C / C++ / MFC Discussion Boards

ashtwin17-Oct-10 23:52

17-Oct-10 23:52

Hi,
I am using SelectObject() in my code but confused with the MSDN description.

CFont font;	
font.CreateFont(-12, 0, 0, 0, 400, FALSE, TRUE, FALSE, ANSI_CHARSET, OUT_CHARACTER_PRECIS, CLIP_CHARACTER_PRECIS, DEFAULT_QUALITY, FIXED_PITCH | FF_MODERN, _T("Courier New"));	
CFont* oldFont = pdc->SelectObject(font);	
pdc->TextOut(0, 0, str);	
pdc->SelectObject(oldFont);	
font.DeleteObject();

As per MSDN we should not use the object return by SelectObject because it is a temporary one.
But if we are restoring the old GDI object in the above way will it cause any problem
because the same dc can be used in some other place.

Re: Problem with SelectObject()

Cedric Moonen18-Oct-10 0:05

Cedric Moonen

18-Oct-10 0:05

MSDN says:

This function may return a pointer to a temporary object. This temporary object is only valid during the processing of one Windows message.

So, it is ok to replace the selected object by the previous object like you did in your case.

Cédric Moonen
Software developer

Charting control [v3.0]
OpenGL game tutorial in C++

Re: Problem with SelectObject()

ashtwin18-Oct-10 0:34

ashtwin

18-Oct-10 0:34

Thanks for the reply. But if i want to use the same DC after this call wiil it be ok.

Re: Problem with SelectObject()

Sauro Viti18-Oct-10 0:42

Sauro Viti

18-Oct-10 0:42

You can reuse the device-context how many times you want. The only thing you should be aware of is that:

each time you select a GDI object (font, pen, brush, etc.) into a device-context you should restore the one that it held originally
you should not save the pointers returned by SelectObject to restore them later, because they could be temporary. To be more specific, the objects pointed could be destroyed during the idle-time of your application (i.e. inside the CWinApp::OnIdle). This means that you should restore these objects before returning from the message handler on which you have called SelectObject

Re: Problem with SelectObject()

ashtwin18-Oct-10 1:49

ashtwin

18-Oct-10 1:49

But if we restore the temporary pointer returned from SelectObject in the DC immediately, the DC may be having some dangling references after the current call. Means if we store the current DC for some other call there are chances of using incorrect memory.

Re: Problem with SelectObject()

Sauro Viti18-Oct-10 1:57

Sauro Viti

18-Oct-10 1:57

ashtwin wrote:
But if we restore the temporary pointer returned from SelectObject in the DC immediately, the DC may be having some dangling references after the current call.

I think you mean: "the device-context hold a pointer that could became invalid". Isn't it?
Don't worry about it: once you have selected back the original object inside the device-context, everything will works fine. What you cannot do is to store that pointer and use it explicitly later.

Re: Problem with SelectObject()

Niklas L18-Oct-10 2:07

Niklas L

18-Oct-10 2:07

There is a difference. The temporary object is a temporary MFC object wrapping the underlying HGDIOBJECT. The gdi object the handle is pointing to is not temporary.

home

Re: Problem with SelectObject()

ashtwin18-Oct-10 2:27

ashtwin

18-Oct-10 2:27

I understood. Thanks you all for your replies.

Integrate ActiveX C# in C++ form

MrKBA17-Oct-10 23:29

MrKBA

17-Oct-10 23:29

Hello,

I created an ActiveXDll on C# and set parameters of COM in its interface.
And now I need to integrate this ActiveXDll in my C++ Form.
So can you help me?

