Re: Access type to member procedure of instance (Object Oriented programming in Ada)

[Adam Beneschan <adam@irvine.com>]

On Tuesday, November 20, 2012 1:38:38 PM UTC-8, Randy Brukardt wrote:
> "Adam Beneschan" wrote in message 
> 
> >If Message had type Message_Type'Class, I'd assume that at some point,
> >Message.Print'Access would need to determine which Print subprogram would 
> >be
> >called, and figure out the address of it.  I don't think it matters whether 
> >it's done when the
> >'Access value is computed or when the call is made, since the tag of an 
> >object can't be
> >changed during the object's lifetime.  I could be wrong, though; maybe I've 
> >missed a case
> >where it matters.  (But I'm not concerned about the possibility of an 
> >object's tag being
> >changed through extralegal means like Unchecked_Conversion.)
> 
> I don't think this is quite true; if the object is a parameter (for 
> instance), it can have different tags for different calls. But I don't see 
> that happening for something like Param.Print'Access -- access values are 
> never dispatching.
> 
> Anyway, since something like Param.Print'Access is illegal in any case, we 
> don't have to decide what this means or how code would be generated for it.

I probably confused things.  Yes, Param.Print'Access is illegal, but I was thinking in terms of "what if" we added a language feature to make it legal--probably by adding a new kind of access type that contained references both to the prefix object and, somehow, to the subprogram.  In that case, I'd want it to be dispatching, if Param's type were classwide.

This would mean that if Param's type were classwide, the actual subprogram referred to by the access value would be determined at runtime, not at compile time.  And I don't think it would matter *when* the subprogram address to which the program will dispatch were actually determined, i.e. when the statement containing the 'Access were executed, or when the access value was dereferenced in order to call the subprogram, since the object's tag shouldn't change in between those times.

But I guess I didn't make it clear that all this was ***if*** we decided to add a new capability to the language and if we did it the way I envision.  Sorry about that.  Hope this helps clear things up.

                                -- Adam