DLL中的单例被破坏

Question

I have C++ wrapped in C dll. The dll is called in my C# project.

In my wrapper functions I call a lot of Singletons, they are setup as follows:

ComponentManager &ComponentManager::_cmpManager()
{
    static ComponentManager ONLY_ONE;
    return ONLY_ONE;
}

The above function is a static function inside my ComponentManager class. Here is the specific problem:

  bool createNewEntity(char *c)
{
    if (ComponentManager::_cmpManager().nameAvailable(c))
    {
        Entity e(c);
        Transform t;
        ComponentManager::_cmpManager().addComponent(c, t);
        SceneNode sc(CMP_MANAGER2.getComponent<Transform>(c));
        SCENE_MANAGER.addSceneNode(sc, e.entityName);
        return true;
    }
    return false;
}

Essentially what this does is the singleton has a Hash Map with a key type string, this function checks to see if this key already exists. The behaviour is always returning true. When I use a global object of type componentManager instead of the singleton it behaves correctly, so something is telling me the singleton keeps leaving scope and deleting itself. Also if I use the singleton in an application exe rather than a dll it behaves correctly. So I have 2 questions,

1. Is there a way to keep my singleton from traveling out of scope? If not.
2. Is there another way of setting up singletons to not be deleted after leaving scope?

Answer 1

C++ static keyword is a bit different from C# static . See https://msdn.microsoft.com/en-us/library/y5f6w579.aspx for description.

In item 2 there it says: 2. When you declare a variable in a function, the static keyword specifies that the variable retains its state between calls to that function.

Try to declare your static not inside the method but in class scope (as per item 3 in above reference).

3. When you declare a data member in a class declaration, the static keyword specifies that one copy of the member is shared by all instances of the class. A static data member must be defined at file scope. An integral data member that you declare as const static can have an initializer.

You will also need to declare that static member at file scope.

Why it works when it is not in a dll -- it is not exactly clear. Probably a peculiar behaviour of linker. If class is declared in a dll it probably tries to instantiate class every time and your static inside the function is a new one every time. But if class is inside the exe file it is somehow the same class every time and when you call your method _cmpManager() it always accesses the same instance of the class.

Just my two cents :-).