C / C++ / MFC

26-Jun-06 2:27

OK... In a pseudo-bet, I am wondering if the MFC developers out there can see what is wrong with the following catch clause extracted from an example. I am hoping that something obvious pops out of it.

catch( CMemoryException *pExMem )
{
	CString	csError;
	TCHAR	caErr[128+1];
 
	pExMem->GetErrorMessage(caErr,128);
	csError.Format(_T("An exception has been encountered:\n%s"),caErr);
	AfxMessageBox(csError,MB_EXCLAMATION);
	pExMem->Delete();
}

Hint - it is not about NUL termination.

Peace!

-=- James

If you think it costs a lot to do it right, just wait until you find out how much it costs to do it wrong!
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Sarath C26-Jun-06 2:39

26-Jun-06 2:39

Either you are trying to delete a stack object or the exception you are throwing might have declared locally.

e.g

try
{
	CMemoryException e;
	throw &e;
}

CMemoryException e;
try
{
	throw &e;
}

Sa^Rath.

"Do Next Thing..."
My Blog | Understanding State Pattern in C++

James R. Twine26-Jun-06 2:45

26-Jun-06 2:45

Well, generally you do not create CException-based objects directly, you are supposed to use the AfxThrow*Exception(...) functions, and CException-based objects are only supposed to be caught by pointer (never by copy or reference), and you are always supposed to call the Delete() function on them unless you are going to re-throw them.

So, no, that is not the problem. Smile | :)

Peace!

-=- James

Zac Howland26-Jun-06 4:15

Zac Howland

26-Jun-06 4:15

This is only the case if you follow the MFC-style exception usage. When using STL-style exceptions, you generally pass them by const-reference. I probably would have passed this exception by const-reference simply because if you are getting a memory exception, something in your memory is royally screwed (meaning, you shouldn't keep using the heap and expect normal results).

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week

Zac

James R. Twine26-Jun-06 4:56

Re: What is wrong with this? [modified]

26-Jun-06 4:56

Zac Howland wrote:
This is only the case if you follow the MFC-style exception usage.

Yep - That is why I specifically mentioned CException.

Zac Howland wrote:
I probably would have passed this exception by const-reference simply because if you are getting a memory exception, something in your memory is royally screwed (meaning, you shouldn't keep using the heap and expect normal results).

Remember that MFC is free to allocate that exception object on another memory area as well, like static and/or global. The special AfxThrow*Exception(...) functions may not just use the "plain" heap to allocate the thrown exception object. And it should always be safe to call Delete() off that pointer (not the delete operator) because the exception object knows if it is heap-based or not.

FWIW, I generally catch other types of exception as const reference as well. You generally do not know what to do with a pointer (should I free it, and if so, how?), and there is that whole "slicing" thing... Smile | :)

Peace!

-=- James

Viorel.26-Jun-06 2:47

Viorel.

26-Jun-06 2:47

Maybe CMemoryException is too catastrophic error, therefore mentioned actions, which requires heap-allocation, cannot be done? In case of fatal errors, the documentation recommends to finish the application with AfxAbort().

-- modified at 8:48 Monday 26th June, 2006

James R. Twine26-Jun-06 3:02

Re: What is wrong with this? [modified]

26-Jun-06 3:02

Thank you! You would be suprised how often I see code like this, where coders are so stupid that they do not realize that if you are in an out-of-memory situation, you should not be trying to allocate memory! Smile | :)

Peace!

-=- James

ovidiucucu26-Jun-06 3:43

ovidiucucu

26-Jun-06 3:43

Yeah indeed that can be the exception of exceptions. Theoretical... Wink | ;)

Practical, appear more "trivial" memory exceptions like (I wrote INT_MAX just as an example of big number, but hopefully you can get the point)

try
{
   char* x = new char[INT_MAX];
}

and no something like

try
{
   char* x = new char[3];
}
catch(CMemoryException* e)
{
   // ugh!!! my heap is finished
}

Ovidiu Cucu
Microsoft MVP - Visual C++

James R. Twine26-Jun-06 5:02

26-Jun-06 5:02

You forgot to call Delete() on that exception pointer. Smile | :)

Peace!

-=- James

ovidiucucu26-Jun-06 3:35

ovidiucucu

26-Jun-06 3:35

I don't see any harmful error there.
Just MB_EXCLAMATION is not a MessageBox flag and that can be observed/fixed (replaced with MB_ICONEXCLAMATION) after the first compilation.
Don't shoot the typist!

Ovidiu Cucu
Microsoft MVP - Visual C++

Sarath C26-Jun-06 3:48

26-Jun-06 3:48

Actually do u know the answer?

