[boost] Re: Virtual function parameter variance

From: David Abrahams (dave_at_[hidden])
Date: 2004-08-08 11:30:51

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)
> _______________________________________________
>
>
>
> _______________________________________________
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>

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com