Re: Private constant of tagged limited controlled type

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org> writes:

> On 10/21/2011 11:14 PM, Malaise wrote:
>> The following complains that "initialization of limited object
>> requires aggregate or function call":
>
> Right.
>
>> with Ada.Finalization;
>> package Pack is
>>    type T is tagged limited private;
>>    C : constant T;
>> private
>>    type T is new Ada.Finalization.Limited_Controlled with record
>>      I : Integer := 0;
>>    end record;
>>    V : T;
>>    C : constant T := V;

That's illegal, because you're trying to copy a limited object.
What if T had a self-referential component?
If the above were allowed, C.Self would incorrectly point
to V, not C.  What if T contained a protected object?
It makes no sense to copy locks.

>> end Pack;
>> Aggregate seems impossible because Limited_Controlled is abstract and
>> anyway T is very complex in fact.
>
> I suggest you read ARM 4.3.1 and 4.3.2. You can write an aggregate of the form
>
> (Ada.Finalization.Limited_Controlled with others => <>)

Right, or:

    C : constant T := (Ada.Finalization.Limited_Controlled with I => 123)

>> A function (returning tagged type) has to be visible and anyway cannot
>> return V (in Ada 2005).
>
> This is an unusual claim. The extended return statement was introduced
> specifically for returning limited types from a function. You ought to
> read ARM 6.5. You could write
>
> return X : T;

Right.  Or it could say

    return (Ada.Finalization.Limited_Controlled with others => <>);

But the comment, "cannot return V" is correct -- no matter whether
V is local or global.

> to return a value of T with all components set to their default values.

I also suggest reading this:

http://www.adacore.com/2007/09/24/ada-gem-10/

and also the following 2 gems, which I wrote to explain
how limited types work in Ada 2005.

- Bob