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

• Microsoft Visual C++, 32-bit Editions, version 5.0

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
Version : 5.0
Platform : NT WINDOWS
Issue type : kbbug