PRB: Bad Pointer from RUNTIME_CLASS with Class from _AFXDLL

MORE INFORMATION

The DECLARE_DYNAMIC macro declares a CRuntimeClass object as follows:

   static AFX_DATA CRuntimeClass class##class_name;
				

Therefore, defining AFX_DATA as __declspec(dllimport) affects the declaration of that object. The rest of this section describes why it is important to do so.

The RUNTIME_CLASS macro is defined in afx.h as follows:

   #define RUNTIME_CLASS(class_name) (&class_name::class##class_name)
				

In other words, this macro returns the address of the static CRuntimeClass member added to the class by the DECLARE_DYNAMIC macro or one of its cousins (DECLARE_SERIAL or DECLARE_DYNCREATE).

Here is some disassembled (X86) code from an .exe file that uses a class named CMyClass, which it imports from an extension DLL. The extension DLL uses its module definition file to export the members of the class, and the header file does not declare the class or any of its members as imported.

m_pMyClass has just been initialized as follows:

   m_pMyClass = new CMyClass;
138:      ASSERT( m_pMyClass->IsKindOf(RUNTIME_CLASS(CMyClass)));
00401a9e   push      00402648
00401aa3   mov       eax,dword ptr [this]
00401aa6   mov       ecx,dword ptr [eax+000000d8]
00401aac   call      ?IsKindOf@CObject@@QBEHPBUCRuntimeClass@@@Z ...
				

The code fails the assertion.

Here is the disassembly of the same code when the header file defines AFX_DATA as __declspec(dllimport):

138:      ASSERT( m_pMyClass->IsKindOf(RUNTIME_CLASS(CMyClass)) );
00401a95   mov       eax,dword ptr
 [_imp_?classCMyClass@CMyClass@@2UCRuntimeClass@@A (00406598)]
00401a9a   push      eax
00401a9b   mov       eax,dword ptr [this]
00401a9e   mov       ecx,dword ptr [eax+000000d8]
00401aa4   call      ?IsKindOf@CObject@@QBEHPBUCRuntimeClass@@@Z ...
				

This code sequence passes the assertion, as expected.

Notice the difference in the way the stack is set up prior to the call to IsKindOf, especially the first item pushed. In the second case, eax is loaded with the address of an item located through the import table, while in the first case, a direct value is used.

Sample Code

/* Compile options needed: None
*/ 

// MYCLASS.H
//   The following class declaration illustrates the CORRECT
//   way to declare the AFX_DATA symbol when exporting a class
//   from an extension DLL

#undef  AFX_DATA
#define AFX_DATA AFX_EXT_CLASS

class CMyClass : public CObject
{
  DECLARE_DYNAMIC()

  // ... other class members omitted for clarity
};

#undef  AFX_DATA
#define AFX_DATA

// end of header file