Multithreading in MFC MDI

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Hi,

I am trying to access globally variables declared in MainFrame of my solution from a thread that starts in my CMyView.cpp. How can I access these parameters without the CMainFrame * _MainFrame; in my structure THREADVIEW in CMyView.h, as below? Any ideas?

C++

typedef struct THREADVIEW{
    * _this;

   //CMainFrame * _MainFrame; /* I cannot have this here */
 
} THREADVIEW;

Many thanks,

Yoldas

addition by the OP:
Hi,
First of all many thanks for your immediate comments. Here are the things in more detail.

C++

///////////////In my view.cpp:
void CView::OnSprint()
{
	CMainFrame *pFrame = (CMainFrame*) AfxGetMainWnd();		
.
.
. 	/* kick off thread */
THREADVIEW *_pview = new THREADVIEW;
_pview->_this = this;
AfxBeginThread(ChangeColorThread, _pview);

}
UINT CView::ChangeColorThread(LPVOID param)
{
	THREADVIEW * thd = (THREADVIEW*)param;
	thd->_this->StateOnThread();

	AfxEndThread(1, FALSE); 

	return 1;
}

void CView:: StateOnThread ()
{
	CMainFrame *pFrame = (CMainFrame*) AfxGetMainWnd();	
	int a = pFrame->AA;
	/* I want to return AA from main frame but I cannot because I cannot declare CMainFrame * pFrame; in my THREADVIEW */
}

///////////// in my view.h

public:

	typedef struct THREADVIEW{
		CView * _this;
		// cannot have CMainFrame * pFrame;
	} THREADVIEW;


	void StateOnThread();

	static UINT ChangeColorThread(LPVOID param);


////////////my mainframe has my in AA:

class CMainFrame : public CMDIFrameWnd
{
	DECLARE_DYNAMIC(CMainFrame)
public:
int AA;
}

which I am trying to access. My thread kicks off from toolbar button after the child frame is displayed but I do not see why this has anything to do with why I cannot access Cmainframe parameters.

I greatly appreciate your input.

Regards,

Yoldas

Posted 1-Nov-12 7:55am

Yoldas Askan

Updated 1-Nov-12 12:59pm

Nelek

v5

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Eugen Podsypalnikov 1-Nov-12 14:52pm

What types of the variables and
what kind of the access (read/write) are wanted, please ? :)

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JackDingler · Accepted Answer · 2012-11-01T08:05:00

You could access it there, but it's a bad idea as you'd need to lock the MainFrame and that can get dirty and complex.

I recommend that you create a singleton class to handle your globals. Keep the member variables protected and provide thread safe access methods for the data.

Something like this.

Header File

C++

class CWorkSpace;

extern CWorkSpace WorkSpace;

class CWorkSpace
{
protected:
   THREADVIEW ThreadView;

public:
   THREADVIEW GetThreadView(void) const
   {
      CLock Lock;
      return ThreadView;
   }

   void SetThreadView(const THREADVIEW & NewThreadView)
   {
      CLock Lock;

      ThreadView = NewThreadView;
   }
}

CPP File

C++

#include "WorkSpace.h"

CWorkSpace WorkSpace;

You'll need to substitute the CLock Lock; code with whatever locking mechanism you choose.

CPallini · Accepted Answer · 2012-11-01T08:09:00

Solution 2

If the variables are global then you can access them (simply declare them extern in your thread source file).

Posted 1-Nov-12 8:09am

CPallini

v2

Comments

[no name] 1-Nov-12 15:49pm

Not sure how, would you kindly look at my question which I have just updated and has more info. Regards.

JackDingler 1-Nov-12 16:05pm

You can access your CMainFrame from a thread, though it's a bad idea and you shouldn't do it.

You shouldn't be accessing globals at all without a locking mechanism, unless you know for a fact that they will never change.

CMainFrame * pMainFrame = (CMainFrame *) AfxGetMainWnd();

CPallini 1-Nov-12 16:07pm

Why cannot you just call AfxGetMainWnd inside the thread?
However, beware worker threads should not 'touch the GUI' see Newcomer's essay:
http://www.flounder.com/workerthreads.htm

[no name] 1-Nov-12 16:09pm

Hi, as you see that's exactly what I have but I cannot access the variables. Changing their values is not an issue. I just want my code to return these parameters. Confusing me.

JackDingler 1-Nov-12 16:41pm

See my comment just above to learn how to do it.

CPallini 1-Nov-12 16:44pm

Why cannot?
What is the problem are you experiencing?

Eugen Podsypalnikov · Accepted Answer · 2012-11-01T10:24:00

1. You have a memory leak now, fix:

C++

// cpp:
void CYourView::OnSprint()
{
  //..
  THREADVIEW sPar = { this }; // on-stack instance, will be automatically "deleted"
  AfxBeginThread(ChangeColorThread, &sPar); // note: you could pass this directly as well
  //..
}

2. Your thread parameter could contain the CMainFrame-pointer:

C++

// cpp:
typedef struct {
  CYourView* pView;
  CMainFrame* pFrame;
} THREADVIEW;

void CYourView::OnSprint()
{
  //..
  THREADVIEW sPar = { this, static_cast<CMainFrame*>(::AfxGetMainWnd()) };
  AfxBeginThread(ChangeColorThread, &sPar);
  //..
}

3. Your StateOnThread could return the value or even the reference of the frame variable:

C++

// cpp:
int /*int&*/ CYourView::StateOnThread()
{
  CMainFrame* pcFrame(static_cast<CMainFrame*>(AfxGetMainWnd()));
  return pcFrame ? pcFrame->AA : 0;
  /*
  static int si(0);
  return pcFrame ? pcFrame->AA : si;
  */
}

4. Do not use this kind of access for the types longer as INT_PTR :)

Multithreading in MFC MDI

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