Re: [boost] Re: Virtual function parameter variance

From: Jeremy Graham Siek (jsiek_at_[hidden])
Date: 2004-08-08 15:47:36

Nope, did now.

On 8/8/04 11:30 AM, "David Abrahams" <dave_at_[hidden]> wrote:

> 
> Did you send this to the OP ("Dave" <better_cs_now_at_[hidden]>)?
> 
> Jeremy Graham Siek <jsiek_at_[hidden]> writes:
> 
>> 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)
>> _______________________________________________
>> 
>> 
>> 
>> _______________________________________________
>> Unsubscribe & other changes:
>> http://lists.boost.org/mailman/listinfo.cgi/boost
>> 

_______________________________________________
Jeremy Siek <jsiek_at_[hidden]>
http://www.osl.iu.edu/~jsiek
Ph.D. Candidate, Indiana University Bloomington
C++ Booster (http://www.boost.org)
_______________________________________________