Memory leak in MFC code at CWnd::SetWindowText

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5.00/5 (1 vote)

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Hi,
I am checking for memory leaks in my MFC SDI app, in the OnBnClickedOK() method for one of my dialogs.
I have tracked down a memory leak to be due to the GetWindowText line in my code below.

void CMyDlg::OnBnClickedOk()
{
		CWnd* pWnd = GetDlgItem(IDC_EDIT1);

		CString sequenceName;

		pWnd->GetWindowText(sequenceName);

Can anyone please tell my why the leak occurs. I am using CMemoryState to check for memory leaks, as shown below, and diffMemState.m_lCounts[1] has a value of 1.

#ifdef _DEBUG
   newMemState.Checkpoint();
   if( diffMemState.Difference( oldMemState, newMemState ) )
   {
	TRACE("Memory leaked!\n" );
      	AfxMessageBox(L"Memory leaked!\n" );
   }
#endif
		pWnd->GetWindowText(sequenceName);
#ifdef _DEBUG
   newMemState.Checkpoint();
   if( diffMemState.Difference( oldMemState, newMemState ) )
   {
	TRACE("Memory leaked!\n" );
	AfxMessageBox(L"Memory leaked!\n" );
   }
#endif

Posted 12-Dec-11 4:44am

Jackie Lloyd

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Albert Holguin 12-Dec-11 13:11pm

You're not checking this right... as Jack states in his solution, you can't even check these things until they go out of scope. Your test is not valid.

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JackDingler · Accepted Answer · 2011-12-12T05:42:00

Solution 1

C++

pWnd->GetWindowText(sequenceName);

Allocates memory in the CString which won't be freed until the string goes out of scope..

CString (varies with version) uses pointer sharing and delayed garbage collection.

So it's possible that the memory may hang around a while even after it goes out of scope.

Memory Leak detection won't necessarily be valid until your program is closing.

******

Put the following macros in every CPP file after your header includes.

C++

#ifdef _DEBUG
#define new DEBUG_NEW
#endif

This will allow the debugger to track every memory allocation and if you have leaks will show you those locations (in the output window) when the program exits.

Posted 12-Dec-11 5:42am

JackDingler

Updated 12-Dec-11 5:47am

v2

Comments

Albert Holguin 12-Dec-11 13:23pm

I mostly agree with you... +4

JackDingler 12-Dec-11 13:53pm

What don't you agree with?

I'm always interested in learning new things.

Albert Holguin 12-Dec-11 15:44pm

Well, for one, there's no such thing as garbage collection in C++... CStrings use reference counting.

JackDingler 12-Dec-11 15:52pm

Good answer on reference counting, the current implementation does use reference counting.

But I could quibble with you on Garbage Collection.

C++ doesn't make it impossible to implement. I've done it.

Albert Holguin 12-Dec-11 23:58pm

Well, just about anything is possible if you make it yourself, not the point. There's no garbage collector in C++ or MFC.

JackDingler 13-Dec-11 3:02am

I don't have access to all of the Microsoft source code, so I can't speak with such certainty.

You have a privileged position.

Andrew Brock 13-Dec-11 7:25am

Source code for MFC is included with Visual Studio for under <install dir="">\VC\atlmfc\src\mfc\ and for the CRT under <install dir="">\VC\crt\src\

There is no garbage collection in there, and C/C++ most defiantly does not provide garbage collection without using 3rd party libraries, or writing it yourself

That said, your answer is still the solution to the question, and is mostly right. 5'd.

Jackie Lloyd 14-Dec-11 10:24am

with regard to the method you mentioned earlier, outting the debug macros in every .cpp file then checking the output window after the program exits, I have done this and have no output other than:
-------- Edit and Continue build started --------

--------------------- Done ----------------------

Could you confirm that this means the memory checks were actually done and that there are none?
Many thanks.

Jackie Lloyd 14-Dec-11 10:36am

Actually, I have now put in a memory leak, which it didn't seem to pick up, have you any idea why? However, when I checked the output before the program exited and it said:
Warning: calling DestroyWindow in CDialog::~CDialog --
OnDestroy or PostNcDestroy in derived class will not be called.
Memory leaked!
can you please tell me if this is my leak or an MFC thing?

JackDingler 14-Dec-11 15:12pm

If you're getting that warning, then you're improperly shutting down the dialog.

Perhaps you're closing the application without closing child windows first?

Your memory cleanup needs to be completed before ExitInstance() ends. Dialogs that go out of scope after this call completes, may report invalid memory leaks.

JackDingler 14-Dec-11 15:08pm

I've used this technique for years. It works well.

Jackie Lloyd 14-Dec-11 16:46pm

Thankyou for your help, that makes a bit of sense to me but I think I need to find some documentation on how to use this from scratch.

JackDingler 14-Dec-11 17:37pm

If you create an initial MFC Project from the wizard, you'll see that all of the CPP files already have these lines installed. They are only missing from files if adding additional components through the wizard.

So create a new MFC application for testing and you'll see the macros in the files.

Jackie Lloyd 12-Dec-11 14:29pm

Thankyou - I see what you mean. I have now managed to get my memory check to show no leaks by putting the checks in better places.