The difference between join and detach

A C++ thread object generally (but not always) represents a thread of execution, which is an OS or platform concept.

When thread::join() is called, the calling thread will block until the thread of execution has completed. Basically, this is one mechanism that can be used to know when a thread has finished. When thread::join() returns, the OS thread of execution has completed and the C++ thread object can be destroyed.

The thread::detach() is called, the thread of execution is "detached" from the thread object and is no longer represented by a thread object - they are two independent things. The C++ thread object can be destroyed and the OS thread of execution can continue on. If the program needs to know when that thread of execution has completed, some other mechanism needs to be used. join() cannot be called on that thread object any more, since it is no longer associated with a thread of execution.

It is considered an error to destroy a C++ thread object while it is still "joinable". That is, in order to destroy a C++ thread object either join() needs to be called (and completed) or detach() must be called. If a C++ thread object is still joinable when it's destroyed, an exception will be thrown.

Some other ways that a C++ thread object will not represent a thread of execution (ie., can be unjoinable):

• A default constructed thread object does not represent a thread of execution, so is not joinable.
• A thread that has been moved from will no longer represent a thread of execution, so is not joinable.

 

join() doesn't kill the thread. Actually it waits until thread main function returns. So if your thread main function looks like this:

while (true) {
}

join() is going to wait forever.

detatch() doesn't kill thread either. Actually it tells std::thread that this thread should continue to run even when std::thread object is destroyed. C++ checks in std::thread destructor that thread is either joined or detached and terminates program if this check fails.

So if you uncomment first line in main function of the following code it will crash. If you uncomment second or third line it will work ok.

#include <thread>

void func() {
}

void fail1() {
    std::thread t(func);
    // will fail when we try to destroy t since it is not joined or detached
}

void works1() {
    std::thread t(func);
    t.join();
}

void works2() {
    std::thread t(func);
    t.detach();
}

int main() {
    // fail1();
    // works1();
    // works2();
}