Version 1.1 of ai05s/ai05-0001-1.txt

From: Martin Dowie
Date: Sunday, Octoher 2, 2005  1:12 PM

I didn't realize that changes were still being proposed/allowed at this
stage until Randy sent in the "extends" email. :-)

I have a suggestion that I hope will be fairly simple and hopefully
useful...

I think that we could extend the Ada.Containers.* hierarchy to include
"Bounded_" variants of each container type by defining that each bounded
package has exactly the same specification as the unbounded version,
except that the container type has a "(Max_Length : Count_Type)" constraint.

Insert/Append routines would raise a Constraint_Error if "Length
(Container) = Max_Length" (or, a "Length_Error : exception;" could be
added to Ada.Containers).

The only 'tricky' bit is what to with:

1) procedure Set_Length
Either check and raise exception or limit to Max_Length even if "Length
 > Container.Max_Length"?

2) each function "&" =>
   a) if Left or Right is the only container then simply check for space;
   b) if Left and Right are elements then return a Max_Length => 2
container;
   c) if Left and Right are containers then append as much of Right to a
new container with Max_Length of Left.Max_Length.

Alternatively for "&" they could return a new container of
Left.Max_Length + Right.Max_Length but that seems against the nature of
bounded things and not so much in keeping with bounded strings. I'm not
too fussed about what the 'actual' rules are/would be, I'm just pointing
out that these are the only things that would need any discussion.

Anyway, I hope I've shown that it would be /simple/ to double the size
of the Ada.Container.* hierarchy with very little effort (and by effort,
I mean 'text'! :-), in much the same way as "Wide_" packages are added
to the RM.

I have a sample "Ada.Containers.Bounded_Vectors" if anyone would like to
see such a beast.

p.s. My $0.02 would choose "check&raise" for what to do with
Set_Length/"&" subprograms, by the way ;-)

****************************************************************

From: Bob Duff
Date: Sunday, October 2, 2005  8:33 PM

> I didn't realize that changes were still being proposed/allowed at this
> stage until Randy sent in the "extends" email. :-)

I think Randy's suggestion was for Ada 201X, not Ada 2005.  ;-)

> I have a suggestion that I hope will be fairly simple and hopefully
> useful...
>
> I think that we could extend the Ada.Containers.* hierarchy to include
> "Bounded_" variants of each container type by defining that each bounded
> package has exactly the same specification as the unbounded version,
> except that the container type has a "(Max_Length : Count_Type)" constraint.

Hmm... If I say:

    X: Bounded_Whatever(Max_Length => 80);
    ...
    X := Y & Z;

where Length(Y) + Length(Z) <= 80, I would like it to work.
I don't know how to do that in Ada.

****************************************************************

From: Jeffrey Carter
Date: Sunday, October 2, 2005  10:04 PM

> I think that we could extend the Ada.Containers.* hierarchy to include
> "Bounded_" variants of each container type by defining that each bounded
> package has exactly the same specification as the unbounded version,
> except that the container type has a "(Max_Length : Count_Type)"
> constraint.

This doesn't work very well, because user-defined assignment doesn't handle
this kind of thing. That's why bounded strings have the limit as a generic
formal constant, not a discriminant on Bounded_String.

Somehow, I doubt anything is being considered for Ada 0X anymore. Randy said
his suggestion was for Ada 1X.

****************************************************************

From: Matthew Heaney
Date: Monday, October 3, 2005  9:05 AM

> I think that we could extend the Ada.Containers.* hierarchy to include
> "Bounded_" variants of each container type by defining that each bounded
> package has exactly the same specification as the unbounded version,
> except that the container type has a "(Max_Length : Count_Type)"
> constraint.

The container type would have to be limited, because of assignment behavior.

What you refer to as Max_Length above is really just capacity.

> 1) procedure Set_Length
> Either check and raise exception or limit to Max_Length even
 > if "Length > Container.Max_Length"?

Just raise CE.

> 2) each function "&" =>
>   a) if Left or Right is the only container then simply check for space;
>   b) if Left and Right are elements then return a Max_Length => 2
> container;
>   c) if Left and Right are containers then append as much of Right to a
> new container with Max_Length of Left.Max_Length.

No.  Operator "&" is gone, because the type has to be limited.

