Pointer to instance of indefinite array?

Pointer to instance of indefinite array?

[NiGHTS]

Suppose I have a package like this...


package Test_Package is

    type Element is 
    record
        ID_1 : Positive;
        ID_2 : Positive;
    end record;

    type Element_Array     is array (Positive range <>) of Element;
    type Element_Array_Ptr is access Element_Array;

    procedure Do_Something_With_Array (
        In_Array : in Element_Array_Ptr
    );

end Test_Package;


In this situation I have an array of "Element" records with an unknown number of elements. The number of elements are to be defined by the user of this package and passed as a pointer to the procedure "Do_Something_With_Array".

So a simple implementation would be something like this...


with Test_Package; use Test_Package;

procedure main is 
    Test_Array : aliased Element_Array (1 .. 20);
begin
    -- Initialize the array, then call this function...
    Do_Something_With_Array (In_Array => Test_Array'Access);
end main;


So this does not compile and with countless errors no matter what configuration I try. I have a feeling my approach is fundamentally wrong. Essentially I am trying to pass a constrained array to a package accepting an array of any length, both of the same element type. I would parse the contents of this array using "for in" or 'First -> 'Last or 'Range. 

What is the best way to achieve this goal without using new / free?

Re: Pointer to instance of indefinite array?

[Jeffrey Carter]

On 08/12/2014 07:07 PM, NiGHTS wrote:
>
> In this situation I have an array of "Element" records with an unknown number
> of elements. The number of elements are to be defined by the user of this
> package and passed as a pointer to the procedure "Do_Something_With_Array".

Why are you using an access type? There is no need for it.

> What is the best way to achieve this goal without using new / free?

Think in Ada and get rid of the access type.

-- 
Jeff Carter
"That was the most fun I've ever had without laughing."
Annie Hall
43

Re: Pointer to instance of indefinite array?

[Shark8]

On 12-Aug-14 20:07, NiGHTS wrote:
> package Test_Package is
>
>      type Element is
>      record
>          ID_1 : Positive;
>          ID_2 : Positive;
>      end record;
>
>      type Element_Array     is array (Positive range <>) of Element;
>      type Element_Array_Ptr is access Element_Array;
>
>      procedure Do_Something_With_Array (
>          In_Array : in Element_Array_Ptr
>      );
>
> end Test_Package;

What are you really trying to do? You can do everything you want in the 
example w/o touching access types:

package Test_Package is
     type Element is
        record
            ID_1 : Positive;
            ID_2 : Positive;
        end record;

     type Element_Array     is array (Positive range <>) of Element;
     procedure Do_Something_With_Array (In_Array : in Element_Array);
end Test_Package;

package body Test_Package is

     procedure Do_Something_With_Array (In_Array : in Element_Array) is
     begin
         for E of In_Array loop
             declare
                 Key	: String renames Positive'Image(E.ID_1);
                 Value	: String renames Positive'Image(E.ID_2);
             begin
                 Ada.Text_IO.Put_Line( "Key:"&Key &ASCII.HT& 
"Value:"&Value &'.' );
             end;
         end loop;
     end Do_Something_With_Array;

end Test_Package;

Re: Pointer to instance of indefinite array?

[NiGHTS]

On Tuesday, August 12, 2014 10:29:04 PM UTC-4, Shark8 wrote:
> On 12-Aug-14 20:07, NiGHTS wrote:
> What are you really trying to do? You can do everything you want in the 
> example w/o touching access types:

On Tuesday, August 12, 2014 10:09:37 PM UTC-4, Jeffrey Carter wrote:
> Think in Ada and get rid of the access type.

Hmm. You're right. It worked! Compiler must be changing it internally to a pointer though since it would be far too inefficient to copy the entire array.

Re: Pointer to instance of indefinite array?

[Jeffrey Carter]

On 08/12/2014 08:06 PM, NiGHTS wrote:
>
> Hmm. You're right. It worked! Compiler must be changing it internally to a
> pointer though since it would be far too inefficient to copy the entire
> array.

The parameter-passing mechanism used by the compiler is generally not something 
we need to worry about.

Having to explicitly pass a pointer to obtain by-reference semantics is a mark 
of low-level languages. It is not a good way to do almost anything.

In a mid-level language, such as Pascal, one often specifies the 
parameter-passing mechanism, and the compiler uses that mechanism. This is 
pretty much what VAR (by reference) and non-VAR (by copy) parameters are in 
Pascal, though they're usually described in terms of whether or not changes to 
the formal parameter also change the actual.

In a high-level language, such as Ada, one specifies how the subprogram uses the 
parameter (in, in out, and out modes) and the compiler uses that and the type 
being passed to determine the parameter-passing mechanism.

