C / C++ / MFC

class CTabCBox : public CComboBox
{
public:
	CTabCBox()	{icTab=0; lpiTab=NULL;}
	~CTabCBox()	{if (lpiTab) free(lpiTab);}

	// subclassed construction
	BOOL SubclassCBox(UINT nID,CWnd* pParent,int icTabs,LPINT lpiTabs);

protected:
	int icTab;		// count of tab stops
	LPINT lpiTab;	// tab stops in logical units

	virtual void MeasureItem(LPMEASUREITEMSTRUCT lpMis);
	virtual void DrawItem(LPDRAWITEMSTRUCT lpDis);
};



// CtestDlg dialog
class CtestDlg : public CDialogEx
{
// Construction
public:
	CtestDlg(CWnd* pParent = NULL);	// standard constructor

// Dialog Data
	enum { IDD = IDD_TEST_DIALOG };

	protected:
	virtual void DoDataExchange(CDataExchange* pDX);	// DDX/DDV support


// Implementation
protected:
	HICON m_hIcon;

	// Generated message map functions
	virtual BOOL OnInitDialog();
	afx_msg void OnSysCommand(UINT nID, LPARAM lParam);
	afx_msg void OnPaint();
	afx_msg HCURSOR OnQueryDragIcon();
	DECLARE_MESSAGE_MAP()
public:
	CTabCBox m_ctrlFontsX;
	CComboBox m_ctrlFonts;
};

// from INITDIALOG
// TODO: Add extra initialization here
	TEXTMETRIC tm;
	int iTab;

	// compute tab stop
	if (m_ctrlFonts.GetDC()->GetTextMetrics(&tm))
		iTab = (33+8) * tm.tmAveCharWidth;

	m_ctrlFontsX.SubclassCBox(IDC_COMBO1,this,1,&iTab);

	CString csS("Font title string\tnnnnnnnn");

	m_ctrlFontsX.AddString(csS);



BOOL CTabCBox::SubclassCBox(UINT nID, CWnd* pParent, int icTabs, LPINT lpiTabs)	
{
	DWORD dwcb;
	int iRc = 0;

	// make very sure all tab members are free and clear
	if (lpiTab)
		free(lpiTab);
	lpiTab = NULL;
	icTab = 0;

	if (icTabs>0)
		{
		// make copy of tab info
		dwcb = icTabs*sizeof(int);
		if (!(lpiTab = (LPINT)malloc(dwcb)))
			return(FALSE);
		memcpy(lpiTab,lpiTabs,(size_t)dwcb);
		icTab = icTabs;
		}
	iRc = SubclassDlgItem(nID, pParent); // THIS CAUSES THE ASSERT
	return(iRc);
} // end of SubclassCBox

It always fails on the second assert (FromHandle....) from wincore.cpp
BOOL CWnd::Attach(HWND hWndNew)
{
	ASSERT(m_hWnd == NULL);     // only attach once, detach on destroy
	ASSERT(FromHandlePermanent(hWndNew) == NULL);
		// must not already be in permanent map