****************************************************************

From: Martin Dowie
Date: Monday, October 3, 2005  10:17 AM

I could live with either "Max_Length" or "Capacity".

Why would "&" have to go? They are present in "Bounded_String"-s...

Can you give me an example of the problems with assignment behaviour?
Because just now I'm sitting in front of something that seems to work
fine with everything I try and throw at it.

****************************************************************

From: Matthew Heaney
Date: Monday, October 3, 2005  10:30 AM

> Why would "&" have to go? They are present in "Bounded_String"-s...

Because there's nothing you can to do with the result, if the type is
limited.

Actually, I should take that back.  Operator "&" could become a
constructor-type function (aka "initializer"), since you would only be
able to use it during the declaration of the bounded contained object
(because the type is limited).

> Can you give me an example of the problems with assignment behaviour?
> Because just now I'm sitting in front of something that seems to work
> fine with everything I try and throw at it.

Assuming the type were non-limited:

declare
    C1 : CT (42);
    C2 : CT (1963);
begin
    C2 := C1;  -- raises CE
end;

(But maybe there were some changes in semantics for Ada 2005, such that
the assignment no longer raises CE.)

****************************************************************

From: Matthew Heaney
Date: Monday, October 3, 2005  11:29 AM

> Hmm... If I say:
>
>     X: Bounded_Whatever(Max_Length => 80);
>     ...
>     X := Y & Z;
>
> where Length(Y) + Length(Z) <= 80, I would like it to work.
> I don't know how to do that in Ada.

Hmmm... Why not?  If the discriminant of the result of Y & Z is the same
as X, then the assignment will succeed.  And the sum of the lengths can
be calculated in the implementation of "&".

Another issue wrt assignment of bounded forms is the (in)efficiency,
since there will be a bitwise copy of both the active and inactive
("free store") elements.

Martin: the Charles library has bounded list forms (having a clever
implementation of the free store, IMHO).  You can play around with those
if you want to get some more data...

****************************************************************

From: Martin Dowie
Date: Monday, October 3, 2005  11:50 AM

Thanks Matt - I'll take a look - is there an easy 'zipped' version of
the head, or do I need to download each file individually? (I've never
really used CVS)

And while I can't do:
declare
   C1 : CT (42);
   C2 : CT (1963);
begin
   C2 := C1;  -- raises CE
end;

I can do:
declare
   C1 : CT (42);
   C2 : CT (1963);
begin
   C2 := C2 & C1;  -- works fine
end;

****************************************************************

From: Bob Duff
Date: Monday, October 3, 2005  12:26 PM

> Hmmm... Why not?  If the discriminant of the result of Y & Z is the same
> as X, then the assignment will succeed.  And the sum of the lengths can
> be calculated in the implementation of "&".

I was thinking of this:

    type Bounded_Sequence(Max_Length: Natural) is
        record
            Elements: Element_Array(1..Max_Length);
            Length: Natural := 0;
        end record;

    X: Bounded_Sequence(Max_Length => 80);
    Y: Bounded_Sequence(Max_Length => 40);
    ...

Now, if Y.Length = 20, I would like to be able to say "X := Y",
since the length of Y is shorter than the Max_Length of X.
But that doesn't work.

Same issue for "X := Y & Z" -- the "&" operation can calculate the right
length for the result, but it can't calculate the right Max_Length.

As you pointed out in another message, this is why Bounded_Strings is
generic on the max length.  That means all objects of a given type will
have the same max length, thus trivially ensuring that, as you said
above, the max length "of the result of Y & Z is the same as X".

> Another issue wrt assignment of bounded forms is the (in)efficiency,
> since there will be a bitwise copy of both the active and inactive
> ("free store") elements.

Good point.  Of course that doesn't matter if the thing is limited,
and if it's limited, the issues about assignment and "&" go away.
In Ada 2005, limited is one good way to go.

In any case, it's not _trivial_ to add these bounded versions,
nor to decide on their exact semantics.  So I think it's unlikely that
they will be included in Ada 2005.  Go for 2015!

