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-   -   Using virtual-C++ vs MFc (http://www.go4expert.com/forums/using-virtual-cpp-vs-mfc-t1039/)

moihere 21Jul2006 15:55

Using virtual-C++ vs MFc
 
Hi,

I am stuck with a doubt! I have been working with MFC based applications for sometime. Decided to read up some theory to strengthen my fundamentals. Naturally, I was reading MFC concepts as well as C++ basics. What struck me was the way we use VIRTUAL in both.

In C++, there goes a standard way to use virtual functions.
- There is a Base class B having a function func() and its derived class D also uses/implements the function func(). If a function creates objects of both B and D and calls this func() through a pointer to B, the base class version of this function will be called UNLESS we declare func() as virtual .right??? So if we want to call func() from D's implemetation, we declare func() as virtual in Base class B , pass a reference of D's object to base class B's pointer and then call func() thru this pointer. I guess i got this right so far???

Now, how do we go about it in MFC(which is also supposed to be C++ at the end of the day)? From what I have used so far..i use virtual funcs like this.

Class CDialog has 3 virtual functions. Lets take OnInitDialog() as the example.
Code: CPP

class CDialog
{
 -----
 virtual BOOL OnInitDialog();
-----
}

In my application, i mostly derive my own class from CDialog,
Code: CPP

eg class CMyDlg : public CDialog
     {
       protected:
    virtual BOOL OnInitDialog();
       }

I override the OnInitDialog() function to suit my initialization needs.
Then, how do i call OnInitDialog()??
Answer: I do it THROUGH DoModal().
Code: CPP

eg: OnBtOpenDlg()
    {
       CMyDlg myDLg;
       if(myDlg.DoModal()==ID_OK)
            { --------}
    }

This takes care of calling the derived class version of OnInitDialog() despite the fact that OnInitDialog is a virtual function inthe base class. We didn't assign any reference of myDlg to a base class pointer or anything?????????????????????????? :confused:

Am i missing something or are they implemented differently in C++ and MFC programming????????

shabbir 21Jul2006 16:46

Re: Using virtual-C++ vs MFc
 
Quote:

Originally Posted by moihere
pass a reference of D's object to base class B's pointer and then call func() thru this pointer

Thats when you need to call the derived class func from the base class but when you want to just call the function using the typecasted pointer its not needed.

In MFC the case is the pointer is typecasted and so when the base class member is virtual correct OnInitDialog is called. Here is the code from MFC.

Code: CPP

BOOL CALLBACK AfxDlgProc(HWND hWnd, UINT message, WPARAM, LPARAM)
{
    if (message == WM_INITDIALOG)
    {
        // special case for WM_INITDIALOG
        CDialog* pDlg = DYNAMIC_DOWNCAST(CDialog, CWnd::FromHandlePermanent(hWnd));
        if (pDlg != NULL)
            return pDlg->OnInitDialog();
        else
            return 1;
    }
    return 0;
}

pDlg is type casted to your dialog instance and so your OnInitDialog is called.

moihere 21Jul2006 19:26

Re: Using virtual-C++ vs MFc
 
So would the same logic be applied to all virtual functions overridden in MFC?(functions of CObject, of CWinApp, InitInstance etc.) Is it correct to say that most of these virtual functions which are overridden are downcasted/typecasted and the correct version of the function is called??

shabbir 21Jul2006 19:32

Re: Using virtual-C++ vs MFc
 
Quote:

Originally Posted by moihere
So would the same logic be applied to all virtual functions overridden in MFC?(functions of CObject, of CWinApp, InitInstance etc.) Is it correct to say that most of these virtual functions which are overridden are downcasted/typecasted and the correct version of the function is called??

Yes.

moihere 21Jul2006 19:34

Re: Using virtual-C++ vs MFc
 
Thanks!! That clears a big missing link !

shabbir 21Jul2006 20:36

Re: Using virtual-C++ vs MFc
 
My pleasure


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