没有适当的默认构造函数可用于 std::unique_ptr

这是我上一篇文章的延续。由于它已经关闭，我决定发表新帖子。我删除了一半的代码以使其更具可读性。

我读过的一些帖子：

使用 SDL 的智能指针 https://stackoverflow.com/questions/24251747/smart-pointers-with-sdl

是否可以将 SDL2 与智能指针一起使用？ https://stackoverflow.com/questions/48672399/is-it-possible-to-use-sdl2-with-smart-pointers

关于 SDL_Window 和 unique_ptr 的几个问题 https://stackoverflow.com/questions/29424877/couple-of-questions-about-sdl-window-and-unique-ptr

class cGraphics
{
public:
    //  Creator functions
    std::unique_ptr<SDL_Window, decltype(&SDL_DestroyWindow)> Create_Window(int xWin, int yWin);

    //  ctor & dtor
    cGraphics() : m_Window(nullptr, SDL_DestroyWindow) {}
    cGraphics(int xWin, int yWin);
    ~cGraphics();
private:
    std::unique_ptr<SDL_Window, decltype(&SDL_DestroyWindow)> m_Window;

};

cGraphics::cGraphics(int xWin, int yWin)
{
    m_Window = std::move(Create_Window(xWin, yWin));

    if (m_Window == nullptr)
    {
        throw "SDL_Window or SDL_Renderer not ready!";
    }
}

cGraphics::~cGraphics()
{
    IMG_Quit();
    SDL_Quit();
}

std::unique_ptr<SDL_Window, decltype(&SDL_DestroyWindow)> cGraphics::Create_Window(int xWin, int yWin)
{
    return std::unique_ptr<SDL_Window, decltype(&SDL_DestroyWindow)>(SDL_CreateWindow("SDL Window", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, xWin, yWin, SDL_WINDOW_SHOWN), SDL_DestroyWindow);
}

编译器抱怨说：

'std::unique_ptr<SDL_Window,void (__cdecl *)(SDL_Window *)>::unique_ptr': no appropriate default constructor available

据我所知，当编译器无法找到某些成员的默认构造函数时，通常会出现此错误。然而这不是真的，因为我明确声明了默认值std::unique_ptr.

如果编译器实际上在抱怨SDL_Window，这是一个不完整的类型（C 结构体），我该怎么办？

A std::unique_ptr<SDL_Window, decltype(&SDL_DestroyWindow)>不可默认构造。这意味着在

cGraphics::cGraphics(int xWin, int yWin) ***
{
    m_Window = std::move(Create_Window(xWin, yWin));

    if (m_Window == nullptr)
    {
        throw "SDL_Window or SDL_Renderer not ready!";
    }
}

当你到达该部分时***编译器将尝试默认构造m_Window因为您没有在成员初始化列表中这样做。编译器对默认构造的尝试m_Window是什么导致了错误。我们可以通过移动来解决这个问题m_Window = std::move(Create_Window(xWin, yWin));从构造函数主体中取出并将其放入成员初始化列表中，例如

cGraphics::cGraphics(int xWin, int yWin) : m_Window(Create_Window(xWin, yWin))
{   
    if (m_Window == nullptr)
    {
        throw "SDL_Window or SDL_Renderer not ready!";
    }
}

如果您不想这样做，那么您可以委托给默认构造函数，然后分配给m_Window就像你原来做的那样。那看起来像

cGraphics::cGraphics(int xWin, int yWin) : cGraphics()
{
    m_Window = Create_Window(xWin, yWin);

    if (m_Window == nullptr)
    {
        throw "SDL_Window or SDL_Renderer not ready!";
    }
}