std::jthread::joinable(3)

💅 std::jthread::joinable(3)

okay so bestie, welcome to the <std::jthread> corner of the C++ Standard Library — this is page std::jthread::joinable(3) and it is, no cap, one of the shortest little baddies in the whole header. literally a vibe check for your thread. 🦋

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✨ NAME ✨

std::jthread::joinable — yes queen, the name is the name, it's std::jthread::joinable 💖

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🌸 Synopsis 🌸

[[nodiscard]] bool joinable() const noexcept;  (since C++20)

so what does it do? it checks if the std::jthread object identifies an active thread of execution. that's the whole assignment, and it ate. 👑

specifically (and this is the part u need for the exam): it returns true if get_id() != std::jthread::id(). which means a default constructed jthread is not joinable. obvious but like, important. 💀

and here's the sneaky lil detail everyone forgets: 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. yes fr fr. done ≠ gone. 😭

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🎀 Parameters 🎀

(none) — she takes nothing, she needs nothing, main character energy ✨

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⭐ Return value ⭐

true if the jthread object identifies an active thread of execution, false otherwise. simple as that, periodt. 💅

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🌈 Example 🌈

// Run this code

#include <chrono>
#include <iostream>
#include <thread>
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';

}

and the output, bestie, she speaks for herself: 🫶

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

look at that journey. default → not joinable. started → joinable. joined → not joinable. detached → not joinable. character development fr. 🌸

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🔥 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]

(the receipts. always cite ur sources, iykyk 💜)

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👑 See also 👑

• get_id — returns the id of the thread (public member function)
• join — waits for the thread to finish its execution (public member function)
• detach — permits the thread to execute independently from the thread handle (public member function)

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lowkey a reminder: joinable() is [[nodiscard]] and noexcept, so don't ignore her return value and don't expect her to throw. she's chill like that. 🧚

okay that's the whole page, 10/10, no notes. this masterpiece is courtesy of http://cppreference.com, version 2024.06.10, dated std::jthread::joinable(3) — mwah, slay, bye bestie 💖✨🎀