Re: recreate a window

".rhavin grobert" <>
Tue, 11 Nov 2008 09:16:35 -0800 (PST)
On 11 Nov., 17:42, Joseph M. Newcomer <> wrote:

See below...

On Tue, 11 Nov 2008 05:51:17 -0800 (PST), ".rhavin grobert" <cl...@yahoo.=

de> wrote:

On 10 Nov., 20:43, Joseph M. Newcomer <> wrote:

See below...
On Mon, 10 Nov 2008 10:26:29 -0800 (PST), ".rhavin grobert" <cl...@yah=> wrote:

On 10 Nov., 18:59, Joseph M. Newcomer <> wrote:

See below...

On Mon, 10 Nov 2008 08:09:56 -0800 (PST), ".rhavin grobert" <cl...@=> wrote:

a CComboBox and CQControl derived control shall recreate itself, i=


the styles given in resource not match the style required for toe
control (here: CBS_OWNERDRAWFIXED must be set). As CComboBox is un=


to correctly modify this style with ModifyStyle(), the recreation =


called and works.

The error is that you did not do this at design time. Essentiall=

y, I treat the failure to

have the wrong styles as an error. Not sure why you are botherin=

g to work around a bug

that shouldn't exist. If this happens, you ASSERT and then fix t=

he dialog at design time,

and omit ALL the code below.
CComboBox is not *supposed* to be able to modify this style after t=

he control is created,

so it is not "unable to correctly modify this style" because it was=

 never defined that

this is even possible.

thats why i recreate it.

My point is, a program which has one of these which is NOT defined as =


is an erroneous program, so you fix it in the dialog template once, an=

d never again worry

about it.

if there's no way to let my code correct the error, i'd assert. but i
thought to use this flags myself, e.g. recreate a control with the
appropriate windowsflags needed for the control, and use the user-
defined flags for my own implementation (in this case, calling a user-
defined drawing routine instead of my own default drawing routine.)

You should not go to extensive effort to correct a defect that can be cor=

rected at design




// recreate the underlying hwnd to match required styles
bool CQControl::QRecreateWindow(DWORD dwStyle, DWORD dwStyleEx)
   CWnd* pWnd = CWnd::FromHandlePermanent(m_hwnd);

This is unnecessary; 'this' is already the CWnd *. So there is n=

o need for the above


no it is not. 'this' is a CQControl that has no CWnd as baseclass! fo=


example, the
CQComboBox is a public CComboBox, public CQControl.

That's a different question than the one you asked. You did not spe=

cify that you are

using multiple inheritance.

i did'nt say i'm not using it;-) But you're right, assuming direct(?)
inheritance is the default.

On the whole, MFC is not multiple-inheritance friendly. You can get in=

to all kinds of

problems using multiple-inheritance in MFC, which is why most of us avoid=


ATL and COM use it, but they use it *very carefully*. And they don't u=

se it for ordinary

controls, but special ATL and COM objects

But in that case, FromHandle is sufficient.

i just wanted to be shure.

****> ASSERT(pWnd != NULL);

   ASSERT(pWnd == CWnd::FromHandle(m_hwnd));
   if (pWnd == NULL)
           return false;

The above statements are meaningless. You already have 'this' or=

 you wouldn't be here. It

doesn't matter about what map it is in; you shouldn't care, either.

see above.

You have to provide *all* relevant information in order to get a corre=

ct answer


Note that I use this technique rather often to create a control in =

a space formerly

occupied by a CStatic, so I'm changing the entire control type. =

This code is FAR too

complicated for such a simple task.

a static in Dialog-Editor would not look like a combobox.

   CRect rc;
   HWND hParent = GetParent(m_hwnd); =

    // [1]

CWnd * parent = GetParent();

wrong. see above.

If you use multiple inheritance, a previously unstated case, then yes.=

  But you can't ask

a question without giving all relevant information and expect to get a=

 relevant answer


   // get position and size of current window
   GetWindowRect(m_hwnd, &rc); =

                        // [2]

I'm surprised this even compiles. There is no need to use the ra=

w HWND. You could just

have written
and get everything you need.

see above;-)

   // following fn's need width and size, we recycle rc
   rc.right = rc.Width(); =

      // [3]

   rc.bottom = rc.Height(); =

                                    // =

This code is incorrect. You must not set the right and bottom in=

 this fashion, because

you are working with the window rect, and the width and height are =

independent of the

coordinate system. Lose the two assignments above.

