Re: MI and clone()
In article <ed3f9270-2398-40eb-a0bb-a6bc3934c736
@k2g2000pro.googlegroups.com>, joshuamaurice@gmail.com says...
On Feb 23, 3:13?pm, Joshua Maurice <joshuamaur...@gmail.com> wrote:
On Feb 23, 2:42?pm, Noah Roberts <d...@reply.com> wrote:> I'm having a very weird problem here that's apparently caused by MI
and/or virtual inheritance that I'd like help with. ?I finally managed
to get the problem to appear in small form. ?I'll leave the code at the
end so I can explain some things more deeply before hand.
[snip example with code]
The last time I checked, MSVC does not support covariant return types
with multiple inheritance. It actually gave me compiler errors IIRC,
saying something like "ambiguous type".
Here's a simple example I just checked vs Comeau Online, gcc version
3.4.3 20050227 (Red Hat 3.4.3-22.1), and msvc 2008.
struct A { virtual A* clone() =0; };
struct B : virtual A { virtual B* clone() =0; };
struct C : virtual A { virtual C* clone() =0; };
struct D : B, C { virtual D* clone() =0; };
Compiles with Comeau Online and gcc 3.4.3. Trying to compile on msvc
2008 gives a compiler error.
1>c:\documents and settings\jmaurice\desktop\cpp_tester\foo.cpp(4) :
error C2250: 'D' : ambiguous inheritance of 'B *A::clone(void)'
I wonder how your example even compiles on msvc 2008. (It does on my
computer.) I find it most odd. My \guess\ is that it's producing
garbage output as it doesn't support covariant return types with
multiple inheritance, and your slightly more complex example hits a
compiler bug, a bug in the sense that it doesn't fail like the
developers of msvc intended to as they don't support covariant return
types with multiple inheritance.
Well, the problem I'm running into doesn't actually use covariant return
types with multiple inheritance. You'll notice that my derived2 class
never inherits from more than one base anywhere along it's path to the
root base. What it does is inherit from that base virtually so that a
totally different branch in the tree can inherit from that base more
than once. So I suspect the fact that I'm not actually inheriting
multiply at that point is the real reason it didn't vomit some error.
Interestingly though both covariance, virtual inheritance, and multiple
inheritence live together just fine if I only attempt to override in the
most derived classes. So in your above example take out the B and C
redefs and leave the D...in my sample it works.
I'm also not using 2008 but 2005 that could be part of the diff maybe.
Without the service pack even because it irreparably destroyed the only
computer we tried to apply it to.
--
http://crazyeddiecpp.blogspot.com