_source object(s).

A default constructed stop_token has no associated stop-state, and thus cannot be stopped; the associated stop-state for which no std::stop_source object(s) exist can also not be stopped if such a request has not already been made.

Contents

[edit] Parameters

(none)

[edit] Return value

false if the stop_token object has no associated stop-state, or it did not yet receive a stop request and there are no associated std::stop_source object(s); true otherwise.

[edit] Notes

If the stop_token object has associated stop-state and a stop request has already been made, this function still returns true.

If the stop_token object has associated stop-state from a std::jthread—for example, the stop_token was retrieved by invoking get_stop_token() on a std::jthread object—then this function always returns true. A std::jthread always has an internal std::stop_source object, even if the thread's invoking function does not check it.

[edit] Example

#include <chrono>
#include <condition_variable>
#include <format>
#include <iostream>
#include <mutex>
#include <string_view>
#include <thread>
using namespace std::chrono_literals;
 
int main()
{
    std::cout << std::boolalpha;
    auto print = [](std::string_view name, const std::stop_token& token)
    {
        std::cout << std::format("{}: stop_possible = {:s}, stop_requested = {:s}\n", 
            name, token.stop_possible(), token.stop_requested()
        );
    };
 
    // A worker thread that will listen to stop requests
    auto stop_worker = std::jthread([](std::stop_token stoken)
    {
        for (int i = 10; i; --i)
        {
            std::this_thread::sleep_for(300ms);
            if (stoken.stop_requested())
            {
                std::cout << "  Sleepy worker is requested to stop\n";
                return;
            }
            std::cout << "  Sleepy worker goes back to sleep\n";
        }
    });
 
    // A worker thread that will only stop when completed
    auto inf_worker = std::jthread([]()
    {
        for (int i = 5; i; --i)
        {
            std::this_thread::sleep_for(300ms);
            std::cout << "  Run as long as we want\n";
        }
    });
 
    std::stop_token def_token;
    std::stop_token stop_token = stop_worker.get_stop_token();
    std::stop_token inf_token = inf_worker.get_stop_token();
    print("def_token ", def_token);
    print("stop_token", stop_token);
    print("inf_token ", inf_token);
 
    std::cout << "\nRequest and join stop_worker:\n";
    stop_worker.request_stop();
    stop_worker.join();
 
    std::cout << "\nRequest and join inf_worker:\n";
    inf_worker.request_stop();
    inf_worker.join();
    std::cout << '\n';
 
    print("def_token ", def_token);
    print("stop_token", stop_token);
    print("inf_token ", inf_token);
}

Possible output:

def_token : stop_possible = false, stop_requested = false
stop_token: stop_possible = true, stop_requested = false
inf_token : stop_possible = true, stop_requested = false
 
Request and join stop_worker:
  Run as long as we want
  Sleepy worker is requested to stop
 
Request and join inf_worker:
  Run as long as we want
  Run as long as we want
  Run as long as we want
  Run as long as we want
 
def_token : stop_possible = false, stop_requested = false
stop_token: stop_possible = true, stop_requested = true
inf_token : stop_possible = true, stop_requested = true