FIX: AfxIsValidString() Returns TRUE for NULL Near Pointers

The information in this article applies to:

• The Microsoft Foundation Classes (MFC) included with:

      - Microsoft Visual C++ for Windows, version 1.0
  

SYMPTOMS

Calling AfxIsValidString() with a near NULL pointer results in a TRUE return value. The return value should be FALSE because a NULL pointer is invalid.

For example, consider the following code:

   char NEAR * npszEmpty = NULL;
   ASSERT(AfxIsValidString(npszEmpty));

CAUSE

AfxIsValidString() takes an LPCSTR, which is defined in WINDOWS.H as:

   typedef const char FAR* LPCSTR; // far pointer to a read-only string

RESOLUTION

To work around this problem, do one of the following:

• Upgrade to Visual C++ version 1.5. The Microsoft Foundation Classes (MFC) version 2.5 supplied with Visual C++ version 1.5 for Windows provides two versions of AfxIsValidString(). The first is the original function that takes an LPCSTR. The second version takes a "const char *". The pointer in the "const char *" version is compared first with NULL, then passed to the LPCSTR version.
• Manually compare the string to NULL along with calling AfxIsValidString(). This is the technique used by MFC version 2.5. For example:

        char NEAR * npszEmpty = NULL;
        ASSERT(npszEmpty != NULL && AfxIsValidString(npszEmpty));
  

STATUS

Microsoft has confirmed this to be a problem in the Microsoft Foundation Classes version 2.0. This problem was corrected in the Microsoft Foundation Classes version 2.5.