Every ZooLib application's client code starts up in ZMain(). ZMain is used rather than main() because different operating systems have different conventions for the parameters to be passed to main, and on Windows, WinMain is used rather than main(). Most ZooLib applications will have a ZMain that is much like ZHelloWorld's:

int ZMain(int argc, char** argv)
{
    try
    {
        ZHelloWorld_App theApp;
        theApp.Run();
    }
    catch (...)
    {}

    return 1;
}
    

The first thing we do is create an instance of our application object, here ZHelloWorld_App. It is a subclass of ZApp. We call the ZApp's Run() method, and when Run() finishes, we exit ZMain, and the program terminates.

There are ordinarily several threads in a ZooLib application. The application object has its own thread, and each window has a thread. The window threads come and go as windows are created and closed, but the application object's thread keeps running the whole time.