Re: Integrate ActiveX C# in C++ form

CPallini18-Oct-10 0:07

CPallini

18-Oct-10 0:07

If you're using MFC, then Visual Studio may generate a wrapper C++ class for you.
Smile | :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke

[My articles]

when hit F5(Run) on C++\MFC proj in VC++.NET 2003- resource file is always out of date

Dorith17-Oct-10 22:59

Dorith

17-Oct-10 22:59

Hello,

When running MFC\C++ project in 2003 VC++.NET env, resource files are always out of date,
even though there are no changes made.
Whenever I hit F5, it gives me the "These project configuration(s) are out of date:"
msgbox.
When building again, I see that all sources are up to date except resources
In the output window I see:
all is "up-to-date"
then "compiling resources..
Linking.."

Can anyone know how to get rid of this annoying msgbox?

Thanks,
Dor

who will create the message loop and message queue?

vasu_sri17-Oct-10 21:40

vasu_sri

17-Oct-10 21:40

in win32, where will keep the all messages are generated by the window? and how do know this message is belongs to this window? and who will maintain the message queue? what happand in background? what is the code in c++? what are the methods are fallows in background?

Regards,
Srinivas

Re: who will create the message loop and message queue?

bleedingfingers17-Oct-10 21:48

bleedingfingers

17-Oct-10 21:48

Every thread has its own message queue that is setup by the system into which it posts the messages. Now that you have a start, dig into MSDN.

...byte till it megahertz...

my donation to web rubbish

Re: who will create the message loop and message queue?

CPallini18-Oct-10 2:12

CPallini

18-Oct-10 2:12

bleedingfingers wrote:
Every thread has its own message queue that is setup by the system into which it posts the messages.

That's not completely true, see The Old New Thing - In pursuit of the message queue.
Smile | :)

Re: who will create the message loop and message queue?

bleedingfingers18-Oct-10 2:40

bleedingfingers

18-Oct-10 2:40

CPallini wrote:
bleedingfingers wrote:
Every thread has its own message queue that is setup by the system into which it posts the messages.

That's not completely true, see The Old New Thing - In pursuit of the message queue.

I know I know. And one would have to use PeekMessage(...) to force a queue to be created but I just gave him a start. Now, let him wipe that e-dust off his MSDN will ya Wink | ;)

...byte till it megahertz...

my donation to web rubbish

Re: who will create the message loop and message queue? [modified]

Rajesh R Subramanian18-Oct-10 6:54

Rajesh R Subramanian

18-Oct-10 6:54

[Edit] I overlooked it - A 5 vote is offered. Smile | :)

[/Edit]

There are some really weird people on this planet - MIM.

modified on Monday, October 18, 2010 1:52 PM

Re: who will create the message loop and message queue?

Sauro Viti17-Oct-10 22:15

Sauro Viti

17-Oct-10 22:15

There are lots of things to say about your question. Let's say:

the operating system handle a message queue for each thread in your process;
it's up to you to write down the message loop for each thread
each time you call the DispatchMessage from your message loop, the message is dispatched to the window procedure of the window to which the message is targeted

To learn about this topic, start from Messages and Message Queues (Windows)[^]

Re: who will create the message loop and message queue?

vasu_sri18-Oct-10 18:40

vasu_sri

18-Oct-10 18:40

who will create a thread and message queue?

what relation ship between operating system, thread, message queue, and application?

Regards,
Srinivas

Re: who will create the message loop and message queue?

Sauro Viti18-Oct-10 21:09

Sauro Viti

18-Oct-10 21:09

vasu_sri wrote:
who will create a thread and message queue?

The first thread is created by the operating system when you start an application. Other threads will be created by the application itself, for example by calling the CreateThread Function (Windows)[^].
A message queue is created and maintained by the operating system; the OS creates it only when needed, i.e. when your thread creates a window or calls any function that involves message queues.

vasu_sri wrote:
what relation ship between operating system, thread, message queue, and application?

It's a very generic question, and answer it requires thousands of words... Have you had a look at the documentation on MSDN that I suggested you?

