typedef redux

[Andrei Alexandrescu]

Sean Kelly wrote:
> Andrei Alexandrescu Wrote:
> 
>> My perception following the discussion around typedef is that we
>> should eliminate it. What we offer is "alias", which is a
>> generalization of C's "typedef", and the change of name is
>> justified by the fact that "alias" defines aliases for more
>> entities than just types.
>> 
>> Does all that sound good?
> 
> I like that typedef purports to offer an inheritance model of sorts
> to concrete types.  Aliasing is all well and good, but it isn't
> always appropriate.  But I really haven't spent much time with
> typedef recently.  From what I've read, it sounds like the
> implementation doesn't live up to the promise.  Is this true?

The current implementation is just broken because there was no 
definition of the feature. It just does the moral equivalent of what an 
unintended switch fall-through does :o).

One basic problem was that Walter and I couldn't even make a water-tight 
case of which direction the inheritance should go: I thought typedef 
should introduce a subtype, and he thought it should introduce a 
supertype. We both had examples. We both saw the other's argument. We 
both realized the shortcomings of the feature, regardless of whatever 
direction we chose. We both saw the costs in the implementation and in 
the language definition, and we realized the paucity of benefits.

So there's not only an implementation that doesn't live up to the 
promise, we even don't have a reasonable promise to fulfill.


Andrei