DOCUMENT:Q174635 17-JUL-2001 [visualc] TITLE :FIX: _variant_t::operator IUnknown*() Fails to AddRef Interface PRODUCT :Microsoft C Compiler PROD/VER:WINNT:5.0 OPER/SYS: KEYWORDS:kbVC500bug kbVC600fix kbVS97sp2fix ====================================================================== ------------------------------------------------------------------------------- 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 ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY. Copyright Microsoft Corporation 2001.