Re: Converting access values

[Mark Lorenzen <mark.lorenzen@ofir.dk>]

Nick Roberts <nick.roberts@acm.org> writes:

> Mark Lorenzen <mark.lorenzen@ofir.dk> wrote:
> 
> > ...
> > My problem is not passing a slice as an actual parameter to a subprogram.
> > I have a task which receives Buf, slices it up and then passes the slices
> > to other tasks using queues implemented as protected types. When passing
> > the slice to a queue, it will probably not be done by copy, but the queue
> > puts the slice into a data structure (list, array etc.) and there we
> > definately have a real assignment and copy.
> 
> Mark, you are a silly sausage! All you do is pass the /bounds/ to the tasks.
> When a task finally comes to actually process a slice, it uses the bounds
> passed to it to obtain the actual slice of the buffer. Since there is only
> one buffer, it can be a global, and parts of it do not need to be passed as
> a parameter anywhere. Would this not work for you?

As I said in another posting, I am going for a design like this:

type Buffer_Slice is
   record
     Data  : Buffer_Ptr;
     First : Ada.Streams.Stream_element_Offset;
     Last  : Ada.Streams.Stream_element_Offset;
   end record;

There will be several buffers allocated/deallocated from a pool, so
the reference is a part of the Buffer_Slice type.

[cut]

> 
> 
> Does this actually solve your problem, or am I missing something?

It solves the problem, but exposes a bit more information to the
clients than necessary. It is very easy for a client to address a
slice outside the bounds:

Buffer.Data (Buffer.First - 10 .. Buffer.Last + 15)

But there is of course also the risk of over-engineering the buffer,
so I will settle with the above design.

> 
> -- 
> Nick Roberts

- Mark Lorenzen