Tech Sujhav : std::thread::joinable()

joinable() function help us to know whether thread object has a ACTIVE thread or not. Thread remain ACTIVE although it has finished its execution.

Term ACTIVE can be more appropriate to explain as

Thread Object is/was assigned with a task by calling constructor.
Operations join() or detach() are not called on that thread object.

Thread created with default constructor is in non ACTIVE.

joinable() returns true if thread is active else false.

Let’s try to understand with the following examples that is easy to understand.

For better understanding you can copy and paste it in online compiler.

#include<iostream>
#include<thread> //Thread Header

using namespace std;

//Following function is the actual task
//to be performed by our thread

void displayActivity()
{
        cout<<"www.techsujhav.com "<<endl;
}
int main()
{
        cout<<"Going to start thread"<<endl;

     //thread object is created with default constructor
//hence joinable output::ACTIVE state is false (0)
        std::thread t_obj;
        cout<<"Thread before task assigned, (ACTIVE) joinable= "<<t_obj.joinable()<<endl;

     //assigned a task with the help of parameterized constructor
//hence joinable output::ACTIVE state is true (1)

        t_obj=std::thread(displayActivity);
        cout<<"Thread after task assigned, (ACTIVE) joinable= "<<t_obj.joinable()<<endl;

       //join operation is called on thread object.

   //hence joinable output::ACTIVE state is false (0)
        t_obj.join();
        cout<<"Thread after join() operation, (ACTIVE) joinable= "<<t_obj.joinable()<<endl;

       //Re-Assigning a NEW task to existing thread.

//hence joinable output::ACTIVE state is true (1)
        t_obj=std::thread(displayActivity);
        cout<<"Thread after new/re-assigned task, (ACTIVE) joinable= "<<t_obj.joinable()<<endl;

       //detach operation is called on thread object.

     //hence joinable output::ACTIVE state is false (0)
        t_obj.detach();
        cout<<"Thread after detach() operation, (ACTIVE) joinable= "<<t_obj.joinable()<<endl;

        cout<<"Main Exit"<<endl;
        return 0;
}

Compile:$ g++ joinable.cpp -o joinable -lpthread

Output:$ ./joinable

Going to start thread
Thread before task assigned, (ACTIVE) joinable= 0
Thread after task assigned, (ACTIVE) joinable= 1
www.techsujhav.com
Thread after join() operation, (ACTIVE) joinable= 0
Thread after new/re-assigned task, (ACTIVE) joinable= 1
Thread after detach() operation, (ACTIVE) joinable= 0
Main Exit

PREV:: std::thread (First Program)

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