****************************************************************

From: Martin Dowie
Date: Monday, October 3, 2005  11:26 PM

>I think Randy's suggestion was for Ada 201X, not Ada 2005.  ;-)
>
>
Apologies for being a bit early with my Ada1Z proposal then! :-)


>Hmm... If I say:
>
>    X: Bounded_Whatever(Max_Length => 80);
>    ...
>    X := Y & Z;
>
>where Length(Y) + Length(Z) <= 80, I would like it to work.
>I don't know how to do that in Ada.
>
Ok, I'm confused by this one as my implementation seems to do the right
thing for this!...

Anyway, I guess all this can wait for the ISO IWA.

-- bounded_integer_vectors.ads
with Ada.Containers.Bounded_Vectors;

package Bounded_Integer_Vectors is
   new Ada.Containers.Bounded_Vectors (Natural, Integer);

-- test_biv.adb
with Ada.Containers; use Ada.Containers;
with Ada.Exceptions; use Ada.Exceptions;
with Ada.Text_IO;    use Ada.Text_IO;

with Bounded_Integer_Vectors; use Bounded_Integer_Vectors;

procedure Test_BIV is
   subtype Vector_3 is Vector (3);
   V1, V2, V3 : Vector_3;
begin
   Put_Line ("Length =>" & Count_Type'Image (V1.Length));
   V1.Append (1);
   Put_Line ("Length =>" & Count_Type'Image (V1.Length));
   V1.Append (3);
   Put_Line ("Length =>" & Count_Type'Image (V1.Length));
   V1.Append (6);
   Put_Line ("Length =>" & Count_Type'Image (V1.Length));
   begin
      V1.Append (10);
   exception
      when Error : others =>
         Put_Line ("!!! " & Exception_Message (Error));
   end;
   Put_Line ("Length =>" & Count_Type'Image (V1.Length));
   Put_Line ("Length =>" & Count_Type'Image (V3.Length));
   V3 := V1 & V2;
   Put_Line ("Length =>" & Count_Type'Image (V3.Length));
end Test_BIV;

-- result.txt
C:\Ada\Bounded_Containers\gps\obj\test_biv.exe
Length => 0
Length => 1
Length => 2
Length => 3
!!! Insert error, already full.
Length => 3
Length => 0
Length => 3

****************************************************************

From: Bob Duff
Date: Tuesday, October 4, 2005  12:01 PM

> Ok, I'm confused by this one as my implementation seems to do the right
> thing for this!...

>    subtype Vector_3 is Vector (3);
>    V1, V2, V3 : Vector_3;

Try it like this:

    V1, V2: Vector_3;
    V3: Vector(6);

6 is plenty big enough to store 3 characters, but the assignment
will raise Constraint_Error.

****************************************************************

From: Martin Dowie
Date: Tuesday, October 4, 2005  12:32 PM

Well, that depends on what I make "&" mean! :-)

   declare
      C1, C2 : Bounded_Integer_Vectors.Vector (3);
      C3     : Bounded_Integer_Vectors.Vector (6);
   begin
      C1.Append (1);
      C1.Append (3);
      C1.Append (7);
      C2.Append (11);
      C2.Append (13);
      C2.Append (17);
      C3 := C1 & C2;
      Put_Line ("+++ No exception raised");
      Put_Line ("Length =>" & Count_Type'Image (C3.Length));
   exception
      when Error : others =>
         Put_Line ("!!! " & Exception_Message (Error));
   end;

-- results.txt
+++ No exception raised
Length => 6

****************************************************************

From: Jeffrey Carter
Date: Tuesday, October 4, 2005  1:00 PM

>   Well, that depends on what I make "&" mean! :-)

Sure, but the point is that there's no way to make

A := B & C;

work for any A, B, & C such that

Length (B) + Length (C) <= A.Max_Length.

Indeed, if you can make it work for your example and for

A : Vector ( 7);
B : Vector (10);
C : Vector (23);

Length (B) = 3
Length (C) = 3

I'll be impressed.

I suspect this discussion should be moved from Ada-Comment.

****************************************************************

From: Bibb Latting
Sent: Thursday, February 9, 2006  1:31 PM

!topic Control over storage pool underlying Ada.Containers packages
!reference Ada 2005 RMA.18(all)
!from Bibb Latting 05-02-09
!keywords Containers, Storage Pools
!discussion
 
The capability to use a derivation of root_storage_pool with a container
is desired to provide flexibility in the management of the storage
underlying the container.  This capability provides the opportunity for
the user to optimize memory allocation for a given application, work-around
fragmentation in the hardware mapping of memory, provide non-core memory
allocation of the container, or to comply with safety/security issues.
 
