Can C++11 tell if std::thread is active?

Question

To my surprise, a C++11 std::thread object that has finished executing, but has not yet been joined is still considered an active thread of execution. This is illustrated in

Answer 1

No, I don't think that this is possible. I would also try to think about your design and if such a check is really necessary, maybe you are looking for something like the interruptible threads from boost.

However, you can use std::async - which I would do anyway - and then rely on the features std::future provides you.

Namely, you can call std::future::wait_for with something like std::chrono::seconds(0). This gives you a zero-cost check and enables you to compare the std::future_status returned by wait_for.

auto f = std::async(foo);
...
auto status = f.wait_for(std::chrono::seconds(0));
if(status == std::future_status::timeout) {
    // still computing
}
else if(status == std::future_status::ready) {
    // finished computing
}
else {
    // There is still std::future_status::defered
}