From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Proposal: Auto-allocation of Indefinite Objects
Date: Tue, 28 Jul 2020 16:59:09 +0200 [thread overview]
Message-ID: <rfpefq$17fm$1@gioia.aioe.org> (raw)
In-Reply-To: rfpcn4$2n6$1@dont-email.me
On 28/07/2020 16:28, Brian Drummond wrote:
> On Mon, 27 Jul 2020 22:02:57 +0200, Dmitry A. Kazakov wrote:
>
>> On 27/07/2020 19:48, Brian Drummond wrote:
>>
>>> 2) can be implemented internally using pointers, but externally appears
>>> to be a data object, just like Unbounded_String does, with similar
>>> semantics.
>>
>> No, the point is that Unbounded_String is exactly opposite to what is
>> required. In no case it should appear as an object of a different type!
>>
>> Compare access to string P with unbounded string U:
>>
>> for I in P'Range loop -- This is OK
>> P(J) := 'a' -- This is OK
>>
>> Now would you do:
>>
>> To_String (U) (J) := 'a' -- Garbage!
>
> That wasn't the aspect of Unbounded I was getting at. I agree ... garbage.
>
> What I meant was that Unbounded doesn't load New, dereferencing,
> deallocation etc onto the programmer, but hides the access details, and
> our indefinite type should do the same (the compiler can probably to a
> better job than the programmer anyway).
>
> I'm suggesting something more like the C++ reference, signalling (perhaps
> by adding a reserved word "indefinite") that fixed size allocation won't
> work;
Equivalent of C++ reference in Ada is renaming.
and implementation is more in line with a controlled type but with
> system-provided Initialise,Adjust,Finalize providing the required
> operations (no need for the programmer to provide them).
>
> A : String := "hello" -- a definite string
> P : access String := new String'("hello");
> Q : indefinite String := "hello";
I think the keyword is misleading. Maybe this:
Q : new String := "hello";
And I don't like initialization. It was a mistake to have limited
return. The syntax must stress that all initialization is strictly
in-place. No copies involved because the pool is fixed.
> ...
> begin
> for I in P'Range loop -- This is OK
> P(J) := 'a'; -- This is OK
> Q(J) := 'a'; -- also OK. But index out of range would raise
> Constraint Error
> ...
> Q := "hello_world"; -- deallocates, allocates with new bounds
> ...
> end; -- deallocate Q here.
The rule could be "same pool" as of the container. In the case of a
block, the pool is the stack. In the case of a record member, the pool
is the pool of where the record itself is allocated. So that you could
allocate all object in the same pool.
> It follows that "indefinite" cannot also be "aliased" unless we want to
> implement smart pointers. For simplicity I'd suggest disallowing "aliased
> indefinite" on the grounds that "access" can (should) be used instead.
It makes sense, but there are use cases for having it aliased:
X : indefinite T;
Y : indefinite S (X'Access); -- Access discriminant
> Records (including tagged, class wide, discriminated) should work the
> same, but probably with shallow copy on assignment if they contain access
> types.
>
> If there is no re-allocation (no different size assignment) the compiler
> is free to substitute direct (stack) storage instead of heap allocation
> and implicit access types. So for example instead of
>
> A constant String := "done";
> ...
> loop
> declare
> P : String := Get_Line;
> begin
> exit when P = A;
> end;
> end loop;
>
> A constant String := "done";
> Q : indefinite String;
> ...
> loop
> Q := Get_Line;
> exit when Q = A;
> end loop;
I am not comfortable with the semantics of this and with possible
implications too. I would keep it simple.
> the implementation can be either an implicit declare block or an implicit
> access type. However, where Q has several reassignments within a block,
> and the compiler can't determine the size, an implicit access type must
> be used. (If it can, it can warn that "indefinite " is unnecessary).
>
>> What if the original object must be a class-wide object, task, protected
>> object, limited object etc?
>>
>> Ada's access types delegate all operations to the target object, except
>> assignment. This is the key property that the proposal in my view must
>> retain.
>
> Indefinite can also be applied to records (discriminated, class wide, etc)
> here the size is indeterminate and may vary on reassignment. Assignment
> would always be shallow copy (where the record contained access types).
That would be inconsistent. IMO, it should be a deep copy, provided such
a component would not make the type limited, of which I am not sure.
--
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de
next prev parent reply other threads:[~2020-07-28 14:59 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-03 22:48 Proposal: Auto-allocation of Indefinite Objects Stephen Davies
2020-04-03 23:45 ` Stephen Leake
2020-04-04 10:54 ` Jeffrey R. Carter
2020-04-04 20:55 ` Stephen Davies
2020-04-04 8:31 ` Dmitry A. Kazakov
2020-07-27 7:47 ` Yannick Moy
2020-07-27 9:21 ` J-P. Rosen
2020-07-27 9:49 ` Dmitry A. Kazakov
2020-07-27 17:48 ` Brian Drummond
2020-07-27 20:02 ` Dmitry A. Kazakov
2020-07-28 14:28 ` Brian Drummond
2020-07-28 14:59 ` Dmitry A. Kazakov [this message]
2020-07-29 15:33 ` Brian Drummond
2020-07-29 16:20 ` Dmitry A. Kazakov
2020-07-30 13:37 ` Stephen Davies
2020-07-30 14:23 ` Dmitry A. Kazakov
2020-07-30 17:04 ` Brian Drummond
2020-07-30 18:28 ` Dmitry A. Kazakov
2020-08-10 0:39 ` Randy Brukardt
2020-08-10 8:57 ` Dmitry A. Kazakov
2020-08-20 0:10 ` Randy Brukardt
2020-08-20 17:49 ` Dmitry A. Kazakov
2020-08-20 20:19 ` Dennis Lee Bieber
2020-08-20 23:33 ` Randy Brukardt
2020-08-21 6:45 ` Dmitry A. Kazakov
2020-08-23 4:52 ` Randy Brukardt
2020-08-23 12:28 ` Dmitry A. Kazakov
2020-08-20 23:30 ` Randy Brukardt
2020-08-21 6:46 ` Dmitry A. Kazakov
2020-08-23 4:48 ` Randy Brukardt
2020-08-23 12:29 ` Dmitry A. Kazakov
2020-08-10 0:31 ` Randy Brukardt
2020-08-10 8:58 ` Dmitry A. Kazakov
2020-08-20 0:13 ` Randy Brukardt
2020-08-20 17:49 ` Dmitry A. Kazakov
2020-08-20 23:25 ` Randy Brukardt
2020-08-21 7:08 ` Dmitry A. Kazakov
2020-08-23 5:03 ` Randy Brukardt
2020-08-23 12:28 ` Dmitry A. Kazakov
2020-07-27 20:31 ` Jeffrey R. Carter
2020-07-31 9:25 ` Stephen Davies
2020-07-31 10:20 ` Dmitry A. Kazakov
2020-08-01 11:22 ` Stephen Davies
2020-08-01 12:58 ` Dmitry A. Kazakov
2020-08-01 20:35 ` Stephen Davies
2020-08-01 20:56 ` Dmitry A. Kazakov
2020-09-03 4:30 ` linda white
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