使用相同类型的静态非空指针初始化后，指向自定义类型的静态指针保持 nullptr [英] static Pointer to Custom Type stays nullptr after initialization with static not-null pointer of same Type

问题描述

我有两个命名空间，每个命名空间都有一个指向其中一个命名空间内的类 Window 的指针.

I have two namespaces with each a pointer to a class Window inside of one of the namespaces.

Graphic.h

namespace Graphic {

    //...
    class Window;

    //...
    void init();
    static Window* window;
}

App.h

namespace App{

    //...
    static Graphic::Window* mainWindow = nullptr;
    //...
    void init();
    void setupGraphic();
    //...
}

我正在尝试使用 static Window* Graphic::window 初始化 static Window* App::mainWindow

I am trying to initialize the static Window* App::mainWindow with the static Window* Graphic::window

Window.h

#include "Graphic.h"
#include <SDL\SDL.h>

class Graphic::Window
{
public:
    Window(const char* pName = "OpenGL Window", 
           unsigned int pWidth = 1000, unsigned int pHeight = 700);
    ~Window();

    const char* name;
    unsigned int width;
    unsigned int height;
    SDL_Window* window;
};

Graphic::window 正在像这样初始化

Graphic.cpp

#include "Graphic.h"

void Graphic::init(){
    window = new Window("Engine");
}

在初始化之后，我尝试用 Graphic::window

And after this initialization i try to initialize the (static Window*)App::mainWindow with Graphic::window

App.cpp

#include "App.h"
#include <Graphic\Graphic.h>
#include <Graphic\Window.h>

void App::setupGraphic()
{
    Graphic::init();
    App::mainWindow = Graphic::window;
}

但是 App::mainWindow 保持 nullptr，即使 Graphic::window 已经成功初始化并且已经在 Graphic::init().没有编译警告/错误，我得到的只是一个异常App::mainWindow was nullptr."

But App::mainWindow stays nullptr, even though Graphic::window has successfully been initialized and has already been worked with in Graphic::init(). There are no compilation warnings/errors, all i get is an exception "App::mainWindow was nullptr."

推荐答案

在Graphic.h"中，你有

In "Graphic.h", you have

static Window* window;

此语句包含在将 #include Graphic.h 的每个翻译单元 (.cpp) 中.因此，每个单元都有其自己的变量window.然后发生的是 Graphic.cpp 分配自己的 window，但是 main.cpp 找到自己的变量 window不变.

This statement is included in every translation unit (.cpp) that will #include Graphic.h. Therefore each unit will have its own variable window. What happens then is that Graphic.cpp assigns its own window, but main.cpp find its own variable window unchanged.

你应该做的是:

在Graphic.h中，声明window但不定义它:

In Graphic.h, declare window but don't define it:

extern Window* window;

并且只定义一次，在 Graphic.cpp 中:

And define it only once, in Graphic.cpp:

Window* Graphic::window = nullptr;

这样所有的翻译单元都会引用同一个全局变量window.

This way all the translation units will refer to the same global variable window.

您应该对 App.h 中定义的变量 Graphic::Window* mainWindow 执行相同的操作.

You should do the same for the variable Graphic::Window* mainWindow defined in App.h.

extern Graphic::Window* mainWindow; // <-- in App.h

和

Graphic::Window* App::mainWindow = nullptr; // <-- in App.cpp

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