Re: asynchronous task communication

[Shark8 <onewingedshark@gmail.com>]

On Sunday, January 6, 2013 4:55:05 PM UTC-6, Charles Hixson wrote:
> On 01/05/2013 12:48 AM, Niklas Holsti wrote:
> 
> It has been suggested that my intended use of Vectors is a mistake.
> That it is unreasonable to store access variables into vectors. This is 
> a pity, as I know of no other Ada construct that would suit my needs.

I think that's some miscommunication, probably because prior to the Containers in Ada the only way to use "vectors" [read dynamically-sized arrays] for indefinite-types was (a) to use access values, or (b) construction via function. (This is really still the case, though (a) is cleaned up with implicit dereferencing.)

Your problem, as described, was that there could be multiple, varying-sized 'cells' which would be known at generation-time, but also might themselves need to grow. (A perfect candidate for Vector.)

You *could* store the access values (I'd recommend a not-null constraint, if possible) in an array, but unless you need to keep a big 'network' active (which sounds dubious with your mailbox/message analogy; though you might need it in a different part than the message-box).

With Indefinite_Vectors you could have something like the following:
Type Message( Length: Positive ) is record
  Data : Array(1..Length) of Character:= ( Others => ' ' );
end record;

Package Message_Box_Pkg is New Ada.Containers.Indefinite_Vector( Positive, Message );

Message_Box : Message_Box_Pkg.Vector;

And that's it.
Indeed, going with the "apartment wall of mailboxes" you could do something like:

Subtype Apt_Address is Positive;
Package Mail_Wall is Ada.Containers.Indefinite_Vector
  ( Apt_Address, Message_Box_Pkg.Vector );

Mail_Center : Mail_Wall.Vector;

As you can see, none of those are using (exposed) access types; which is generally a good thing.

For using vectors w/ access values, you could use something like this:

Type Cell(<>);
Type Link is Not Null Access Cell;
Type Data is Array(Positive Range <>) of Link;

Type Cell( References: Natural ) is
  case References is
    When 0 => Null;
    -- You might even be able to change Links to a Vector_Package.Vector
    When others => Links : Data(1..References);
  end case;
end record;

Package Vector_Package is new Ada.Containers.Vectors(Positive, Link);

Petri_Dish : Vector_Package.Vector;