Thread joinable() function in C++

Thread::joinable is an in-built function in C++ std::thread. It is an observer function which means it observes a state and then returns the corresponding output and checks whether the thread object is joinable or not.

A thread object is said to be joinable if it identifies/represent an active thread of execution.

A thread is not joinable if:

• It was default-constructed
• If either of its member join or detach has been called
• It has been moved elsewhere

Syntax:

std::thread::joinable()

Parameters: This function does not accepts any parameters.

Return Value: It is a boolean type function and returns true when the thread object is
joinable. It returns false if the thread object is not joinable.

The following program demonstrate the use of std::thread::joinable()

Note: On the online IDE this program will show error. To compile this, use the flag “-pthread” on g++ compilers compilation with the help of command “g++ –std=c++14 -pthread file.cpp”.

// C++ program to demonstrate the use of
// std::thread::joinable()

#include <chrono>
#include <iostream>
#include <thread>

using namespace std;

// function to put thread to sleep
void threadFunc()
{
    std::this_thread::sleep_for(
        std::chrono::seconds(1));
}

int main()
{
    std::thread t1; // declaring the thread

    cout << "t1 joinable when default created? \n";
    // checking if it is joinable
    if (t1.joinable())
        cout << "YES\n";
    else
        cout << "NO\n";

    // calling the function threadFunc
    // to put thread to sleep
    t1 = std::thread(threadFunc);

    cout << "t1 joinable when put to sleep? \n";

    // checking if t1 is joinable
    if (t1.joinable())
        cout << "YES\n";
    else
        cout << "NO\n";

    // joining t1
    t1.join();

    // checking joinablity of t1 after calling join()
    cout << "t1 joinable after join is called? \n";
    if (t1.joinable())
        cout << "YES\n";
    else
        cout << "NO\n";

    return 0;
}

Output:

t1 joinable when default created? 
NO
t1 joinable when put to sleep? 
YES
t1 joinable after join is called? 
NO

Note: The third output will appear 1 sec later because the thread was put to sleep for 1 minute.