MFC 虚函数

// Test.cpp : Defines the class behaviors for the application.
//

#include "stdafx.h"
#include "Test.h"
#include "TestDlg.h"

#ifdef _DEBUG
#define new DEBUG_NEW
#undef THIS_FILE
static char THIS_FILE[] = __FILE__;
#endif

/////////////////////////////////////////////////////////////////////////////
// CTestApp

BEGIN_MESSAGE_MAP(CTestApp, CWinApp)
//{{AFX_MSG_MAP(CTestApp)
// NOTE - the ClassWizard will add and remove mapping macros here.
//    DO NOT EDIT what you see in these blocks of generated code!
//}}AFX_MSG
ON_COMMAND(ID_HELP, CWinApp::OnHelp)
END_MESSAGE_MAP()

/////////////////////////////////////////////////////////////////////////////
// CTestApp construction

CTestApp::CTestApp()
{
// TODO: add construction code here,
// Place all significant initialization in InitInstance
}

/////////////////////////////////////////////////////////////////////////////
// The one and only CTestApp object

CTestApp theApp;

/*
1.全局函数（程序入口）：
_tWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPTSTR lpCmdLine, int nCmdShow)
{
// call shared/exported WinMain
return AfxWinMain(hInstance, hPrevInstance, lpCmdLine, nCmdShow); // 全局函数
}
看见了吗？里面调用了AfxWinMain(),也就说MFC把对WinMain()的调用转化为对AfxWinMain()的调用，这更加说明，即使在MFC中，
也没有什么其它神奇的方法能够跳过或换掉对WinMain()的调用。

2.在WINMAIN.CPP文件中，实现了AfxWinMain()。
AfxWinMain()才实际调用CWinApp->InitApplication()、CWinApp->->InitInstance()完成系统初始化。

int AFXAPI AfxWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
LPTSTR lpCmdLine, int nCmdShow)
{
ASSERT(hPrevInstance == NULL);

int nReturnCode = -1;
CWinThread* pThread = AfxGetThread(); // 得到进程指针
CWinApp* pApp = AfxGetApp();  // CWinApp 是 CWinThread 的爸

// AFX internal initialization
if (!AfxWinInit(hInstance, hPrevInstance, lpCmdLine, nCmdShow))
goto InitFailure;

// App global initializations (rare)
if (pApp != NULL && !pApp->InitApplication()) // 虚函数 子类实现了用子类的 子类没实现 用父类的
goto InitFailure;

// Perform specific initializations
if (!pThread->InitInstance())            // // 虚函数 子类实现了用子类的 子类没实现 用父类的
{
if (pThread->m_pMainWnd != NULL)
{
TRACE0("Warning: Destroying non-NULL m_pMainWnd\n");
pThread->m_pMainWnd->DestroyWindow();
}
nReturnCode = pThread->ExitInstance();
goto InitFailure;
}
nReturnCode = pThread->Run();

InitFailure:
#ifdef _DEBUG
// Check for missing AfxLockTempMap calls
if (AfxGetModuleThreadState()->m_nTempMapLock != 0)
{
TRACE1("Warning: Temp map lock count non-zero (%ld).\n",
AfxGetModuleThreadState()->m_nTempMapLock);
}
AfxLockTempMaps();
AfxUnlockTempMaps(-1);
#endif

AfxWinTerm();
return nReturnCode;
}

*/

//DEL BOOL CTestApp::InitInstance()
//DEL {
//DEL 	// Standard initialization
//DEL 	// If you are not using these features and wish to reduce the size
//DEL 	//  of your final executable, you should remove from the following
//DEL 	//  the specific initialization routines you do not need.
//DEL
//DEL 	CTestDlg dlg;
//DEL 	m_pMainWnd = &dlg;
//DEL 	int nResponse = dlg.DoModal();
//DEL 	if (nResponse == IDOK)
//DEL 	{
//DEL 		// TODO: Place code here to handle when the dialog is
//DEL 		//  dismissed with OK
//DEL 	}
//DEL 	else if (nResponse == IDCANCEL)
//DEL 	{
//DEL 		// TODO: Place code here to handle when the dialog is
//DEL 		//  dismissed with Cancel
//DEL 	}
//DEL
//DEL 	// Since the dialog has been closed, return FALSE so that we exit the
//DEL 	//  application, rather than start the application's message pump.
//DEL 	return FALSE;
//DEL }

BOOL CTestApp::InitApplication()
{
// TODO: Add your specialized code here and/or call the base class

return CWinApp::InitApplication();
}

BOOL CTestApp::InitInstance()
{
// TODO: Add your specialized code here and/or call the base class

return CWinApp::InitInstance();
}

虚函数用法就是 

1.使用子类定义对象

CTestApp theApp;

2.使用父类指针指向子类对象

CWinThread* pThread = AfxGetThread(); // 得到进程指针 0x0052d6c0
CWinApp* pApp = AfxGetApp(); // CWinApp 是 CWinThread 的爸 0x0052d6c03.用父类对象调用虚函数(实现到哪个子类，就会调用哪个子类哪个虚函数)
#include <iostream>
using namespace std;
class Parent
{
protected:
int i;
int j;
public:
virtual void Show()
{
printf("Parent()\n");
}

void fun(Parent & p)
{
Show();
}
};

class Child1 : public Parent
{
public:
virtual void Show()
{
printf("Child1\n");
}
};

class Child2 : public Child1 // 在父类中的构造函数中条用了虚函数 所以当子类实现了 因为多肽就会使用子类的 当子类不实现 就使用父类的
{
public:
// virtual void Show()
// {
// printf("Child2\n");
// }
};

int main()
{
Child1 c1;
Child2 c2;

// Parent *p[] = {&c1, &c2};
//
// p[0]->fun(c1);
// p[1]->fun(c2);
// p[0]->Show();
// p[1]->Show();
Parent &p = c2;
p.fun(c2);
return 0;
}如果Child2中不实现那个虚函数，在调用的时候回调用父类的函数，所以就像MFC一样，给了用户一个借口，如果用户不实现，程序会默认去使用父类实现的虚函数，如果子类中实现了虚函数，那就使用子类默认的函数.
就像MFC CTestApp::InitInstance()中如果在 CTestApp中没有被实现，就会使用

BOOL CWinApp::InitInstance()
{
return TRUE;
}