关于深入浅出mfc中frame2示例的问题_百度知道

[whypp]

第三章里面《六大关键技术之仿真》frame2范例中，最后调到的是CFrameWnd::PreCreateWindow
而不是CWnd::PreCreateWindow，
据作者解释是因为“第2章的「Object slicing 与虚拟函数」
一节所说提「虚拟函数的一个极重要的行为模式」”，

根据我的理解（也来自第二章），Object slicing是在把派生类对象强制转化成父类使用的时候，编译器会将父类的虚拟表以及成员变量，成员函数复制一份给该派生类对象，使之完全成为一个其父类的对象，通过该对象调的成员函数，都是父类的成员函数。

但是在第三章这个例子里，并不存在子类强制转化为父类使用的情况，而是CFrameWnd继承了父类CWnd的CreateEx()函数，其this指针是指向CFrameWnd类的对象的，所以才调到了CFrameWnd::PreCreateWindow（）。

侯老师却说是因为Object slicing，我就糊涂了，高手请解惑。

MFC.H

#define BOOL int
#define TRUE 1
#define FALSE 0

#include <iostream.h>

class CObject
{
public:
  CObject::CObject()  {
                      }
  CObject::~CObject() {
                      }
};

class CCmdTarget : public CObject
{
public:
  CCmdTarget::CCmdTarget()  {
                            }
  CCmdTarget::~CCmdTarget() {
                            }
};

class CWinThread : public CCmdTarget
{
public:
  CWinThread::CWinThread()  {
                            }
  CWinThread::~CWinThread() {
                            }

  virtual BOOL InitInstance() {
                                cout << "CWinThread::InitInstance \n";
                                return TRUE;
                              }
  virtual int Run() {
                      cout << "CWinThread::Run \n";
                      return 1;
                    }
};

class CWnd;

class CWinApp : public CWinThread
{
public:
  CWinApp* m_pCurrentWinApp;
  CWnd* m_pMainWnd;

public:
  CWinApp::CWinApp()  {
                        m_pCurrentWinApp = this;
                      }
  CWinApp::~CWinApp() {
                      }

  virtual BOOL InitApplication() {
                                   cout << "CWinApp::InitApplication \n";
                                   return TRUE;
                                 }
  virtual BOOL InitInstance()    {
                                   cout << "CWinApp::InitInstance \n";
                                   return TRUE;
                                 }
  virtual int Run() {
                      cout << "CWinApp::Run \n";
                      return CWinThread::Run();
                    }
};


class CDocument : public CCmdTarget
{
public:
  CDocument::CDocument()   {
                           }
  CDocument::~CDocument()  {
                           }
};


class CWnd : public CCmdTarget
{
public:
  CWnd::CWnd()   {
                 }
  CWnd::~CWnd()  {
                 }

  virtual BOOL Create();
  BOOL CreateEx();
  virtual BOOL PreCreateWindow();
};

class CFrameWnd : public CWnd
{
public:
  CFrameWnd::CFrameWnd()   {
                           }
  CFrameWnd::~CFrameWnd()  {
                           }
  BOOL Create();
  virtual BOOL PreCreateWindow();
};

class CView : public CWnd
{
public:
  CView::CView()   {
                   }
  CView::~CView()  {
                   }
};


// global function

CWinApp* AfxGetApp();

[whypp]

MY.H

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

class CMyWinApp : public CWinApp
{
public:
  CMyWinApp::CMyWinApp()   {
                           }
  CMyWinApp::~CMyWinApp()  {
                           }

  virtual BOOL InitInstance();
};

class CMyFrameWnd : public CFrameWnd
{
public:
  CMyFrameWnd();
  ~CMyFrameWnd()  {
                  }
};

[whypp]

MFC.cpp

#include "my.h"  // it should be mfc.h, but for CMyWinApp definition, so...

extern CMyWinApp theApp;

BOOL CWnd::Create()
{
  cout << "CWnd::Create \n";
  return TRUE;
}

BOOL CWnd::CreateEx()
{
  cout << "CWnd::CreateEx \n";
  PreCreateWindow();
  return TRUE;
}

BOOL CWnd::PreCreateWindow()
{
  cout << "CWnd::PreCreateWindow \n";
  return TRUE;
}

BOOL CFrameWnd::Create()
{
  cout << "CFrameWnd::Create \n";
  CreateEx();
  return TRUE;
}

BOOL CFrameWnd::PreCreateWindow()
{
  cout << "CFrameWnd::PreCreateWindow \n";
  return TRUE;
}


CWinApp* AfxGetApp()
{
  return theApp.m_pCurrentWinApp;
}

[whypp]

MY.cpp

#include "my.h"

CMyWinApp theApp;

BOOL CMyWinApp::InitInstance()
{
    cout << "CMyWinApp::InitInstance \n";
    m_pMainWnd = new CMyFrameWnd;
    return TRUE;
}

CMyFrameWnd::CMyFrameWnd()
{
    cout << "CMyFrameWnd::CMyFrameWnd \n";
    Create();
}

//------------------------------------------------------------------
// main
//------------------------------------------------------------------
void main()
{

CWinApp* pApp = AfxGetApp();

pApp->InitApplication();
pApp->InitInstance();
pApp->Run();
}
//------------------------------------------------------------------