C++ const safety

Question

In C++11, I create threads with std::thread. And I have some global variables:

const int max_size = 78;
const int cat[5] = {1, 20, 3, 40, 5};

Now, if my threads read those variables, is there any chance of unidentified behavior?

Answer 1

As long as those variables are never written to (yes it is possible to write to const variables in C++ via pointer manipulation, but for what reason?) then no.

If you are worried about undefined or unspecified behavior (with respect to thread safety), you could always use a mutex.

An OK example:

// Globals
const int max_size = 78;
const int cat[5] = {1, 20, 3, 40, 5};

void call_from_thread() {
    std::cout << "Max Size: " << max_size << std::endl;
    std::cout << "cat[0]: " << cat[0] << std::endl;
    //..
    std::cout << "cat[4]: " << cat[4] << std::endl;
}

int main() {
    //Launch some threads
    std::thread thread1(call_from_thread);
    std::thread thread2(call_from_thread);

    //Join the threads with the main thread
    thread1.join();
    thread2.join();

    //..

    *((int*)&cat[3])=15; // Since threads are joined, this is OK as far as the write is concerned
    return 0;
}

In the above example, writing to a const variable can cause undefined behavior. So it is still a REALLY BAD idea.

A really really bad example:

int main() {
    //Launch some threads
    std::thread thread1(call_from_thread);
    std::thread thread2(call_from_thread);

    *((int*)&cat[3])=15; // BAD idea for thread safety as well as what is discussed above.

    //Join the threads with the main thread
    thread1.join();
    thread2.join();

    //..

    return 0;
}