Please feel free to change the above discussion as desired.
 
****************************************************************

From: Tucker Taft
Sent: Thursday, February 9, 2006  1:58 PM

The general consensus on this suggestion was that it
is good, but would involve significant analysis to
get it right at this point.  A recommendation was to
leave it out of the standard, but encourage implementations
to support the capability.  In general the Containers
packages were designed to be a starting point, not
the final story.  Ordered_ and Hashed_ might easily be
augmented with numerous other capabilities, including
"Bounded_...", "Protected_...", "Pooled_...", etc.
So long as all the packages have nearly identical
contents, it isn't too bad to have implementation-specific
code at the point of the instantiation.  The large
bulk of the user's code will depend only on the package
contents, not its instantiation parameters, and reasonable
portability can be preserved.

****************************************************************

From: Randy Brukardt
Sent: Thursday, February 9, 2006  3:22 PM

The deadline for informal comments on Ada 2005 was last Friday. At this
point, only editorial comments will be accepted. There may be an opportunity
for formal comments at some point later in the future (although the primary
effect of such comments would be to endanger the future of the Amendment and
potentially of Ada itself).

So, at the point we're primarily talking about future work (beyond Ada
2005).

In any case, there are a number of reasons that user-defined storage pools
are inappropriate for the unbounded containers that Ada 2005 has:

1) There is no good way to name the default storage pool. Thus, there is no
good way to specify the default case of using the default storage pool. We
definitely do not want to make instantiating these containers harder.

Note that we tried early in the Ada 2005 process (see AI-300) to find a way
to make the default pool available. However, it ran into opposition from
some ARG members, who noted that some implementations used multiple default
storage pools, tailored to the characteristics of the type. That model
doesn't make it easy to name the default pool. The issue died because no
solution that worked for such a model could be found.

2) Although it's counter-intuitive, allowing user-defined storage pools
makes the containers less safe. The containers are almost certainly going to
be implemented as controlled types. If the storage pool has some method
other than Deallocate for freeing memory (before the pool itself ceases to
exist) -- and many user-defined pools do -- that could free memory belonging
to container objects and subparts. When those objects are later finalized,
the world will end, as the finalization information could very well have
been destroyed. (For instance, Janus/Ada chains together controlled objects;
if the memory is prematurely freed, those chains can be destroyed, and
almost anything could happen, none of it good.)

This problem cannot happen with the standard storage pool.

One could try to mitigate this problem by trying to define when it would be
safe to do premature deallocation, but such wording would also have the
effect of banning useful implementation techniques (such as caching of
nodes).

We'd also have to add wording to take into account this possibility. That
would make the containers *appear* less safe than they currently are, by
adding additional cases of erroneous behavior. Some reviewers were extremely
concerned about erroneous behavior of the containers, and we spent a lot of
effort eliminating as much such behavior as possible.

Aside: We'd also need wording to allow any operation to call Allocate, and
to require the propagation of any exceptions it generates. We don't need
special wording for the standard storage pool, because all it can do is
propagate Storage_Error, and *any* operation in Ada is *always* allowed to
do that. So no extra wording is needed.

3) The unbounded forms can (and often have to) allocate pretty much any size
or shape of memory as part of pretty much any operation. For instance, a
vector container (depending on it's implementation) could allocate
individual elements, small "chunks" of elements, or the entire data array of
elements at one time. Putting limits on that would be inappropriate for the
unbounded containers - we want to encourage innovative implementations.
Thus, any pool used with the unbounded containers would need to allow
arbitrary-sized allocations.

The effect of (2) and (3) is that many useful storage pools couldn't be used
in the context of the unbounded containers. A pool that is used to manage
memory would be inappropriate; it could only manage memory when no container
instantiation exists, and at that point no memory is allocated anyway. A
pool that does not allow any size allocation also cannot be used. For
instance, a pool that only allows specific sizes of allocations to eliminate
fragmentation could not be used in the context of the unbounded containers.

That doesn't leave many useful pools that could be used with the unbounded
containers. A pool that simply records the memory use could be used, and an
alternative heap could be used, but that's about it. That didn't seem like
enough to be worth the additional complication.

