Safe singletons

Gamedev Framwork (gf) provides two classes to define singletons: gf::Singleton and gf::SingletonStorage. It implements a variant of the singleton pattern.

How to use singleton classes?

gf::Singleton is a wrapper around a pointer. The pointer is provided by a gf::SingletonStorage that is allocated in the stack. As soon as the storage is allocated, the singleton can be used like a global variable.

// a global variable
gf::Singleton<gf::MessageManager> gMessageManager;

int main() {
  // allocate storage for the singleton
  gf::SingletonStorage<gf::MessageManager> storageForMessageManager(gMessageManager);

  // from now on, you can access the singleton like a global variable
  gMessageManager().sendMessage<Ping>(nullptr);
}

But beware: singletons are bad. You should try to create as few singletons as possible. Game development is one example where singletons can be used. Just because a game is a very complex piece of code and that singletons sometimes make the code more readable. Just know what you are doing.

Why two classes?

Short answer: to prevent the static initialization order fiasco.

Long answer. Global variables are initialized at the start of the main function. The order of initialization depends on the compiler so that it can be quite messy if a global variable needs another global variable. This is called the static initialization order fiasco. There is no direct and simple workaround for this problem.

One solution is to use two classes. The first class is gf::Singleton and simply holds a pointer. Global variables of this type can be safely defined as the order of initialization is not relevant. The second class is gf::SingletonStorage and is allocated in the main function. It serves two purposes: provide an allocated object to the singleton; handle the lifetime of the object. Indeed, objects can be initialized like any other variables so the order is well specified. A bonus is the automatic deallocation at the end of the program.