Click here to Skip to main content
14,217,717 members

Saving Drawing Contexts

Rate this:
5.00 (14 votes)
Please Sign up or sign in to vote.
5.00 (14 votes)
27 Mar 2001
GDI objects selected in a DC can't be deleted - even when you call DeleteObject. This handy class makes these potential leaks a thing of the past.


There are some problems with maintaining resources for drawing, such as pens, brushes, etc. When you select an object into a DC, that object becomes undeletable. You might exit the context in which it was allocated, and its destructor is called, and DeleteObject is ultimately called on it, but, surprise!, it hasn't really gone away. Because it was selected into a DC, there is a flag saying it is in active use, and therefore your DeleteObject can't work. I don't know the actual details of how this works internally, but I know the external manifestation. The pen, brush, etc. simply stays in the "GDI heap".

If you are working on one of those horrid 16-bit windows-kludge-on-top-of-MS-DOS systems like Windows 3.1, Windows 3.11, Win98, Win98, or WinME (and if you think that Win9x/ME is a 32-bit OS, you have (a) not read the API documentation carefully (b) believed Microsoft's consumer-level documentation and thought it was technical documentation), there is a very limited system-wide pool out of which all pens and brushes are allocated, and eventually not even your desktop can allocate the resources it needs to draw. On a real, 32-bit operating system such as NT/2000/XP, the GDI heap is much larger, and is per-process, so eventually your program won't be able to draw, but the rest of the system will work fine, since you are only allowed to shoot yourself in the foot.

Here's a classic case:

void CMyView::OnPaint()
    CPaintDC dc(this);
    CFont f;
    f.CreateFont(...); // parameters not shown
    dc.TextOut(...); // whatever...
   } // destructors called here...

Looks pretty good, right? Wrong. Look at what happens. The destructors are called when the context exits. This means that the DC will be freed (in the case of a CPaintDC, this means that ::EndPaint will be called), and the destructor for the CFont will be called, which means that a ::DeleteObject will be called. (This has other implications, for example, if you are doing a CWnd::SetFont call, where the font has to have a lifetime beyond the lifetime of the variable; see my essay on this topic).

Strictly speaking, there is no specified order in which the destructors are known to be called. I checked the C++ standard, and although all sorts of issues are specified in order of execution of destructors, the order in which they are called for auto variables (that is, ordinary stack variables) seems to be unspecified. In practice, it appears to be in the inverse order of their declaration, that is, the font, which is declared after the DC, will be destroyed first, then the DC will be destroyed. This loses, because when the font is destroyed, it is still selected into the font. (You may suspect that by declaring your fonts before the CPaintDC will solve this. I would consider this an egregious programming blunder. For one thing, I'm not sure that deleting the DC first properly sets the values in the font so that it knows it is no longer in a DC (it could be selected into several DCs). I would never attempt this.

The proper thing to do is to restore the state of the DC before destroying it. Typically, this is done by saving the contents of the DC when you do a SelectObject and then restoring them, for example:

void CMyView::OnPaint()
    CPaintDC dc(this);
    CFont f;
    f.CreateFont(...); // parameters not shown
    CFont * oldfont = dc.SelectObject(&f);
    dc.TextOut(...); // whatever...
   } // destructors called here...

This will now work properly. When the destructor for the font is called, it is no longer in a DC, and it will be deleted.

In fact, the principle that hikers follow, "Leave gates as you found them", is important in DC management. When you write a function that takes an incoming DC, you have to make sure that every change you make is restored. Note that every operation that modifies a DC returns an object (or value) that can be used to restore the state of the DC (the MFC calls wrap certain values, such as HFONT values, in a corresponding "wrapper class", such as CFont, and return a pointer to that MFC object). However, there are a lot of parameters to a DC, and this means you end up, in a serious drawing routine, having a lot of odd variables to keep track of.

However, it is simpler than this. There is a serious underappreciated function, ::SaveDC, and its partner, ::RestoreDC, which are available as methods CDC::SaveDC and CDC::RestoreDC. Using these, you can now change the code to avoid the use of any gratuitous variables:

void CMyView::OnPaint()
    CPaintDC dc(this);
    CFont f;
    f.CreateFont(...); // parameters not shown
    int save = dc.SaveDC();
    dc.TextOut(...); // whatever...
   } // destructors called here...

Actually, it doesn't matter how many changes you made in the DC; when you do the RestoreDC call, the DC is restored to whatever state it was just before the SaveDC. This means that all the objects selected into it after the SaveDC, all the changes in the text color, background color, styles, etc. are all erased.

There are interesting ways of using the integers to SaveDC in creative ways, but I'm not going to go into those, primarily because I don't do them myself. Instead, I'm going to show a class that makes all this very easy. There's no download for this code; it is so simple you can copy-and-paste it into your own file.


class CSaveDC {
       CSaveDC(CDC & dc) { sdc = &dc; saved = dc.SaveDC(); }
       CSaveDC(CDC * dc) { sdc = dc; saved = dc->SaveDC(); }
       virtual ~CSaveDC() { sdc->RestoreDC(saved); }
       CDC * sdc;
       int saved;

That's all there is to it! Note that it has two constructors, one if you have a CDC * and one if you have a CDC or CDC &. All you do is declare a dummy variable. But there's an important trick, illustrated below.

void CMyView::OnPaint()
    CPaintDC dc(this);
    CFont f;
    f.CreateFont(...); // parameters not shown
    { /* save context */
     CSaveDC sdc(dc);
     dc.TextOut(...); // whatever...
    } /* save context */
   } // destructors called here...

