MFC, CreateCompatibleDC question.

Code:

---

       
               
                CDC twdc;
                twdc.CreateCompatibleDC(this->GetDC());

        //=== gen pattern ===//
        for(        int x=0 ; x<99 ; x++ ){
                for( int y=0 ; y<99 ; y++ ){
                        twdc.SetPixelV( x,y, x*x+y*y );
                }
        }
        TRACE( "%d", twdc.GetPixel(4,4) );  //  any number for the (4,4).

---

i do this in MFC, why it always TRACE out -1 ?? and alsoo , there is nothing printed when use BitBlt to print it out.

i think i have problem with the CreateCompatibleDC().

help me please.

Try creating a bitmap and loading that into the dc first. I think that when you use CreateCompatibleDC (the win32 api function which the mfc is just a wrapper for) the default bitmap created with it is a 1x1 monochrome bitmap.

Ok, now i know what is wrong in my program. thank you very much!!

i know that the CDC obj. is 1x1 at first.

how to resize it ??
i can use SelectObject( BMP ) ; to resize, but i know this is a dump method.

i have do this,

Code:

---


        twdc.CreateCompatibleDC(this->GetDC());
 
        CRgn tRgn;        tRgn.CreateRectRgn( 0,0,20,20 );
        twdc.SelectClipRgn( &tRgn, RGN_COPY );

---

but not working.

how to ??

P.S.
Macau has a first SARS case tonight !!! :(

CreateCompatibleBitmap()
SelectObject() [catch the returned BMP and replace before deleting the DC or a GDI memory leak will result]

>>twdc.CreateCompatibleDC(this->GetDC());

this is a GDI memory leak as you never can release the DC returned from GetDC() (and in your code snippet do not DeleteDC() the compatible DC creating another leak)

O, something new to me.

what about the
int AfxMessageBox(......);

and

char * itoa( .... .. ) ;

i always use them without catching the returns.
will they cause any memory leakage ?

Its GDI memory created by your app (using a Create....function have to be deleted, those you Get... have to be released).

GDI objects can not (generally) be deleted while selected into a DC.

"A DC is a structure that defines a set of graphic objects and their associated attributes, and the graphic modes that affect output."

"This handle is saved so that the original brush can be selected back into the DC after the last painting operation has been completed with the new brush. After the original brush is selected back into the DC, the new brush is deleted, freeing memory in the GDI heap. "


As to the returns of
AfxMessageBox(......)
isn't that the control pressed by the user? Not a GDI object.
But if is a dynamically created variable would have to be freed.


Do you use memory re/allocating functions without checking the return?