OTOH, it makes perfect sense for other forms of container to provide this
sort of functionality. The intent (as Tucker pointed out) is that the
containers in Ada 2005 provide a starting point for Ada containers, not be
the end point. We certainly hope that additional forms of containers are
developed for future use. (Hopefully, that would contain a solution to (1),
as well.)

One of the things that we'll be doing in future months is considering where
we need future work. Certainly, there has been a lot of interest in other
containers and other forms of containers (such as Bounded), so that will be
considered. And, as these are separate packages, there is no need to wait
another 10 years to standardize them. Moreover, these packages are
relatively easy for implementers to provide, so I would expect that most
implementers will do so. But we'll need energetic people (like Matt Heaney
was for the current set of containers) to participate and draft all of the
tedious wording needed. (A thick skin, to survive the constant nit-picking
at their hard work, also is a prerequisite.)

****************************************************************

From: Robert A. Duff
Sent: Thursday, February 9, 2006  4:54 PM

>...The containers are almost certainly going to
> be implemented as controlled types. If the storage pool has some method
> other than Deallocate for freeing memory (before the pool itself ceases to
> exist) -- and many user-defined pools do -- that could free memory belonging
> to container objects and subparts.

The container type has finalization.  The heap-allocated memory blocks do not
(unless the actual element type passed in does).  So it would be OK
to use a user-def pool, so long as the element type does not have
controlled parts.

Anyway, I agree with what Tuck and Randy said: allowing user-def pools is a
good idea, but for the future (after Ada 2005).

****************************************************************

From: Randy Brukardt
Sent: Thursday, February 9, 2006  5:10 PM

Huh? If the heap-allocated blocks are linked together (certainly will be
true in the list container, probably in most of the others too), when the
finalization routine walks the blocks whose memory has been reused to free
them, the results won't be pretty.

We certainly don't want to be mandating implementations that don't include
any links (that would be a major change for the unbounded containers, and
would make it very hard to implement the intended semantics). So I don't see
how one could expect such a pool to work at any time than when no container
objects exist. And even that prevents node caching.

****************************************************************

From: Robert A. Duff
Sent: Thursday, February 9, 2006  5:53 PM

Yes, you're right.  I was confused.

****************************************************************

From: Bibb Latting

...
> 1) There is no good way to name the default storage pool. Thus, there is
> no good way to specify the default case of using the default storage pool.

It seems to me that the use of multiple default pools implies an
"overloading" of a "root" default storage pool.  How does this
implementation support a user-defined storage pool?  Are all collections
assigned to the pool allocated from the same user-pool or are there
implementation restrictions that require the user to support an "overloaded"
set of user-pools?

From this perspective, an implementation would be free to do what it
currently does with the default pool, while the use of a user-defined pool
would operate in the same manner and with the same restrictions as the
current implementation.  The implementation always has the option of
exposing the specialized allocation mechanism in some way for their
customer's use.

> 2) Although it's counter-intuitive, allowing user-defined storage pools
> makes the containers less safe.

This problem exists for any controlled type allocated within a user-defined
pool; the coupling between user-defined pools and controlled types is really
what needs to be addressed.  It would seem that the use of a user-defined
pool with any controlled type would be unsafe.

> 3) The unbounded forms can (and often have to) allocate pretty much any
size
> or shape of memory as part of pretty much any operation. For instance, a
> vector container (depending on it's implementation) could allocate
> individual elements, small "chunks" of elements, or the entire data array
of
> elements at one time. Putting limits on that would be inappropriate for
> the
> unbounded containers - we want to encourage innovative implementations.
> Thus, any pool used with the unbounded containers would need to allow
> arbitrary-sized allocations.

Agreed, although the user-defined pool may be attempting to correct
underlying memory issues rather than addressing performance.  Of course this
could be overcome by customizing the implementation of the root pool, if an
interface has been provided by the implementation.

****************************************************************

From: Randy Brukardt

...
> > 1) There is no good way to name the default storage pool. Thus, there is
> > no good way to specify the default case of using the default
> storage pool.
>
> It seems to me that the use of multiple default pools implies an
> "overloading" of a "root" default storage pool.  How does this
> implementation support a user-defined storage pool?  Are all collections
> assigned to the pool allocated from the same user-pool or are there
> implementation restrictions that require the user to support an
> "overloaded" set of user-pools?