Sa^Rath.

"Do Next Thing..."
My Blog | Understanding State Pattern in C++

Naveen26-Jun-06 3:52

Naveen

26-Jun-06 3:52

u didn't see this?

http://www.codeproject.com/script/comments/forums.asp?msg=1548920&forumid=1647#xx1548920xx[^]

nave

Sarath C26-Jun-06 4:25

26-Jun-06 4:25

Actually, in his code, there's no allocation...

so how do we know what's happening outside. could you pls explain it?

Sa^Rath.

"Do Next Thing..."
My Blog | Understanding State Pattern in C++

James R. Twine26-Jun-06 5:00

Re: What is wrong with this? [modified]

26-Jun-06 5:00

You are missing the point of the post. The problem is that if you ever catch that exception, you should never try to handle it in the way that the snippet demonstrates.

There are only two reasons to get that exception (ignore the fact that some developers call AfxThrowMemoryException() for the wrong reasons): [1] A heap request cannot be allocated due to an out of memory condition, and [2] the heap is corrupted and Thinks it is out of memory Smile | :)

.

For either case, you probably should not be then trying to allocate memory again.

Peace!

-=- James

ovidiucucu26-Jun-06 5:06

ovidiucucu

26-Jun-06 5:06

Well, sometime happens. Making confusion between heap and stack, we are in a hurry calling others "stupid" then abort (call AfxAbort). Smile | :)

Ovidiu Cucu
Microsoft MVP - Visual C++

James R. Twine26-Jun-06 4:42

26-Jun-06 4:42

Of course I do...

Peace!

-=- James

how we caan add files to listbox

Chris Meech26-Jun-06 4:25

Chris Meech

26-Jun-06 4:25

I can just see a developer, patting himself on the back with having put together an exception handler and then stress testing to see it invoked and wondering why the messagebox is never displayed.

This is a really good example of not seeing the forest for the trees solution to a problem. Smile | :)

Chris Meech

I am Canadian. [heard in a local bar]

When no one was looking, every single American woman between the ages of 18 and 32 went out and got a tatoo just above their rumpus. [link[^]]

happy_ram26-Jun-06 1:18

happy_ram

26-Jun-06 1:18

if we select the folder automatically all the files in the folder should be loaded in the listbox.iam able to select the folder but i don't know how to get the total number of files in folder and how to load this files into listbox,
please help me,
sree.

Hamid_RT26-Jun-06 1:25

Hamid_RT

26-Jun-06 1:25

if you want to get files or folders if you use ListBox see DlgDirList and use m_ListBox.GetCount()

whitesky

happy_ram26-Jun-06 1:34

happy_ram

26-Jun-06 1:34

actually my intension just like winamp application,in winamp we have two option add file or add folder.i write for files and i get a problem when i dealing with folders.
how we select the folder and how to add contentsof that folder to listbox.
thank u,
please help me,urgent .
sree

Sarath C26-Jun-06 1:31

26-Jun-06 1:31

here is the sample snippet

CFileFind finder;
 BOOL bWorking = finder.FindFile("*.*"); // give full path, else it will search in the current directory
 while (bWorking)
 {
    if (finder.IsDots())
       continue;

    // if it's not a directory, add it
    if (!finder.IsDirectory())
    {
        bWorking = finder.FindNextFile();
        m_ListBox.AddString((LPCTSTR) finder.GetFileName());
    }
 }

Sa^Rath.

"Do Next Thing..."
My Blog | Understanding State Pattern in C++

Sarath C26-Jun-06 1:33

26-Jun-06 1:33

Another way to do the same thing

// The pointer to my list box.
extern CListBox* pmyListBox;

// Add all the files and directories in the windows directory.
TCHAR lpszWinPath[MAX_PATH], lpszOldPath[MAX_PATH];
::GetWindowsDirectory(lpszWinPath, MAX_PATH);

::GetCurrentDirectory(MAX_PATH, lpszOldPath);
::SetCurrentDirectory(lpszWinPath);

pmyListBox->ResetContent();
pmyListBox->Dir(DDL_READWRITE|DDL_DIRECTORY, _T("*.*"));

::SetCurrentDirectory(lpszOldPath);

Sa^Rath.

"Do Next Thing..."
My Blog | Understanding State Pattern in C++

Sarath C26-Jun-06 1:34

26-Jun-06 1:34

Modify

SaRath C wrote:
::GetWindowsDirectory(lpszWinPath, MAX_PATH);

with your selected path

Sa^Rath.

"Do Next Thing..."
My Blog | Understanding State Pattern in C++