Couple of questions about SDL_Window and unique_ptr

Question

I currently had a problem with storing a SDL_Window pointer as a std::unique_ptr.
What I tried was:

std::unique_ptr<SDL_Window> window_;

The solution:

std::unique_ptr<SDL_Window, void(*)(SDL_Window*)> window_;

The first attempt kept throwing errors in the memory header, saying SDL_Window is an incomplete type. Well I know that SDL_Window is a struct and can't be instanciated with

SDL_Window* window_ = new SDL_Window();

therefore the instanciation is done with SDL_CreateWindow(params).

Questions are:

1. Why can't I call the default constructor (or any other) for SDL_Window?

2. Why does the unique_ptr needs a deleter in this case, but not here:

   renderSystem_ = std::unique_ptr<Renderer::RenderSystem>(new Renderer::RenderSystem());
   

   RenderSystem being a class with just a default constructor, destructor.
   Is it because the unique_ptr can access the destructor, which acts as the deleter and doesn't need to come as a template argument?

Answer 1

The SDL_Window type is, just like the compiler says, incomplete.

The SDL library is using a common pattern in C language: pointers to incomplete type.

At the point you create the unique pointer, the SDL_Window type looks to the compiler like this:

struct SDL_Window;

That's one way you can create an incomplete type.

The compiler doesn't know anything except that SDL_Window is a type, and not a global variable or a function. This also means it cannot assume what size it has, it cannot assume that it has any constructors or a destructor.

As for the unique pointer to the SDL_Window, another way is to use this:

struct SDLWindowDestroyer
{
    void operator()(SDL_Window* w) const
    {
        SDL_DestroyWindow(w);
    }
};

std::unique_ptr<SDL_Window, SDLWindowDestroyer> window_;

Now you don't need to provide a function in a constructor to the window_