No appropriate default constructor available for std::unique_ptr

Question

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.

Some of the posts I read:

Smart pointers with SDL

Is it possible to use SDL2 with smart pointers?

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);
}

The compiler complains that:

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

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.

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

Answer 1

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!";
    }
}

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 to 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 intp 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!";
    }
}

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!";
    }
}