Arrays with discriminated task components

[AdaMagica]

I've got a task type with a discriminant:

type Index is range 1 .. N;

task type T (D: Index);

Now I want an array of these tasks, where each task knows its identity (the index) via the discriminant, an iterated_component_association:

Arr: array (Index) of T := (for I in Index => ???);

How can I do this?

This works with access, but I find this extremely ugly:

Arr: array (Index) of access T := (for I in Index => new T (I));

Alternatively, I could use the traditional method with a Start entry with the index as parameter:

task type T is
entry Start (D: Index);
end T;

[Niklas Holsti]

On 2022-12-24 13:44, AdaMagica wrote:
> I've got a task type with a discriminant:
>
> type Index is range 1 .. N;
>
> task type T (D: Index);
>
> Now I want an array of these tasks, where each task knows its
> identity (the index) via the discriminant, an
> iterated_component_association: >
> Arr: array (Index) of T := (for I in Index => ???);
>
> How can I do this?

One way is to give the discrimant a default value that is a function
call that returns a new identifier on each call:

Next_Index : Index := Index'First;
-- The value returned by the next call of New_Index.

function New_Index return Index
-- Returns a unique Index value (up to N).
is
Result : constant Index := Next_Index;
begin
if Next_Index < Index'Last then
Next_Index := Next_Index + 1;
-- else report error?
end if;
return Result;
end New_Index;

task type T (D: Index := New_Index);

Then you can declare the array without any initial value:

Arr: array (Index) of T;

and the initialization of each task in the array makes its own call to
New_Index and gets its own identifier value.

A bit sneaky, but has the advantage that it extends automatically to
two arrays of tasks, or one array and some separate single declarations
of tasks, etc.

[Jeffrey R.Carter]

On 2022-12-24 19:05, Niklas Holsti wrote:
> On 2022-12-24 13:44, AdaMagica wrote:
>>
>> Now I want an array of these tasks, where each task knows its identity (the
>> index) via the discriminant, an
>> iterated_component_association: >
>> Arr: array (Index) of T := (for I in Index => ???);
>

> One way is to give the discrimant a default value that is a function call that
> returns a new identifier on each call:

No, this does not guarantee that the task's discriminant is its index in the
array, which is a requirement of the question.

--
Jeff Carter
"My mind is aglow with whirling, transient nodes of
thought, careening through a cosmic vapor of invention."
Blazing Saddles
85

[Niklas Holsti]

On 2022-12-25 0:41, Jeffrey R.Carter wrote:
> On 2022-12-24 19:05, Niklas Holsti wrote:
>> On 2022-12-24 13:44, AdaMagica wrote:
>>>
>>> Now I want an array of these tasks, where each task knows its
>>> identity (the index) via the discriminant, an
>>> iterated_component_association: >
>>> Arr: array (Index) of T := (for I in Index => ???);
>>
>> One way is to give the discrimant a default value that is a function
>> call that returns a new identifier on each call:
>
> No, this does not guarantee that the task's discriminant is its index in
> the array, which is a requirement of the question.

Right, I did not take that requirement into account, apologies for my
inattention. Indeed the tasks in the array are initialized in an
arbitrary order, not necessarily in increasing index order.

[Niklas Holsti]

On 2022-12-24 13:44, AdaMagica wrote:

This seems to work with gnat, but I'm not entirely sure if it is legal
(could there be a conflict between the default value of the task
discriminant, which is the same for all tasks in the array, and the
actual discriminants which are different for each task in the array?):

N : constant := 10;

type Index is range 1 .. N;

task type T (D: Index := Index'First);
-- A default value for D is needed to make the type constrained, as
-- required by the Arr declaration below.

function New_T (I : in Index)
return T
is
begin
return TI : T (D => I)
do
null;
end return;
end New_T;

Arr: array (Index) of T := (for I in Index => New_T(I));

Whether this is any less ugly than the heap allocation method is doubtful.

[AdaMagica]

Thanx, Niklas and Jeffrey. I just didn't think of the generator function.
Christoph