Re: [boost] Virtual function parameter variance

From: Jeremy Graham Siek (jsiek_at_[hidden])
Date: 2004-08-08 10:09:15

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)
_______________________________________________