Re: Dispatching and generics - language lawyer question

[Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov>]

adam@irvine.com (Adam Beneschan) writes:

> I have a couple questions about what's supposed to happen when a
> generic is instantiated with a class-wide type as an actual type.  I'm
> hoping that someone who really understands the intricacies of the
> language will be kind enough to answer, and provide chapter and verse
> from the RM to explain the correct answer.
> 
> (1) What is the correct output of the program below?

GNAT 3.15a1 says "TRUE". Not a definitive answer, but possibly helpful :).

> (2) What would be the correct output if "private" in line [A] is
> changed to "tagged private"?

GNAT 3.15a1 also says "TRUE" for this. 

> I'm pretty sure the answer to (1) is FALSE, since the "=" on line
> [B] in the instance denotes the predefined "=" declared for T1 in
> the generic, and not the "=" "defined" for the class-wide type, by
> 8.3(13). 

I think the controlling paragraph is 12.5.1 (21). I think the behavior
exhibited by GNAT 3.15a1 agrees with this. Note that both variations
of your formal type declaration are "formal private types"; the
presence of "tagged" does not matter here. There is different behavior
for a "formal derived type", which is indicated by the keyword "new".

There is a problem with "reemergence" of predefined operators for
generic formal types, but I don't have an example of when it occurs. I
didn't see a discussion of this in Cohen's "Ada as a Second Language",
either. Nor in the Rationale. But maybe I just didn't look hard
enough.

You might try reading the Rationale for generics; it has some good
stuff in it. It's online at www.adapower.com; click the "resources"
button (I hate html frames!).

-- 
-- Stephe