SDL_Rect box = { 10, 10, 50, 10 };

while (!quit) {
    if (SDL_PollEvent(&event)) {
        switch (event.type) {
            case SDL_QUIT:
                quit = true;
                break;
        }
    }

    // clear render
    SDL_SetRenderDrawColor(renderer, 0, 0, 0, 0);
    SDL_RenderClear(renderer);

    box.w += 1;

    SDL_SetRenderDrawColor(renderer, 255, 255, 255, 0);
    SDL_RenderFillRect(renderer, &box);
    SDL_SetRenderDrawColor(renderer, 255, 0, 0, 0);
    SDL_RenderDrawRect(renderer, &box);

    // render
    SDL_RenderPresent(renderer);
    SDL_Delay(10);
}
