FIX: Invalid String Displayed by CToolTipCtrl (133257)

CAUSE

The ToolTip common control stores string information about the tooltips it is going to display. When you specify the string to use for a tooltip, you pass a pointer to the buffer, which contains this string.

Unlike a list or combo box, a ToolTip control stores the pointer to the string buffer instead of copying the string to its own local memory area. Therefore, the string buffer passed in must be maintained as long as the tooltip exists.

For example, the following code sequence will not work correctly:

   int CMyWnd::OnCreate()
   {
     ...
     CString str("Helpful Info");
     m_ToolTip.Create(this);
     m_ToolTip.AddTool(this,str);
     ...
   }

The CString object used in the call to AddTool is temporary. As soon as the function exits, the string buffer is freed and the tooltip is pointing to an invalid address. The overloaded version of CToolTip::AddTool, which takes a resource ID as an argument, has this problem. This is easy to see in the implementation of the function:

BOOL CToolTipCtrl::AddTool(CWnd* pWnd, UINT nIDText,
                           LPCRECT lpRectTool, UINT nIDTool)
{
  ASSERT(nIDText != 0);
  CString str;
  VERIFY(str.LoadString(nIDText));
  return AddTool(pWnd, str, lpRectTool, nIDTool);
}

RESOLUTION

You can either use hard-coded strings that will never be de-allocated or you must store the string data for later use. One possible approach to doing this is to create your own CToolTipCtrl-derived class, and store the strings in it. You can add a OnAddTool handler for the TTM_ADDTOOL message for this class. In the OnAddTool handler, allocate space for the string. Then retrieve the permanent address of the stored string, and pass that in the TOOLINFO structure to the default Window procedure.

An example of such a CToolTipCtrl-derived class is included in the "Sample Code" section of this article.

MORE INFORMATION

NOTE: This problem may not show up on Windows NT when using Visual C++ version 2.1 or 2.2 because even though the string is passed in as ANSI, Windows NT converts it to UNICODE by thunking it and has to hang on to it. However, if the application is compiled using UNICODE, the problem will appear on Windows NT.

Sample Code Showing Example Workaround

// TTFIX.H
// 
// CFixToolTipCtrl
// 
//    CToolTipCtrl-derived class that duplicates the strings
//    passed in to AddTool and SetToolInfo so that they will be
//    properly stored until the tooltip is destroyed.
// 

class CFixToolTipCtrl : public CToolTipCtrl
{
protected:
   CPtrList m_lstTips;
public:
     CFixToolTipCtrl();
     virtual ~CFixToolTipCtrl();

   // Generated message map functions
protected:
   //{{AFX_MSG(CFixToolTipCtrl)
// NOTE - the ClassWizard will add and remove member functions here.
   //}}AFX_MSG
      afx_msg LRESULT OnAddTool(WPARAM wParam, LPARAM lParam);
   DECLARE_MESSAGE_MAP()
};
// END OF TTFIX.H
///////////////////////////////////////////////////////////////////// 


//////////////////////////////////////////////////////////////////// 
// TTFIX.CPP
// 
#include "stdafx.h"
#include <afxcmn.h>
#include "ttfix.h"


BEGIN_MESSAGE_MAP(CFixToolTipCtrl, CToolTipCtrl)
   //{{AFX_MSG_MAP(CFixToolTipCtrl)
  // NOTE - the ClassWizard will add and remove mapping macros here.
   //}}AFX_MSG_MAP
     ON_MESSAGE(TTM_ADDTOOL, OnAddTool)
END_MESSAGE_MAP()


CFixToolTipCtrl::~CFixToolTipCtrl()
{
    while (!m_lstTips.IsEmpty())
       free(m_lstTips.RemoveHead());
}

LRESULT CFixToolTipCtrl::OnAddTool(WPARAM wParam, LPARAM lParam)
{
      TOOLINFO ti = *(LPTOOLINFO)lParam;
      if ((ti.hinst == NULL) && (ti.lpszText != LPSTR_TEXTCALLBACK)
            && (ti.lpszText != NULL))
      {
            char *pStr = _tcsdup(ti.lpszText);
            m_lstTips.AddTail(pStr);
            ti.lpszText = pStr;
      }
      return DefWindowProc(TTM_ADDTOOL, wParam, (LPARAM)&ti);
}

// END OF TTFIX.CPP