I believe that the implementation uses nested default pools to better handle
memory management. Anyway, I don't understand their implementation very
well, so I can't comment too intelligently.

> From this perspective, an implementation would be free to do what it
> currently does with the default pool, while the use of a user-defined pool
> would operate in the same manner and with the same restrictions as the
> current implementation.  The implementation always has the option of
> exposing the specialized allocation mechanism in some way for their
> customer's use.

Well, the problem is how to represent that default pool in a generic
specification. Surely, the "root" containers have to work with the default
pool; we certainly don't want to require every user to define their own
storage pool! For instance, if we had a container that looked like:

    generic
        type Element_Type is private;
        Pool : in out Root_Storage_Pool'Class;
    package Pool_Unbounded_List is

That would work fine for a user-defined pool, but what would the
instantiation be for the default pool? There is no name for such a thing.
You could use the awful kludge:

    type Junk_Ptr is access all Integer;
    package My_List is new Pool_Unbounded_List (Integer, Junk_Ptr'Storage_Pool);

but the default pool here might not support the allocation of the larger
blocks needed by the list.

> > 2) Although it's counter-intuitive, allowing user-defined storage pools
> > makes the containers less safe.
>
> This problem exists for any controlled type allocated within a user-defined
> pool; the coupling between user-defined pools and controlled types is really
> what needs to be addressed.  It would seem that the use of a user-defined
> pool with any controlled type would be unsafe.

Correct, but the problem is worse with the containers, because the whole
point of the containers is to hide the storage allocation behavior. Thus,
there is no clear point where it is safe to do deallocations via the pool,
and it is less obvious that such deallocations are dangerous.

In any case, this whole area needs work (garbage collection appears to not
be allowed if any controlled objects are in the memory), so it seems that
this will be cleaned up. But not this go-round. (We've only discussed this
problem at dinner; it's never been on the agenda.)

> > 3) The unbounded forms can (and often have to) allocate pretty much any size
> > or shape of memory as part of pretty much any operation. For instance, a
> > vector container (depending on it's implementation) could allocate
> > individual elements, small "chunks" of elements, or the entire data array of
> > elements at one time. Putting limits on that would be inappropriate for the
> > unbounded containers - we want to encourage innovative implementations.
> > Thus, any pool used with the unbounded containers would need to allow
> > arbitrary-sized allocations.
>
> Agreed, although the user-defined pool may be attempting to correct
> underlying memory issues rather than addressing performance.  Of course this
> could be overcome by customizing the implementation of the root pool, if an
> interface has been provided by the implementation.

I agree that there are *some* uses of such pools; there just aren't that
many. The common use of such pools (tracking down storage leaks) shouldn't
be necessary with the predefined containers -- vendors will fix such
problems quickly if they ever existed. In any case, users shouldn't need to
debug the containers implementations.

I have a pool that bypasses all of the heap stuff to directly allocate
memory using the Windows memory allocation functions. It was designed to
allocate arrays that could be expanded in place rather than having to copy
them as more memory is needed. Such a pool would be useful for vectors in
some cases: but it isn't general enough to use for the arbitrary-sized
allocations of the containers library. I don't know how many pools are that
general (unless they fall back to the default pool somehow).

Anyway, it's not that allowing user-defined pools is bad idea, but rather
that such use would work better either with a container tailored for that
use and/or with bounded forms. I think we'll get there, but not immediately.

****************************************************************

From: Bibb Latting

...
>    generic
>        type Element_Type is private;
>        Pool : in out Root_Storage_Pool'Class;
>    package Pool_Unbounded_List is
>
> That would work fine for a user-defined pool, but what would the
> instantiation be for the default pool? There is no name for such a thing.
> You could use the awful kludge:
>
>    type Junk_Ptr is access all Integer;
>    package My_List is new Pool_Unbounded_List (Integer, Junk_Ptr'Storage_Pool);
>

Indeed, we can cajole the address of the default pool out of the system by
using a variety of kludges.  I'm compelled to confess that I've used several
myself.  In AI-300, there are two aspects of the problem that seem
interesting: 1) a way to name a default pool (not necessarily the root pool;
note the special implementation), and 2) a syntax change to support some
sort of auto-switching in a generic.  I don't think the second is worth the
complexity introduced; the first represents a philosophical problem for the
language.  It seems that supporting some level of visibility for the root
pool would be worthwhile.  However, this exposes the root pool to easier
manipulation (corruption) by a programmer.

