NAME¶

std::stop_token::stop_possible - std::stop_token::stop_possible

Synopsis¶

[[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.

Parameters¶

(none)

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.

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.

Example¶

// Run this code

#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