Re: Overriding/Redefining a Non-Virtual Function in Derived Classes
Michael K. O'Neill ha scritto:
In Scott Meyers' "Effective C++", item 36 states "Never redefine an
inherited non-virtual function". This admonition is repeated at item 23.8
of C++ FAQ Lite ("[23.8] Should a derived class redefine ("override") a
member function that is non-virtual in a base class?" at
http://www.parashift.com/c++-faq-lite/strange-inheritance.html#faq-23.8 ).
I only just now recognized that in the MFC framework, this is done all the
time. Particularly with respect to message handlers for CWnd-derived
classes, it's common to see code like:
BOOL CDerivedWnd::OnEraseBkgnd(CDC* pDC)
{
// call base class function, which is not a virtual function
CWnd::OnEraseBkgnd(pDC);
// ... code specific to derived class
return TRUE;
}
I recognize that MFC is not a paragon of C++ good-programming practices. It
is also my understanding that the programming of message handlers for
CWnd-derived classes was an attempt to *avoid* virtual functions.
All the message handler machinery is just an attempt to have
virtual-like functions in a more manageable way (i.e.: more suitable to
address the specific problem) than the "standard" C++ virtual keyword.
So, in certain sense, MFC is still using some kind of virtuality. MFC
can't be taken as a general counter-example of EC++ item 36.
But given the fact that it is used so extensively by the MFC framework, is
it truly such a bad idea to override/redefine non-virtual functions in
derived classes? Is it truly something that should be left to "experienced
C++ programmers", as mentioned in the C++ FAQ Lite?
Well... Not that I like or endorse MFC, but you need a quite experienced
programmer to design something like the MFC message handler machinery.
The fact that the machinery is provided in library form so that even an
unexperienced programmer can use it out-of-the-box, doesn't mean the
advice of item 36 or the C++ FAQ Lite is wrong or invalid.
Just my opinion,
Ganesh
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