BUG: LNK2001 on Operator>> with Class from MFC Extension DLL

• The Microsoft Foundation Classes (MFC) included with: - Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1, 4.2, 5.0

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)

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);

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);

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. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

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