   // translate into parents coordinates
   ScreenToClient(hParent, &rc.TopLeft()); =

               // [5]

Why are you not doing something as simple as
                CRect rc;
this is all you need. You have a complex and convoluted solution=

 to a simple problem.

Just translating the top/left coordinate makes no sense, because it=

 will produce a window

that has the wrong size.

no. everything works pretty well. i you insist, one could of course
define a struct RECT_ that has let, top, width and height members, bu=


apart from that, my way is completely ok.

Why do you need to define a new structure? You can just do
        CRect rc;
        ::GetWindowRect(m_hWnd, &rc);
        ::ScreenToClient(parent, &rc);
where, now that I know you are using multiple inheritance, is still si=

mpler than the

complex code you wrote. And by setting the right and bottom to the =

Width() and Height(),

BEFORE transforming the top and left, you still get the wrong size.

=E4hm, no.. both rects are in pixel.

Consider the following: the rect is in the more-or-less middle of the scr=

een, and

somewhere inside of the window.

Let the screen coordinates be 600,400, 700, 500, that is, it is a 100x100=


physically at 600,400.

You set rc.right=rc.Width() and rc.bottom=rc.Height(). So we have
        rc.right = 100;
        rc.bottom = 100;

Let the (0,0) of the client area of the window that contains the control =

be at 400, 200 in

screen coordinates.

Now we transform with ScreenToClient the initial coordinates, just top an=

d left, to client

coordinates. In client coordinates, 600,400 translates to 200, 200 in =

client coordinates.

So you have a rectangle that starts at 200,200 and ends at 100,100, when =

your goal is to

have one that starts at 200,200 and goes to 300,300. Since right < lef=

t and bottom < top,

what you have is actually an illegal rectangle.

in the view of "rectangle", you're right. but i have now something
different: i have struct that has x,y,width,height as needed by
CreateWindowEx(). but to dont make thinks to complicated for other i
now use the following struct:

struct SQDim {
    UINT nLeft;
    UINT nTop;
    UINT nWidth;
    UINT nHeight;

    inline SQDim(UINT left = 0, UINT top = 0, UINT width = 0, UINT heigh=
= 0) :
        nLeft(left), nTop(top), nWidth(width), nHeight(height) {};

    inline SQDim const& operator =(SQDim const& rDim)
        {memcpy(this, &rDim, sizeof(this));};

    inline SQDim const& operator =(RECT const& rRect)
        nLeft = rRect.left;
        nTop =;
        nWidth = rRect.right - nLeft;
        nHeight = rRect.bottom - nTop;

    inline operator RECT() const
        {RECT rc = {nLeft, nTop, nWidth + nLeft, nHeight + nTop}; return

Had you just done ScreenToClient on the whole rectangle, you would have g=


        200,200, 300, 300
which is what you would want. right and bottom are not distances, but =

absolute positions

within the coordinate system in question, which in this case is the clien=

t coordinate

system (after translation). But you translate left and top as coordina=

tes and right and

bottom as distances. This is erroneous.

You have five overly complicated lines doing what two trivial lines=

 will accomplish.


   // let derived class modify rc

I have no idea what this does because there is no handler called On=

CreationRect and you

have not given the code for this, but there's nothing I know that n=

eeds to be done to this

rectangle once you have it.

in this case, the derived class uses this virtual as following:

void CQComboBox::OnCreationRect(RECT& rc)
   RECT rcDropped;
   rc.bottom = rcDropped.bottom -;

Using the 'On' prefix, which is used for handlers, is confusing. Ca=

lling it something

meaningful like ComputeDroppedRect might have made it less ambiguous a=

s to what was going


   // create a new window at current windows coordinates
   HWND hNew = CreateWindowEx(dwStyleEx, QGetClassName(), "=

", dwStyle,

           rc.left,, rc.right, rc.bottom, hPar=

ent, 0,

           ::AfxGetInstanceHandle(), NULL);

Why are you using the raw CreateWindowEx call? You can do this t=

rivially in MFC:

        CWnd temp;
                temp.CreateEx(dwStyleEx, QGetClassN=

ame(), _T(""), dwStyle, rc,

                        GetDlgCtrlId(), par=


Note how simple this is. For that matter, does CQControl have a =

Create method that would

be better?

