Re: Return by reference

[Brian Rogoff <bpr@shell5.ba.best.com>]

On Wed, 3 Nov 1999, Matthew Heaney wrote:
> In article <Pine.BSF.4.10.9911021905050.1386-100000@shell5.ba.best.com> 
> , Brian Rogoff <bpr@shell5.ba.best.com>  wrote:
> 
> > I was going to use the "Rosen trick" to modify data in the Fmt each time
> > "&" is called. Unfortunately, that requires making the full view of
> > Format_Type limited and you run into the problem I described.
> >
> > Is there a cleaner way to implement that interface?
> 
> 1) Privately implement the type as tagged, to make it a by-reference
> type.
> 
> 2) This allows you to invoke RM95 13.3 (16), to take the address of
> parameter.
> 
> 3) Pass the address to an instantiation of
> System.Address_To_Access_Conversions, which gives you a variable view of
> the parameter.

Interesting idea. I'll try it, but it still does more copying than I'd
have liked, since it isn't a return-by-reference type. You'll end up
copying on the way out. 

> Note that there has been a subtle error running throughout this thread.
> There is nothing wrong with returning a "by-reference" type as a
> function return type; the error is returning a *limited* type.

No, I was quite clear in my post. The issue is about "return-by-reference" 
types (see 6.5.11) and why returning an argument of such a type is the
same as returning a local variable of such a type (it is the same, in that 
it raises Program_Error). You can certainly return a (limited)
return-by-reference type from a function, it just has to be global to that 
function. 

-- Brian

> 
> 
> --
> Science is, foremost, a method of interrogating reality: proposing
> hypotheses that seem true and then testing them -- trying, almost
> perversely, to negate them, elevating only the handful that survive to
> the status of a theory. Creationism is a doctrine, whose adherents are
> interested only in seeking out data that support it.
> 
> George Johnson, NY Times, 15 Aug 1999
> 
>