[boost] Re: Virtual function parameter variance

From: Dave (better_cs_now_at_[hidden])
Date: 2004-08-08 13:17:51

OK, I appreciate the clarification!  Great lib and book BTW!

"Jeremy Graham Siek" <jsiek_at_[hidden]> wrote in message
news:BD3BAC4B.302C%jsiek_at_osl.iu.edu...
> Hello Dave,
>
> Sorry about the confusion. I should have distinguished between how this
> works in C++, and what is theoretically possible. The contravariance rule
I
> spoke of describes what makes sense theoretically. However, C++ is more
> conservative in this case and restricts parameter types of overloads to be
> the same type. This is a similar issue to covariant return types. Early
on,
> C++ did not allow them, even though it made sense theoretically, then
later
> on covariant return types were allowed.
>
> Best Regards,
> Jeremy
>
>
> On 8/8/04 2:09 AM, "Dave" <better_cs_now_at_[hidden]> wrote:
>
> >
> > Hello all,
> >
> > Quoting from page 24 of "The Boost Graph Library; User Guide and
Reference
> > Manual":
> >
> > "It turns out that by the contravariance subtyping rule, the parameter
type
> > in the derived classes member function must be either the same type or a
> > base class of the type as the parameter in the base class."
> >
> > Now please consider this code:
> >
> > #include <iostream>
> >
> > struct base_1 {};
> > struct derived_1: base_1 {};
> >
> > struct base_2
> > {
> >    virtual void foo(derived_1 *p) {std::cout << "base_2::foo()\n";}
> > };
> >
> > struct derived_2: base_2
> > {
> >    virtual void foo(base_1 *p) {std::cout << "derived_2::foo()\n";}
> > };
> >
> > int main()
> > {
> >    base_2 *ptr = new derived_2;
> >    ptr->foo(new derived_1);
> > }
> >
> > This outputs base_2::foo().  Why?
> >
> > The quoted passage implies derived_2::foo() should override
base_2::foo().
> > Clearly, it does not.  Furthermore, I cannot find anything in the
Standard
> > that indicates it should.  So, I am clearly misinterpreting the quoted
> > passage.  Can anybody explain to me what was meant in that passage?
> >
> > Thank you,
> > Dave
> >
> >
> >
> >
> >
> > _______________________________________________
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>
> _______________________________________________
> Jeremy Siek <jsiek_at_[hidden]>
> http://www.osl.iu.edu/~jsiek
> Ph.D. Candidate, Indiana University Bloomington
> C++ Booster (http://www.boost.org)
> _______________________________________________
>
>
>
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