DECLARE_DYNCREATE and pure vritual

[venkat456]

Hi,
Is there any way one can have a class that implements "DYNCREATE" and also has a pure virtual function?

Is there a work around?

Thanks in advance

Regards
Venkatesh

[jck]

The reason an abstract class (that is, a class with at least one virtual function) won't work with DYNCREATE is that DECLARE_DYNCREATE contains the declaration:


static CObject* CreateObject();




which IMPLEMENT_DYNCREATE implements as:


CObject* PASCAL CMyClass::CreateObject()
{
return new CMyClass;
}




Now, of course, new CMyClass

fails for an abstract base class. I believe that these lines of code are the only differences between the DYNCREATE and the DYNAMIC macros -- is there any reason you couldn't simply use the DYNAMIC macros?

In Shepherd and Wingo's excellent "MFC Internals" (p. 173), they discuss abstract base classes in the context of the SERIAL macros and suggest redefining the IMPLEMENT_SERIAL macro as follows to accommodate abstract base classes:


#define IMPLEMENT_SERIAL_ABC(class_name, base_class_name, wSchema) \
_IMPLEMENT_RUNTIMECLASS(class_name, base_class_name, wSchema, NULL) \
CArchive& AFXAPI operator>>(CArchive& ar class_name* & pOb) \
{ pOb = (class_name*) ar.ReadObject(RUNTIME_CLASS(class_name));\
return ar; }




This new "ABC" (abstract base class) version of the macro eliminates the CreateObject() reference in the _IMPLEMENT_RUNTIMECLASS macro (replacing it with NULL) so that the constructor call for the abstract base class is never made.

Hope this helps.

[jck]

In the first line of test I omitted the word "pure" -- an abstract base class is a class with at least one pure virtual function.