Invalid application of 'sizeof' to incomplete type 'SDL_Window'

Question

Creating a pointer to an SDL_Window struct and assigning it to a shared_ptr, the mentioned error results.

Part of the class:

#include <SDL2/SDL.h>

class Application {
    static std::shared_ptr<SDL_Window> window;
}

Definition:

#include "Application.h"
std::shared_ptr<SDL_Window> Application::window{};

bool Application::init() {  
    SDL_Window *window_ = nullptr;
    if((window_ = SDL_CreateWindow(title.c_str(),
                                  SDL_WINDOWPOS_UNDEFINED,
                                  SDL_WINDOWPOS_UNDEFINED,
                                  window_width,
                                  window_height,
                                  NULL)
        ) == nullptr) {
        std::cerr << "creating window failed: " << SDL_GetError() << std::endl;
    }

    window.reset(window_);
}

The error appears at 'window.reset()'. What is the reason and how to fix this behavior?

Answer 1

By default, shared_ptr will release the managed resource using delete. However, if you're using a resource that needs releasing another way, you'll need a custom deleter:

window.reset(window_, SDL_DestroyWindow);

_{NOTE: I'm fairly sure this will work, but I don't have an SDL installation to test it with.}

Answer 2

As said previously by Mike, you'll have to specify your deleter with shared_ptr. For a unique_ptr, however, you'll probably want to create a special type for your deleter so that it can be cleanly used as a template parameter. I used this struct:

struct SDLWindowDeleter {
    inline void operator()(SDL_Window* window) {
        SDL_DestroyWindow(window);
    }
};

Then include the deleter via template parameter:

std::unique_ptr<SDL_Window, SDLWindowDeleter> sdlWindowPtr = SDL_CreateWindow(...);