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Q174635: FIX: _variant_t::operator IUnknown*() Fails to AddRef Interface

Article: Q174635
Product(s): Microsoft C Compiler
Version(s): WINNT:5.0
Operating System(s): 
Keyword(s): kbVC500bug kbVC600fix kbVS97sp2fix
Last Modified: 17-JUL-2001

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

- The C/C++ Compiler (CL.EXE), included with:
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 5.0 
   - Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
-------------------------------------------------------------------------------

SYMPTOMS
========

When using _variant_t::operator IUnknown*(), the returned interface is not
appropriately AddRef'd. This can cause unpredictable results when using the
returned interface. For instance, subsequent calls through the interface may
inexplicably fail or may cause an Access Violation.

RESOLUTION
==========

Write a function to convert a _variant_t to an IUnknown*. A method based on
_variant_t::operator IDispatch*() is shown in the sample below.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This bug has been fixed in Visual Studio 97 Service
Pack 3 and Visual C++ version 6.0.

For more information, please see the following article in the Microsoft Knowledge
Base:

  Q170365 INFO: Visual Studio 97 Service Packs - What, Where, and Why

MORE INFORMATION
================

The code below should be called instead of allowing _variant_t::operator
IUnknown*() to be implicitly called to work around this problem. The conversion
operator will be called whenever a conversion from _variant_t to IUnknown* is
required. One place this occurs is with assignment statements, another is
passing a _variant_t to a function that expects an IUnknown*.

Sample
------

    IUnknown* f( const _variant_t & vt )
    {
        if (V_VT(&vt) == VT_UNKNOWN)
        {
            V_UNKNOWN(&vt)->AddRef();
            return V_UNKNOWN(&vt);
        }
        _variant_t vttmp;

        vttmp.ChangeType(VT_UNKNOWN, &vt);

        V_UNKNOWN(&vttmp)->AddRef();
        return V_UNKNOWN(&vttmp);
     }
     void g()
     {
       _variant_t vt;
       IUnknown * punk=0;
       // ...some code...
       punk = vt;    // will cause _variant_t::operator IUnknown*()
                     // to be called. Comment this out.
       //punk = f(vt); // call f instead. Uncomment for workaround
       // ...more code...
     }

Additional query words: refcount Release

======================================================================
Keywords          : kbVC500bug kbVC600fix kbVS97sp2fix 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : WINNT:5.0
Issue type        : kbbug
Solution Type     : kbfix

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

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