Re: Converting access values

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

Jeffrey Carter <spam@spam.com> writes:

> Mark Lorenzen wrote:
> 
> > Hello,
> > Imagine that we have a program that reads large amount of data from
> > (for example) a network connection into buffers. The data is just to
> > be seen as a sequence of octets.
> > We now want to copy parts ("slices") of the data to other tasks that
> > may do something interesting with these slices. The rate of data is
> > too high to simply copy the wanted slices, so instead we make "cheap
> > copies".
> > The idea is that these cheap copies simply point to slices within the
> > large buffer and this way all copies refer to a common buffer and no
> > real copying is going on.
> 
> The answer, as is often the case with Ada, is that you don't need to
> use access types to do what you want. Assuming you have a big
> Storage_Array and want to pass a slice of it somewhere, just pass the
> slice:
> 
> Buf : Stream_Element_Array (Really_Big);
> ...
> procedure Do_Something (Data : in [out] Stream_Element_Array);
> ...
> Do_Something (Data => Buf (Low .. High) );
> 
> I don't know of any compiler that will pass Data by copy; if yours
> does, you should seriously consider changing compilers. What will be
> passed is a slice descriptor, which may be bigger than an access value
> but will still be very small. Actual access to the buffer will be by
> reference.

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.
 
> 
> -- 
> Jeff Carter
> "It's all right, Taggart. Just a man and a horse being hung out there."
> Blazing Saddles
> 34

- Mark Lorenzen