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Q168079: FIX: _com_ptr_t::operator=(const _variant_t&) AddRefs Twice

Article: Q168079
Product(s): Microsoft C Compiler
Version(s): 5.0
Operating System(s): 
Keyword(s): kbcode kbVC500bug kbVS97sp1fix
Last Modified: 17-JUL-2001

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The information in this article applies to:

- Microsoft Visual C++, 32-bit Enterprise Edition, version 5.0 
- Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
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SYMPTOMS
========

When you use an instantiation of the _com_ptr_t template class and assign a
_variant_t to it, the associated COM object may not be released when it should
be.

CAUSE
=====

_com_ptr_t::operator=(const _variant_t&) calls another member function,
QueryStdInterfaces, which ultimately AddRef's upon success. The problem is that
operator= also AddRef's upon success.

RESOLUTION
==========

Call Release() explicitly before the object goes out of scope.


STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This bug has been corrected in Visual Studio 97
Service Pack 1.

For additional information about the Visual Studio 97 Service Pack 1, please see
the following article in the Microsoft Knowledge Base:

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

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

The sample code below demonstrates the problem with operator=(const
_variant_t&) of the class template _com_ptr_t. The parameter vt is an
IUnknown*. To offset the result of the second AddRef, you need to call punk-
>Release before leaving the function below.

Sample Code
-----------

     // Compile options needed: none
     // Make an IUnknownPtr
     _COM_SMARTPTR_TYPEDEF( IUnknown, __uuidof(IUnknown));
     void f( const _variant_t & vt )
     {
         IUnknownPtr punk;
         punk = vt;
     }

Additional query words:

======================================================================
Keywords          : kbcode kbVC500bug kbVS97sp1fix 
Technology        : kbVCsearch kbAudDeveloper kbVC500 kbVC32bitSearch kbVC500Search
Version           : 5.0
Issue type        : kbbug
Solution Type     : kbfix

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