Method thread2

dlib/test/threads.cpp:110–115

        void thread1() { thread(1); }
        void thread2() 
        { 
            thread(2); 
            if (is_dlib_thread() == false)
                failure = true;
        }
        void thread3() { thread(3); }
        void thread4() { thread(4); }
        void thread5() { thread(5); }