Re: Preventing memory and resource leaks with GDI Objects ???

"Peter Olcott" <>
Sat, 13 Mar 2010 22:31:18 -0600
"Joseph M. Newcomer" <> wrote in

Note, however, that in all versions of Windows I'm
familiar with (which is up through
Vista) a font which is selected into a DC will not be
deleted if ::DeleteObject is called,
whether explicitly via CGdiObject::DeleteObject or via the
implicit destructor, so if the
shared pointer reference count goe to 0, the delete method
will end up calling
::DeleteObject, and if the font is selected into a DC it
will not actually delete it and
there will be a genuine GDI resource leak.

I tried to explain this several times, but he keeps
confusing MFC behavior with actulal
system behavior, and they are not the same!

According to my understanding about what you said about this
the below code could not work, and yet it does work. The
CFont object is deleted repeatedly even when selected into
the MemoryDC. It must be getting deleted because when I
comment out the DeleteObject() statement it fails with an
assertion. If I leave it in, then it does not fail with the
assertion. I don't see how this could possibly mean that the
CFont object is not getting deleted while it is selected.

It could very well be (and probably is) that my
understanding is less than complete. What am I missing here?

// Displays a Font Selection DialogBox with
// default "Times New Roman", Bold, Italic, 8 Point
inline void ScreenCaptureType::SelectFont() {

  int PointSize = 8;
  int FontHeight = -MulDiv(PointSize,
GetDeviceCaps(MemoryDC.m_hDC, LOGPIXELSY), 72);
  LOGFONT LogFont = {FontHeight, 0, 0, 0, FW_BOLD, 1, 0, 0,
FF_DONTCARE, L"Times New Roman"};
  CFontDialog dlg(&LogFont);
  if (dlg.DoModal() == IDOK)

On Fri, 12 Mar 2010 00:02:17 -0800 (PST), Goran
<> wrote:

On Mar 10, 6:56 pm, "Peter Olcott" <>

I am have built a general purpose bitmap/image handling
class and want to add TextOut() capability to this
class. To
do this I must convert some of my GDI local function
variables into GDI object member variables. This means
I must be able to re-use several GDI objects, instead of
constructing them and destroying them after a single

Well, certainly nothing stops you from doing that, e.g.

(Warning: works with head-compiler and in head-debugger,

typedef shared_ptr<CGdiObject> SPGdiObject;
class COhSoGreat
typedef KeyType int;
std::map<KeyType, SPGdiObject> m_gdiObjects;
 struct CTempGdiObjectSelection : noncopyable
   CTempGdiObjectSelection(CDC& dc, const CGdiObject& o)
: m_dc(dc),
m_new(o) { m_pOld = dc.SelectObject(&o); }
   ~CTempGdiObjectSelection() {
m_dc.SelectObject(m_pOld); }
  CDC& m_dc;
  CGdiObject* m_pOld;
  CGdiObject& m_new

 void SoGreatDrawingCode(CDC& dc) const /*an important
   { CTempGdiObjectSelection SelectSomething(dc,
     // Draw, baby, draw.
     { CTempGdiObjectSelection SelectSomething(dc,
       // Draw, baby, draw.

 void OtherFunctionsThatMustFillYourGdiObjectContainer()
 { // e.g.
   SPGdiObject P(new CFont);
   if (!P->CreateXYZ(params))
   m_gdiObjects[some_key] = P;

Use an approach like this and it will be rather hard to
leak any

You also said:

"The main thing that I want to do is to manually free any
memory or resources that the CBitmap object is using. I
don't know what resources it is using, and I don't know
to free them."

Just let CBitmap C++ object be destroyed. As Joe said,
don't try to
get smart with leaving it selected in a DC while
destroyed. That's why
I proposed CTempGdiObjectSelection up here. If you can
make your gdi
object container constant while drawing (note the "const"
qualifier on
"SoGreatDrawingCode"), you're practically locked out of a
leak (bar limitless ingenuity in shooting oneself in the
foot :-) ).

You say that you tried destroying gdi object while
selected and that
this works, but did you verify this with some GDI handle
program? There are some on the internet. Try this first.

What issues are raised with resource and memory leaks by
using CDC and CBitmap objects? How are these issues

Listening to Joe gets you far :-).


Joseph M. Newcomer [MVP]
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