I have no idea why you have chosen to change the control ID by sett=

ing it to 0; this kills

off all your handlers in the parent window. For a brief moment y=

ou will have two controls

with the same ID, but this will be changed momentarily, and causes =

no harm.

oh, i thought it was hMenu.

Note that it is specified as being the hMenu for a top-level window an=

d the control ID for

a child window. From the documentation:

"For a child window, hMenu specifies the child-window identifer, an in=

teger value..."

It will have to be cast to an HMENU to satisfy the compiler.

But what you didn't do was make sure it is in the right place in th=

e Z-order

        temp.SetWindowPos(this, 0, 0, 0, 0, SWP_NOMOVE | SW=



All this use of raw API and HWNDs is unneccessay.

   // destroy old window and attach new one
   m_hwnd = hNew;

Get rid of all the above lines.

The convoluted nature of what you are doing is probably what leads =

to the assertion

failure. I wouldn't even try to debug this code, I would rewrite=

 it as indicated. Then,

and only then, if there is still a bug, would I worry about it.

Since MFC does not normally support multiple inheritance, there can be=

 some confusing side

effects here that may account for it. What is the type of the objec=

t displayed by the


it displayes the expected type (CQComboBox)

Then you should not be seeing the error. But, as I indicated, MFC does=

 not play well with

multiple inheritance, and virtual destructors in a multiple inheritance s=

ituation can have

interesting consequences. You're off into uncharted territory here.

Is there a reason you chose to use multiple inheritance over direct in=

heritance? You are

working in unsupported areas here.

of course;-) i have a baseclass (CQControl) from with CQButton,
CQTree, CQComboBox and so on derive. Each derived class uses multiple
inheritance to derive from CQControl and its MFC-class. By doind this,
i can do the following:

void SomeFunction(CQControl* pCtrl)
   // add an red blinking element with id ELEMENT_ID to the control
   // set text of element ELEMENT_ID with detail id DETAIL_ID to "S=


   // and add an image
   pCtrl->QDtlSet(ELEMENT_ID, DETAIL_ID, _T("Some Text"),
   // Select element ELEMENT_ID

if pCtrl is a CQListView, you set element-row ELEMENT_ID, detail-
column DETAIL_ID to image ELEMENT_IMAGE, detail DETAIL_ID. if pCtrl is
a CQButton and you have more than one element, you get a multibutton.
if you dont have "columns" (Button, ListBox,..), the details appear
tab-separated. So you have the same syntax for all controls. Use in
programm: guess you have a function that shall fill a selection-
control with user-names and some additional infos (an icon per user or
a user-state-color and so on). this function just gets a CQControl*
and fills it. if it fills a Listview, a Treeview, a ListBox, a
ComboBox or even a Button just depents on the design of the GUI. More:
CQControl uses a special worker thread and does all the syncronisation
needed, so you could call the above SomeFunction() from ANY Thread;-)

Yes, it is a cool idea, but as I've indicated, you are out into seriously=


territory here, and most of the ideas are not supported by the MFC framew=

ork, which

aggressively is single-inheritance.

You can have the same syntax for all controls without requiring multiple =

inheritance, but

I agree it is painful to duplicate code. As far as I know, no one has =

succeeded in doing


;-))))))))))))) What i wrote is no "i plan to do this". IT
WORKS! ;-)))

  I tried it myself some years ago for the same purposes, and after a
lot of

newsgroup discussions, the verdict at that time was "You are never going =

to win at this,

don't bother", and I gave it up. Nothing has changed in the intervenin=

g decade since I

tried this, so it probably doesn't work today, either.

It's not that you have a bad idea, but that you have an idea that is prob=

ably incompatible

with how MFC is designed to work.

Joseph M. Newcomer [MVP]
MVP Tips:

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