Re: Rational for not making cursor tagged in Containers

["Randy Brukardt" <randy@rrsoftware.com>]

"Anh Vo" <anhvofrcaus@gmail.com> wrote in message
news:1176998738.656903.141250@q75g2000hsh.googlegroups.com...
> I enjoy using the prefixed (object) notation when and where ever I
> can. However, when it comes to Containers, I can not use this notation
> on cursor because it is not tagged.
>
> It seems that it is not balanced between Container types and its
> cursors. I just want to know what rationale behind it for not making
> cursor a tagged type.

I think there are two reasons:

(1) The original designer of the containers library intended for cursors to
be implemented as a bare access value; adding a tag makes that messier and
doubles the size of the cursors. I personally don't find this too important.

(2) Ada does not allow an operation to be primitive for two tagged types.
(I'm sure we'll hear at length from Dmitry on this. ;-) That would mean that
operations like:
     procedure Delete (Container : in out Vector; Index : in Cursor);
would be illegal. That could be worked around by making the cursor
classwide:
     procedure Delete (Container : in out Vector; Index : in Cursor'Class);
but now you don't have much taggedness left. On most operations, you're not
even losing prefix notation (as the cursor is the second parameter).

Indeed, the real reason for your problem is that container is implicit in
Cursor operations that read from the container:
     function Element (Position : Cursor) return Element_Type;

A prefix call like
    My_Cursor.Element
doesn't make much sense, because a cursor is just an accessor, you're not
extracting the element from it! You really ought to be able to say:
   My_Container.Element (Position => My_Cursor)
which makes it clear where the element is coming from.

I admit we didn't think much about these issues. When it was suggested that
perhaps the container type should be visibly tagged, I recall someone noted
that would allow the prefix notation. And that seemed good; but I don't
think anyone noted that it doesn't work sometimes.

                                    Randy.