Application crashes at RichEditCtrl.StremOut(), It says attempt to read write protected memory. this often an indication that the other memory is corrupt.

ptr_Electron17-Oct-10 17:47

ptr_Electron

17-Oct-10 17:47

Application crashes at RichEditCtrl.StremOut(), It says attempt to read write protected memory. this often an indication that the other memory is corrupt.

May I know some path to find the issue and fix the issue. Any idea ?

.....

if (0 != file.Open(m_str, CFile::modeCreate | CFile::modeWrite, &e))
	{		
	    EDITSTREAM es = {0};		
    	    es.pfnCallback = COleElement::CallFunction;	
	    es.dwError = 0;		
	    es.dwCookie = (DWORD)&file;		
	    lRet = m_cRichEdCt.StreamOut(SF_RTF, es);
	}..........

DWORD CALLBACK CMyClass::CallFunction(DWORD dwCookie, LPBYTE lpBuf,LONG nCount, LONG* Write)	
{
      CFile* pFile = (CFile*)dwCookie;
      if (pFile)	
      {		
          pFile->Write(lpBuf, nCount);
      }	
       *Write = nCount;	
       return 0;
}

Re: Application crashes at RichEditCtrl.StremOut(), It says attempt to read write protected memory. this often an indication that the other memory is corrupt.

Richard MacCutchan17-Oct-10 21:26

Richard MacCutchan

17-Oct-10 21:26

I have already responded to this question here[^]. Did you follow my suggestion and run it with the debugger?

Just say 'NO' to evaluated arguments for diadic functions! Ash

Re: Application crashes at RichEditCtrl.StremOut(), It says attempt to read write protected memory. this often an indication that the other memory is corrupt.

ptr_Electron19-Oct-10 20:40

ptr_Electron

19-Oct-10 20:40

I tried to debug and got the below values for es

es	{dwCookie=1231056 dwError=0 pfnCallback=0x05151631 }	_editstream

and the blreadk poing in the call back function is not getting hit.

Please advice

Re: Application crashes at RichEditCtrl.StremOut(), It says attempt to read write protected memory. this often an indication that the other memory is corrupt.

Richard MacCutchan19-Oct-10 23:04

Richard MacCutchan

19-Oct-10 23:04

ptr_Electron wrote:
the blreak point in the call back function is not getting hit.

This suggests (possibly) that the callback address is not valid, and the error occurs because of this. I have just noticed that in your code your callback is defined as :

DWORD CALLBACK CMyClass::CallFunction(DWORD dwCookie, LPBYTE lpBuf,LONG nCount, LONG* Write)

but your setting in the EDITSTREAM structure defines it as :

es.pfnCallback = COleElement::CallFunction;

which looks to be a different class. Are you sure this is correct?

Just say 'NO' to evaluated arguments for diadic functions! Ash

Converting to the web

hamilton12317-Oct-10 11:49

hamilton123

17-Oct-10 11:49

Hi, I have a C++ program working on my C drive. Now I have uploaded it (using XP IE as my FTP client) to my website; together with the files that it needs to read. I want to able to run from a second PC thro' the web. I can just about get the program to start and find the files. But (a) I am probably not doing this in the best way. And (b) my program crashes with an access violation before doing anything useful. I am a complete newbie in this field, so what should I be looking out for?

Regards, George.

Re: Converting to the web

Cedric Moonen17-Oct-10 21:33

Cedric Moonen

17-Oct-10 21:33

It's not clear what you are trying to do. As far as I understood, you would like other people to download your program (with all the files which are needed by it) and run it on their own machine. Is that correct ? So, there's nothing really related to web (except that they download the files).
Make sure that your program is compiled in release mode, that the target machine has all the required dll's. Which IDE did you use to create your program ?

Usually, if you have to supply an executable with additional files (like configuration files), it is best to provide an installer that installs everything at once. Take a look at InnoSetup[^] for instance.

Cédric Moonen
Software developer

Charting control [v3.0]
OpenGL game tutorial in C++

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