Bruno Medeiros wrote:
> Mike Parker wrote:
>>
>> This line is the key:
>>
>> Foo foo = new Bar();
>>
>> foo is an instance of Foo, not an instance of Bar, 
> 
> No, that is wrong, foo *is* an instance of Bar. (yet, it's type contract is only that it is a Foo)
> 

Yes, yes. Underneath the foo is an instance of Bar. But because, as you say, the type contract is that of Foo then you are effectively dealing with a Foo and not a Bar. It should only be treated as a Bar when you downcast.

Uh, I didn't know the issue of "covariance versus contravariance" is a topic of continuing debate:

http://c2.com/cgi/wiki?ContraVsCoVariance

and especially

http://www.cs.trinity.edu/~mlewis/CSCI3294-F01/Papers/p431-castagna.pdf

where I found on page 10:

"At this point, we are able to make precise the roles played
by covariance and contravariance in subtyping: contravariance
is the correct rule when you want to substitute a function
of a given type for another one of a different type;
covariance is the correct condition when you want to specialize
(in object-oriented jargon "override") a branch of an overloaded
function by one with a smaller input type."

Regards,
Markus

Markus Kranz wrote:
> Uh, I didn't know the issue of "covariance versus contravariance" is a topic of
> continuing debate:
> 
> http://c2.com/cgi/wiki?ContraVsCoVariance
> 
> and especially
> 
> http://www.cs.trinity.edu/~mlewis/CSCI3294-F01/Papers/p431-castagna.pdf
> 
> where I found on page 10:
> 
> "At this point, we are able to make precise the roles played
> by covariance and contravariance in subtyping: contravariance is the correct rule when you want to substitute a function of a given type for another one of a different type; covariance is the correct condition when you want to specialize (in object-oriented jargon "override") a branch of an overloaded function by one with a smaller input type."
> 
> Regards,
> Markus
> 
> 

I finally took the time to read it, it is indeed a good article.

-- 
Bruno Medeiros - CS/E student
http://www.prowiki.org/wiki4d/wiki.cgi?BrunoMedeiros#D