Q111923: HOWTO: Compare a CString to the Empty String

Article: Q111923
Product(s): Microsoft C Compiler
Version(s): winnt:1.0,2.0,2.1,4.0,5.0
Operating System(s): 
Keyword(s): kbMFC kbString kbVC kbGrpDSMFCATL
Last Modified: 07-MAY-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used 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          : kbMFC kbString kbVC kbGrpDSMFCATL 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:1.0,2.0,2.1,4.0,5.0
Issue type        : kbhowto

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.