I think there are language features in place (Ravenscar is built on the features)
that provide the basis for a mechanism to control what language features are
allowed for a given application.  So, in terms of a default storage pool, I guess
my starting position advocates the presentation of a method to obtain a reference
to a default pool.  What each implementation does with the default reference is
up to the implementation; any derivations would work the same way as user-pools
currently do for an implementation.

****************************************************************

From: Randy Brukardt
Sent: Tuesday, March 14, 2006  4:30 PM

I'm posting this question here because I don't want to post someone's
contact information on comp.lang.ada (that's a certain way to increase the
amount of spam you get), and it's at least tangentially related to the
Amendment. Please cc Dr. Barry if you respond, since I don't believe that he
is joined here.


---------

From: Dr Alwyn Barry

Hi,

Could you forward this question to the appropriate person?

I have been using Ada 2005 to rewrite some Python code.  In translating
the List processing using the new Doubly_Linked_Lists container class
I constantly have to use:

C : Cursor := First(L);
...
while Has_Element(C) loop
   ... do something using Element(C) or Update_Element(..)
   Next(C);
end loop;

Given the inherent insecurity with First()...Next(), is there a better
iterator, such as the 'natural' extension of the For Loop construct
to iterators ... eg:

for C in L loop
   ... do something using Element (C) or Update_Element(..)
end loop;

Also, the GNAT version binds a cursor to the created list, which
means that when the Move() procedure is called, cursors on the moved
list cannot be used!  Since move is a _genuine_ move, this is very
limiting.  It is impossible to overcome this limitation because the
Node_Type is hidden and so there is no other way to reference into
a list.

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From: Randy Brukardt
Sent: Wednesday, March 15, 2006  10:42 PM

(I'd hoped someone else would answer this, but since no one has, I'll take a
stab at it...)

A.M. Barry writes:

> I have been using Ada 2005 to rewrite some Python code.  In translating
> the List processing using the new Doubly_Linked_Lists container class
> I constantly have to use:
>
> C : Cursor := First(L);
> ...
> while Has_Element(C) loop
>    ... do something using Element(C) or Update_Element(..)
>    Next(C);
> end loop;
>
> Given the inherent insecurity with First()...Next(), is there a better
> iterator, such as the 'natural' extension of the For Loop construct
> to iterators ... eg:
>
> for C in L loop
>    ... do something using Element (C) or Update_Element(..)
> end loop;

We talked about this "natural" extension a little bit, and it's pretty clear
that it's not obvious what is natural. See the e-mail in AC-112:
http://www.ada-auth.org/cgi-bin/cvsweb.cgi/AIs/AC-00112.TXT (warning, this
link is going to change in the next couple of days to:
http://www.ada-auth.org/cgi-bin/cvsweb.cgi/ACs/AC-00112.TXT - the site is
being reorganized to make room for Ada 2005 AIs and ASIS AIs.)

However, there are the Iterate and Reverse_Iterate procedures in the
containers packages. Is there a reason that you can't use them?? They're not
quite as convinient as the loop syntax given above, but they're just as
safe.

> Also, the GNAT version binds a cursor to the created list, which
> means that when the Move() procedure is called, cursors on the moved
> list cannot be used!  Since move is a _genuine_ move, this is very
> limiting.  It is impossible to overcome this limitation because the
> Node_Type is hidden and so there is no other way to reference into
> a list.

Cursors include the container; that's a fundamental property of them in this
model. That's because you can do operations on them without giving the
container. Also, cursors are not pointers per se, although they may be
implemented that way. We wanted to allow alternative implementations that
don't necessarily use explicit pointers.

In any case, Move destroys all of the cursors; the language defines them as
"invalid", and any future use is erroneous. (If the error was detected, you
are lucky.) In this sense, Move is very similar to destroying the container
itself (say, by calling Unchecked_Deallocation on it).

But Move isn't intended for cases where you need to preserve cursors. The
four parameter Splice is designed for that, as it returns the new cursor for
where the element(s) are inserted. If you want to preserve the original
container, just use ":=".

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