没有合适的默认构造函数可用于 std::unique_ptr [英] No appropriate default constructor available for std::unique_ptr

问题描述

这是我上一篇文章的延续.既然已经关了，我决定发个新帖.我删除了一半代码以使其更具可读性.

This is a continuation of my previous post. Since it has already been closed I decided to make a new post. I removed half of the code to make it more readable.

我读过的一些帖子:

使用 SDL 的智能指针

是否可以将 SDL2 与智能指针一起使用?

关于 SDL_Window 和 unique_ptr 的几个问题

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:

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

我知道当编译器找不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 std::unique_ptr 的默认值.

I understand that this error usually shows up when the compiler cannot locate a default constructor for some of the members. However this is not true as I explicitly declared a default value for the std::unique_ptr.

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

If the compiler is actually complaining about SDL_Window, which is an incomplete type (a C struct), what can I do about this?

推荐答案

A std::unique_ptr 不是默认可构造的.这意味着在

A std::unique_ptr<SDL_Window, decltype(&SDL_DestroyWindow)> is not default constructable. That means in

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!";
    }
}

当您到达 *** 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.来自编译器 ro 默认构造 m_Window 的尝试是导致错误的原因.我们可以通过将 m_Window = std::move(Create_Window(xWin, yWin)); 移出构造函数体并将其放入成员初始化列表中来解决这个问题，如

When you reach the part *** the compiler is going to try and default construct m_Window since you didn't do so in the member initialization list. That attempt from the compiler ro default construct m_Window is what causes the error. We can fix this by moving m_Window = std::move(Create_Window(xWin, yWin)); out of the constructor body and putting it in the member initialization list like

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，然后像最初那样分配给 m_Window.那看起来像

If you don't want to do that then you can delegate to the default constructor and then assign to m_Window like you were doing originally. That would look like

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!";
    }
}

这篇关于没有合适的默认构造函数可用于 std::unique_ptr的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！