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Q141447: FIX: dbDao’s CdbException Uses Private Constructor

Article: Q141447
Product(s): Microsoft C Compiler
Version(s): 3.0,3.5,3.51,3.6
Operating System(s): 
Keyword(s): kbcode kbDAOsearch kbDatabase kbMFC kbVC
Last Modified: 30-JUL-2001

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

- The DAO SDK, versions 3.0, 3.5, 3.51, 3.6 
-------------------------------------------------------------------------------

SYMPTOMS
========

It is difficult to explicitly throw a CdbException from the dbDAO class library.
It is possible, depending on how the exception class is thrown, to generate this
error:

  C2248: 'CdbException::CdbException' : cannot access private member declared
  in class 'CdbException'

CAUSE
=====

This is because the constructor for the class is implicitly private. It does not
inherit, nor are any classes inherited from CdbException, and it does not define
any friends for the class.

The dbDao classes, however, are successfully able to throw the exception, but
only because of another bug within the Visual C++ 4.0 compiler that allows you
to throw a temporary instance of a class with only a private constructor.

RESOLUTION
==========

The error is generated if you declare an instance of CdbException and then
attempt to throw it.

     HRESULT hr;
     CdbException myException( hr );
     throw myException;          // Generates error C2248

However, you can successfully throw the instance if you throw a temporary
instance of CdbException.

     HRESULT hr;
     throw CdbException( hr );

Now it is possible to catch this exception using C++ exception handling.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This bug was corrected in Visual C++ 32- bit Edition
version 4.1.

Additional query words: kbVC400bug 3.00 4.00 4.10 vcfixlist410

======================================================================
Keywords          : kbcode kbDAOsearch kbDatabase kbMFC kbVC 
Technology        : kbAudDeveloper kbDAOsearch kbSDKDAOSearch kbSDKSearch kbSDKDAO300 kbSDKDAO350 kbSDKDAO360 kbSDKDAO351
Version           : :3.0,3.5,3.51,3.6
Issue type        : kbbug
Solution Type     : kbfix

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