BUG: You receive an error LNK2001 when you try to use the >> operator to serialize one of the extension DLL classes in Visual C++

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This article was previously published under Q152254

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.

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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 >>.

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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.

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STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article.

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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.

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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)

   ...

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