MFC collection class CTypedPtrList problem

Rigeto Zhao -- rigeto.zhao@gauging.eurotherm.com
Thursday, January 16, 1997

Environment: MSVC++ 4.2b, WinNT4.0

I am building an application (SDI) that stores the configuration info from a
dialog box for 4 scanners. i am using CTypedPtrList to store the info about
each scanner.
My CScanner class is defined in AppConfigDoc.h file as follow:

//////////////////////////////////////////////////////////////
// CScanner class
class CScanner : public CObject
{
protected:
	char*	m_strName;    //scanner name
	BOOL	CHECK;        //TRUE: scanner is selected
	CString m_strLineNum;
public:
	CScanner(char* szName):m_strName(szName){}
	CScanner(const CScanner& s):m_strName(s.m_strName){}
	const CScanner& operator =(const CScanner& s){
		m_strName=s.m_strName;
	return *this;
	}

	BOOL operator ==(const CScanner& s) const
	{
		if (m_strName==s.m_strName)
			return TRUE;
		else 
			return FALSE;
	}

	BOOL operator !=(const CScanner& s) const
	{
		return !(*this==s);
	}

protected:	
	CScanner();
	DECLARE_SERIAL(CScanner)
public:
	virtual void Serialize(CArchive& ar);
};

////////////////////////////////////////////////////
//AppConfigDoc class
class CAppConfigDoc : public CDocument
{
friend class MyDlg;              //my dialog used for input
..........................
protected: // create from serialization only
	
	char* m_strName;	//@@@ current scanner

//@@@ Attributes
public:
	CTypedPtrList  m_scannerList;
......................................
}

//////////////////////////////////////////////////////////////////
//In MyDlg.cpp
void MyDlg::OnEumNext() // event handler for the "Next" Button onthe dialog
{
	.......................
	//@@@ store data in a new CScanner object
	CAppConfigDoc* m_pDoc=GetDoc();
	if(m_bScan1==TRUE){//var associated w/ dialog check box
		m_pDoc->m_strName="Scan1";
		m_pCurScan=m_pDoc->NewScanner(); //see implementation below
        }
.......
}


The implementation for NewScanner() is as follow (a member function of
CAppConfigDoc)
//////////////////////////////////////////////////////////////
// 
CScanner* CAppConfigDoc::NewScanner()
{
	CScanner* pScannerItem = new CScanner(m_strName);
	m_scannerList.AddTail(pScannerItem); // <-------------Error here 
	SetModifiedFlag();  // Mark the document as having been modified,
       			    // for purposes of confirming File Close.
	return pScannerItem;

}

============================================================================
The program compiles fine but I get an Unhandled exception error right at
the statement
        m_scannerList.AddTail(pScannerItem);
in CAppConfigDoc::NewScanner() function. (see <-------------Error here )

Any idea what is going on?
Thanks in advance for your help.

---

SCS.007@mch.scn.de
Saturday, January 18, 1997


I have tried it in MSVC++ 4.0 and NT 4.0 and it works perfectly.

But the documentation says that the function ConstructElements(...) is called.

// Extract from the help files

You should override these functions if their default action is not suitable 
for your collection class. The default implementation of ConstructElements 
sets to 0 the memory that is allocated for new elements of the collection; no 
constructors are called. The default implementation of DestructElements does 
nothing.
In general, overriding ConstructElements is necessary whenever the collection 
stores objects that require a call to a constructor (or other initializing 
function), or when the objects have members requiring such calls. Overriding 
DestructElements is necessary when an object requires special action, such as 
the freeing of memory allocated from the heap, when the object is destroyed.
For example, you might override ConstructElements for an array of CPerson 
objects as follows:
  
CPerson : public CObject { . . . };
CArray< CPerson, CPerson& > personArray;

void ConstructElements( CPerson* pNewPersons, int nCount )
{
    for ( int i = 0; i < nCount; i++, pNewPersons++ )
    {
        // call CPerson default constructor directly
        new( pNewPersons )CPerson;
    }
}

// End of extract.

I haven't overridden this function and yet it works fine for me.... Try 
stepping thru the AddTail() function and find out where exactly the 
assertion_failure is coming.

Chandru.

---

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