MFC对象的创建顺序

     MFC中所有的对象以CObject为根，CObject派生出一些类：CCmdTarget、CWnd、CDocument。具体的层次关系如下：

               图（1）MFC的层次关系

      本案例以MFC程序代码为蓝本，采用Win32 Console Application程序（即控制台应用程序），来仿真MFC的内部行为，当然省略了消息映射和消息循环。

  新建一个Console工程，名称为Frame1，详细代码如下：

//mfc.h

#include <iostream.h>

class CObject
{
public:
	CObject::CObject(){cout<<"CObject Constructor \n";}
	CObject::~CObject(){cout<<"CObject Destructor \n";}
	
};

class CCmdTarget:public CObject
{
public:
	CCmdTarget::CCmdTarget(){cout<<"CCmdTarget Constructor \n";}
	CCmdTarget::~CCmdTarget(){cout<<"CCmdTarget Destructor \n";}
};

class CWinThread:public CCmdTarget
{
public:
	CWinThread::CWinThread(){cout<<"CWinThread Constructor \n";}
	CWinThread::~CWinThread(){cout<<"CWinThread Destructor \n";}
};

class CWinApp:public CWinThread
{
public:
	CWinApp* m_pCurrentWinApp;
public:
	CWinApp::CWinApp(){
		m_pCurrentWinApp=this;
		cout<<"CWinApp Constructor \n";
	}
	CWinApp::~CWinApp(){cout<<"CWinApp Destructor \n";}
};

class CDocument:public CCmdTarget
{
public:
	CDocument::CDocument(){cout<<"CDocument Constructor \n";}
	CDocument::~CDocument(){cout<<"CDocument Destructor \n";}
};

class CWnd:public CCmdTarget
{
public:
	CWnd::CWnd(){cout<<"CWnd Constructor \n";}
	CWnd::~CWnd(){cout<<"CWnd Destructor \n";}
};

class CFrameWnd:public CWnd
{
public:
	CFrameWnd::CFrameWnd(){cout<<"CFrameWnd Constructor \n";}
	CFrameWnd::~CFrameWnd(){cout<<"CFrameWnd Destructor \n";}
};

class CView:public CWnd
{
public:
	CView::CView(){cout<<"CView Constructor \n";}
	CView::~CView(){cout<<"CView Destructor \n";}
};

//global function
CWinApp* AfxGetApp();

//mfc.cpp

#include "my.h"
extern  CMyWinApp theApp;
CWinApp* AfxGetApp()
{
	return theApp.m_pCurrentWinApp;
}

//my.h

#include<iostream.h>
#include "mfc.h"

class CMyWinApp:public CWinApp
{
public:
	CMyWinApp::CMyWinApp(){cout<<"CMyWinApp Constructor \n\n";}
	CMyWinApp::~CMyWinApp(){cout<<"CMyWinApp Destructor \n";}
};

//my.cpp

#include "my.h"

CMyWinApp theApp; //global object

void main()
{
	CWinApp* pApp = AfxGetApp();

}

运行效果如下：

   解析：

        于是，你看到了，Frame1并没有new 任何对象，反倒是有一个全局对象 theApp存在，C++规定：全局对象的构建将比程序进入点（在DOS环境为main，在Windows环境为WinMain）更早，所以theApp 的构造函数将早于main()函数。即，上述构造函数的输出操作(XXX Constructor )都是在main()函数之前完成的。

       main()函数调用全局函数AfxGetApp()来获得theApp的对象指针，这完全是仿真MFC程序的手法。

参考文献：侯俊杰.深入浅出MFC(第二版).武汉.华中科技大学出版社,2001