HOWTO: Compare a CString to the Empty String

ID: Q111923

The information in this article applies to:

The Microsoft Foundation Classes (MFC), included with:
- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51, 1.52
- Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 5.0

SUMMARY

The correct way to test a CString to see whether it is empty (or not empty) is to use the IsEmpty() member function as shown below:


   CString x = "This is a sample CString";
   if (x.IsEmpty())
      AfxMessageBox("The CString is EMPTY");
   else
      AfxMessageBox("The CString is not EMPTY");

MORE INFORMATION

The CString member function IsEmpty() tests a CString for the empty condition. The function returns nonzero if the string has zero length.

If the CString equality operators (==, !=) are used in an attempt to test whether a CString is NULL, a general protection (GP) fault may occur. The following code fragment demonstrates this:


   CString x = "This is a sample CString";
   if (x != NULL)  // GP Fault for (x == NULL) condition, as well
      AfxMessageBox("The CString is not EMPTY");
   else
      AfxMessageBox("The CString is EMPTY");

Because x is a CString object, not a pointer, it is inappropriate to compare x to a null pointer.

Testing a CString against the empty string, as shown below, is allowed. Because a temporary CString object is constructed, this method is less efficient than calling IsEmpty():


   CString x = "This is a sample CString";
   if (x != "")
      AfxMessageBox("The CString is not EMPTY");
   else
      AfxMessageBox("The CString is EMPTY");

Additional query words: CString inequality 2.50 2.51 2.52 3.00 3.10 gpf NULL gp-fault

Keywords : kbnokeyword kbMFC kbVC
Version : WINDOWS:1.0,1.5,1.51,1.52; WINDOWS NT:1.0,2.0,2.1,4.0,5.0;
Platform : NT WINDOWS
Issue type : kbhowto