Question

Can C++ objects have block scope? For example, is this ok: (it crashes)

(Go easy, I’m still learning C++)

__block Poco::Thread* lastThread;

dispatch_async(dispatch_get_main_queue(), ^
{
    for (int i = 1; i <= 5; i++)
    {
        Poco::Runnable* worker = new  Worker(_counter, "worker" + Poco::NumberFormatter().format(i));
        Poco::Thread* workerThread = new Poco::Thread();
        workerThread->start(*worker);
        lastThread = workerThread;
    }
});

lastThread->join(); //wait so we can watch what happens. 

Answer 1

Your code is valid, nothing wrong with declaring that pointer __block scope. But your code will crash because lastThread does not point to any object when join() is invoked. You run that block asynchronously, so almost certainly lastThread->join() is reached before lastThread will point to the worker thread.