Note that the save context you create by using the CSaveDC class must be in a scope that is smaller than the objects selected into the DC. Thus the /* save context */ block guarantees that the CSaveDC destructor is guaranteed to be called before the destructor for the font. All you have to do is declare your fonts, pens, brushes, and regions outside the /* save context */ block and you can be guaranteed that their destructors will be called in a context where they are not selected into the active DC.

Note that CSaveDCs can nest (because the ::SaveDC can nest). The nesting can be static or dynamic. For example:

void CMyView::OnPaint()
    CPaintDC dc(this);
    CFont f;
    f.CreateFont(12,...); // most parameters not shown
    { /* save context */
     CSaveDC sdc(dc);
     dc.TextOut(...); // whatever...
   } // destructors called here

void CMyView::drawboxes(CDC & dc)
    CPen RedPen(PS_SOLID, 0, RGB(255, 0, 0));
    CBrush GreenBrush(RGB(0, 255, 0);
    CFont f;
    f.CreateFont(6, ...); // most parameters not shown
    { /* save context */
    } /* save context */

Note that the save context in drawboxes contains lots of changes; these will be undone when you leave the save context, so that when drawboxes returns to OnPaint, the DC will have the correct font (12-pixel), background mode, etc.

The views expressed in these essays are those of the author, and in no way represent, nor are they endorsed by, Microsoft.

Copyright © 1999 <!--webbot bot="Substitution" s-variable="CompanyLongName" startspan -->CompanyLongName <!--webbot bot="Substitution" endspan i-checksum="22147" --> All Rights Reserved.


This article has no explicit license attached to it, but may contain usage terms in the article text or the download files themselves. If in doubt, please contact the author via the discussion board below.

A list of licenses authors might use can be found here.


About the Author

Joseph M. Newcomer
United States United States
PhD, Computer Science, Carnegie Mellon University, 1975
Certificate in Forensic Science and the Law, Duquesne University, 2008

Co-Author, [i]Win32 Programming[/i]

Comments and Discussions

QuestionA small question Pin
Gornix5-May-08 17:21
memberGornix5-May-08 17:21 
AnswerRe: A small question Pin
Joseph M. Newcomer5-May-08 17:35
memberJoseph M. Newcomer5-May-08 17:35 
GeneralCSaveDC Pin
uy quoc5-Dec-06 14:34
memberuy quoc5-Dec-06 14:34 
GeneralRe: CSaveDC Pin
Joseph M. Newcomer5-Dec-06 17:36
memberJoseph M. Newcomer5-Dec-06 17:36 
Generalthanks a lot Pin
uy quoc28-Nov-06 14:56
memberuy quoc28-Nov-06 14:56 
GeneralRe: thanks a lot Pin
Joseph M. Newcomer28-Nov-06 16:58
memberJoseph M. Newcomer28-Nov-06 16:58 
GeneralRe: thanks a lot Pin
uy quoc1-Dec-06 15:12
memberuy quoc1-Dec-06 15:12 
GeneralRe: thanks a lot Pin
Joseph M. Newcomer1-Dec-06 17:32
memberJoseph M. Newcomer1-Dec-06 17:32 
GeneralRe: thanks a lot Pin
uy quoc5-Dec-06 14:21
memberuy quoc5-Dec-06 14:21 
GeneralRe: thanks a lot Pin
tonamy28-Nov-06 21:02
membertonamy28-Nov-06 21:02 
GeneralThanks! Pin
Wes Aday25-Aug-06 4:11
professionalWes Aday25-Aug-06 4:11 
GeneralCPaintDC Question Pin
hoang van hiep18-Jun-06 18:03
memberhoang van hiep18-Jun-06 18:03 
GeneralRe: CPaintDC Question Pin
Joseph M. Newcomer26-Jun-06 17:48
memberJoseph M. Newcomer26-Jun-06 17:48 
GeneralI am looking for a simple MFC Control with Draw Area Pin
p2002ad25-May-05 6:22
memberp2002ad25-May-05 6:22 
GeneralRe: I am looking for a simple MFC Control with Draw Area Pin
Joseph M. Newcomer28-May-05 4:40
memberJoseph M. Newcomer28-May-05 4:40 
GeneralFYI Pin
Vlad Vissoultchev29-Dec-02 15:27
memberVlad Vissoultchev29-Dec-02 15:27 
Generalvery good!! Pin
Zhou Hang19-Nov-02 6:38
memberZhou Hang19-Nov-02 6:38 
GeneralNeat :-) Pin
Nish Nishant9-Jun-02 0:29
sitebuilderNish Nishant9-Jun-02 0:29 
QuestionWhy not subclass CPaintDc ? Pin
27-Oct-01 3:40
suss27-Oct-01 3:40 
GeneralA great leak stopper! Pin
Bitshifter30-Aug-01 4:55
memberBitshifter30-Aug-01 4:55 

General General    News News    Suggestion Suggestion    Question Question    Bug Bug    Answer Answer    Joke Joke    Praise Praise    Rant Rant    Admin Admin   

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.

Posted 27 Mar 2001


30 bookmarked