Microsoft KB Archive/111923

Knowledge Base

How To Compare a CString to the Empty String

Article ID: 111923

Article Last Modified on 11/21/2006

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APPLIES TO

• Microsoft Foundation Class Library 4.2, when used with:
  • Microsoft Visual C++ 1.0 Professional Edition
  • Microsoft Visual C++ 1.5 Professional Edition
  • Microsoft Visual C++ 1.51
  • Microsoft Visual C++ 1.52 Professional Edition
  • Microsoft Visual C++ 1.0 Professional Edition
  • Microsoft Visual C++ 2.0 Professional Edition
  • Microsoft Visual C++ 2.1
  • Microsoft Visual C++ 4.0 Standard Edition
  • Microsoft Visual C++ 5.0 Standard Edition

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This article was previously published under Q111923

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: kbhowto kbstring KB111923

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