The examples in the documentation show the declaration of an instance of
Gtk::Main inside main() such as:

   Gtk::Main kit(argc, argv);

Such a declaration led me to believe that more than one could be instantiated. 
I believe I read somewhere that Gtk::Main is a singleton and so this isn't
allowed.  Am I correct, or am I misremembering?  If I am correct, why are
instances allowed to be created like this instead of something like the below
code that makes it clear that it's a singleton.  Am I just misunderstanding
something?

--- Header file ---
class Main {
public:
  Main* getInstance(int& argc, char**& argv, bool set_locale=true);
private:
  Main(int& argc, char**& argv, bool set_locale = true);
  Main* s_instance;
}

--- Code file ---
Main * Main::s_instance = NULL;
Main* Main::getInstance(int& argc, char**& argv, bool set_locale=true)
{
  if (s_instance == NULL)
    s_instance = new Main(argc, argv, set_locale);

  return s_instance;
}