Tech Sujhav : thread::join [Advanced] - Why do we need to call join() on thread object?

Question:-1, Why do we need to call join() on thread object, although created thread has already finished its task?

Question:-2, Why does program lead to abort if join() is not called?

Question:-3, Is it necessary to call join() or detach() on a thread Object?

Explanation:

Thread remain in ACTIVE state even though it has finished its assigned task. [Just scroll through the joinable () section]

Now if we do not call join() then state remains active i.e. joinable() == true and hence once thread-object goes out of scope it will call it’s destructor and inside the destructor there is check that if joinable() == true then call std::teriminate. Our program will stopped abruptly there itself.

NOTE:- It is important to observe that every event/operation (join/detach/joinable etc.) happening with thread-object, not with thread that Operating System is running.

Failure Case	Success case
#include<iostream> #include<thread> //Thread Header using namespace std; //actual task to be performed //by our thread void displayActivity() { cout<<"www.techsujhav.com : "<<endl; } int main() { cout<<"Going to start thread"<<endl; {//New scope started. std::thread t_obj(displayActivity); //Below for loop just to consume time //to show that "displayActivity" finished for(int i=0; i<900000000;i++); cout<<"Going out of scope"<<endl; }//<--program aborted here only cout<<"Main Exit"<<endl; return 0; }	#include<iostream> #include<thread> //Thread Header using namespace std; //actual task to be performed //by our thread void displayActivity() { cout<<"www.techsujhav.com : "<<endl; } int main() { cout<<"Going to start thread"<<endl; {//New scope started. std::thread t_obj(displayActivity); cout<<"Before Join, (ACTIVE) joinable=" <<t_obj.joinable()<<endl; t_obj.join(); cout<<"After Join, (ACTIVE) joinable=" <<t_obj.joinable()<<endl; cout<<"Going out of scope"<<endl; }//<--End of scope cout<<"Main Exit"<<endl; return 0; }
Output: $ ./join Going to start thread www.techsujhav.com : Going out of scope terminate called without an active exception Aborted	$ ./join Going to start thread Before Join, ACTIVE (joinable())=1 www.techsujhav.com : After Join, ACTIVE (joinable())=0 Going out of scope Main Exit
Important to note that “Main Exit” is not displayed in output.	Now it clearly visible that destructor has performed it’s activity without calling std::terminate.