Why is it necessary to use the joinable() check condition?

It is necessary to check whether a thread object is associated with an actual execution thread.

#include <thread>

void my_function() {
    // ...
}

int main() {
    std::thread t; // Initially not associated with an execution thread

    if (!t.joinable()) {
        // Thread is not associated, can assign or initialize
        t = std::thread(my_function);
    }

    if (t.joinable()) {
        // Thread is associated, can safely call join() or detach()
        t.join();
    }

    return 0;
}

Main reasons:

• Preventing Undefined Behavior: Calling join() or detach() on a std::thread object that is not associated with a thread (!t.joinable()) leads to undefined behavior.
• Resource Management: Allows determining whether the std::thread object owns resources of an actual thread and whether they need to be released (join() or detach()).
• Conditional Handling: Provides the ability to perform actions with the thread (waiting for completion, detaching) only if it is active.
• Initialization and Assignment: Enables safe reassignment of a std::thread object to a new thread only if the previous one was not associated.