DOCUMENT:Q152254 07-FEB-2002 [visualc] TITLE :BUG: LNK2001 on Operator>> with Class from MFC Extension DLL PRODUCT :Microsoft C Compiler PROD/VER::4.0,4.1,4.2,5.0,6.0 OPER/SYS: KEYWORDS:kbProgramming kbDLL kbMFC kbVC kbVC400bug kbVC410bug kbVC420bug kbVC500bug kbVC600bug k ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - The Microsoft Foundation Classes (MFC), used with: - Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1 - Microsoft Visual C++, 32-bit Enterprise Edition, versions 4.2, 5.0, 6.0 - Microsoft Visual C++, 32-bit Professional Edition, versions 4.2, 5.0, 6.0 - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 - Microsoft Visual C++.NET (2002) ------------------------------------------------------------------------------- SYMPTOMS ======== When attempting to build an application that uses classes from an MFC extension DLL, the linker reports an error of the form: mainfrm.obj : error LNK2001: unresolved external symbol "class CArchive & __stdcall operator>> (class CArchive &,class CClassInExtDLL * &)" (??5@YGAAVCArchive@@AAV0@AAPAVCDummyDoc@@@Z) Specifically, this problem occurs for code that attempts to use the >> operator to serialize one of the extension DLL classes that use the DECLARE_SERIAL macro. CAUSE ===== If an exported class is declared in an extension DLL and is made serializable by the use of the macros DECLARE_SERIAL and IMPLEMENT_SERIAL, the global function: friend CArchive& AFXAPI operator>>(CArchive& ar, CMyObject* &pOb); is not automatically exported along with the class. Therefore, if an application tries to serialize an object of that class type by the use of the >> operator, a LNK2001 unresolved external error occurs on operator >>. RESOLUTION ========== Create a new macro based on the DECLARE_SERIAL macro: #define DECLARE_SERIAL_EXTDLL(class_name) \ _DECLARE_DYNCREATE(class_name) \ AFX_EXT_API friend CArchive& AFXAPI \ operator>>(CArchive& ar, class_name* &pOb); After defining this macro, use this new DECLARE_SERIAL_EXTDLL macro in place of DECLARE_SERIAL in the class declaration. Rebuild the DLL and link its new import library with the application. In this resolution, the AFX_EXT_API prefix resolves the LNK2001 error by explicitly exporting the function operator>> when building the extension DLL. STATUS ====== Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. MORE INFORMATION ================ In order to make a class serializable by the use of a CArchive object, the class must be derived from CObject and must implement the DECLARE_SERIAL and IMPLEMENT_SERIAL macros. These macros are needed because they define an overloaded function, operator>>, explicitly for that class. In a case where that class is defined in an extension DLL and is exported by using the AFX_EXT_CLASS macro, the overloaded function, operator>>, does not get exported along with the class. Thus, if the main application instantiates an object of that class type and tries to serialize that object by the use of a CArchive object, a LNK2001 unresolved external error will occur. Sample Code ----------- Declare your class in the extension DLL this way: In the header (.h) file for CMyObject: #define DECLARE_SERIAL_EXTDLL(class_name) \ _DECLARE_DYNCREATE(class_name) \ AFX_EXT_API friend CArchive& AFXAPI \ operator>>(CArchive& ar, class_name* &pOb); class AFX_EXT_CLASS CMyObject : public CObject { DECLARE_SERIAL_EXTDLL(CMyObject) ... }; In the implementation (.cpp) file for CMyObject: ... IMPLEMENT_SERIAL(CMyObject, CObject, YOUR_SCHEMA_NUMBER_HERE) ... REFERENCES ========== For additional information on serialization and how to export classes from extension DLL's, please see: - MFC TechNote #2 in the Books Online - MFC TechNote #33 in the Books Online For additional information, please see the following article in the Microsoft Knowledge Base: Q131946 PRB: Bad Pointer from RUNTIME_CLASS with Class from _AFXDLL Additional query words: kbvc400bug kbvc410bug kbvc420bug kbvc500bug kbvc600bug ====================================================================== Keywords : kbProgramming kbDLL kbMFC kbVC kbVC400bug kbVC410bug kbVC420bug kbVC500bug kbVC600bug kbGrpDSMFCATL Technology : kbAudDeveloper kbMFC Version : :4.0,4.1,4.2,5.0,6.0 Issue type : kbbug ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. 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