std::jthread::joinable

[[nodiscard]] bool joinable() const noexcept;

Checks if the std::jthread object identifies an active thread of execution. Specifically, returns true if get_id() != std::jthread::id(). So a default constructed jthread is not joinable.

A thread that has finished executing code, but has not yet been joined is still considered an active thread of execution and is therefore joinable.

Parameters

(none).

Return value

true if the jthread object identifies an active thread of execution, false otherwise.

Example

#include <iostream>
#include <thread>
#include <chrono>
using namespace std::chrono_literals;
 
void foo()
{
    std::this_thread::sleep_for(500ms);
}
 
int main()
{
    std::cout << std::boolalpha;
 
    std::jthread t;
    std::cout << "before starting, joinable: " << t.joinable() << '\n';
 
    t = std::jthread{foo};
    std::cout << "after starting, joinable: " << t.joinable() << '\n';
 
    t.join();
    std::cout << "after joining, joinable: " << t.joinable() << '\n';
 
    t = std::jthread{foo};
    t.detach();
    std::cout << "after detaching, joinable: " << t.joinable() << '\n';
 
}

before starting, joinable: false
after starting, joinable: true
after joining, joinable: false
after detaching, joinable: false

References

• C++23 standard (ISO/IEC 14882:2023):
• 33.4.4.3 Members [thread.jthread.mem]
• C++20 standard (ISO/IEC 14882:2020):
• 32.4.3.2 Members [thread.jthread.mem]

See also