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 window_;


        

Answer 1

I find this cleaner:

    auto window = std::unique_ptr<SDL_Window, std::function<void(SDL_Window *)>>(
            SDL_CreateWindow("Test", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, kWindowWidth, kWindowHeight, 0),
            SDL_DestroyWindow
    );

Answer 2

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_