-- 
Jeff Carter
"That was the most fun I've ever had without laughing."
Annie Hall
43

Re: Pointer to instance of indefinite array?

[NiGHTS]

On Wednesday, August 13, 2014 12:28:14 AM UTC-4, Jeffrey Carter wrote:
> On 08/12/2014 08:06 PM, NiGHTS wrote:
> 
> >
> 
> > Hmm. You're right. It worked! Compiler must be changing it internally to a
> 
> > pointer though since it would be far too inefficient to copy the entire
> 
> > array.
> 
> 
> 
> The parameter-passing mechanism used by the compiler is generally not something 
> 
> we need to worry about.
> 
> 
> 
> Having to explicitly pass a pointer to obtain by-reference semantics is a mark 
> 
> of low-level languages. It is not a good way to do almost anything.
> 
> 
> 
> In a mid-level language, such as Pascal, one often specifies the 
> 
> parameter-passing mechanism, and the compiler uses that mechanism. This is 
> 
> pretty much what VAR (by reference) and non-VAR (by copy) parameters are in 
> 
> Pascal, though they're usually described in terms of whether or not changes to 
> 
> the formal parameter also change the actual.
> 
> 
> 
> In a high-level language, such as Ada, one specifies how the subprogram uses the 
> 
> parameter (in, in out, and out modes) and the compiler uses that and the type 
> 
> being passed to determine the parameter-passing mechanism.
> 
> 
> 
> -- 
> 
> Jeff Carter
> 
> "That was the most fun I've ever had without laughing."
> 
> Annie Hall
> 
> 43

Is it possible to have a constrained array of indefinite arrays without using access? In my target program, the "Do_Something_With_Array" function is actually an interface to a FIFO queue where groups of indefinite arrays are temporarily buffered for use by another task. Of course this will be via an entry to a protected object but those details aside how can I construct a constrained array of indefinite arrays?

Re: Pointer to instance of indefinite array?

[Jeffrey Carter]

On 08/13/2014 09:06 AM, NiGHTS wrote:
>
> Is it possible to have a constrained array of indefinite arrays without using
> access? In my target program, the "Do_Something_With_Array" function is
> actually an interface to a FIFO queue where groups of indefinite arrays are
> temporarily buffered for use by another task. Of course this will be via an
> entry to a protected object but those details aside how can I construct a
> constrained array of indefinite arrays?

If I need something like that, I'll usually use an array of an unbounded-array 
data structure, which in Ada is found in Ada.Containers.Vectors.

-- 
Jeff Carter
"C++ is vast and dangerous, a sort of Mordor of
programming languages."
Jason R. Fruit
120

Re: Pointer to instance of indefinite array?

[Shark8]

On 13-Aug-14 10:06, NiGHTS wrote:
> Of course this will be via an entry to a protected object but
> those details aside how can I construct a constrained array
> of indefinite arrays?

For that you *do* need accesses, this is because the compiler needs to 
know the size of the elements. Of course this implementation detail can 
be hidden away from clients:
Package Test_UCA is

     Type Positive_Array is Array (Positive Range <>) of Positive;
     Type Array_Array(Len : Positive) is Private;

     -- Obj's Entries should be used as follows:
     -- O.Test( P ); -- Saves P into J.
     -- ...
     -- O.Result_Length( Temp ); -- Gets the length of J.
     -- declare
     --   New_Array : Positive_Array(1..Temp);
     -- begin
     --    O.Result( New_Array ); -- Copies J into New_Array & clears J.
     --  --... (Whatever processing you need.)
     -- end;
   Protected type Obj(Len : Positive) is
     Entry Test		( P	: Positive_Array );
     Entry Result_Length	( Len	: out Positive );
     Entry Result	( R	: out Positive_Array );
   Private
     J : access Positive_Array;
     K : Array_Array(Len);
   End Obj;

Private
   Type Internal_Array_Array is Array (Positive Range <>) of not null 
access Positive_Array;
   Type Array_Array(Len : Positive) is record
     Data :  Internal_Array_Array(1..Len);
   end record;


   Default : Constant Internal_Array_Array:= ( 1..0 => New 
Positive_Array'(1,1) );
End Test_UCA;


Package Body Test_UCA is
     Protected Body Obj is

         Entry Test		( P	: Positive_Array ) when True is
         Begin
             J:= new Positive_Array'(P);
         End Test;

         Entry Result_Length	( Len	: out Positive ) when J /= null is
         Begin
             Len:= J'Length;
         End Result_Length;

         Entry Result	( R	: out Positive_Array ) when J /= null is
         Begin
             R:= J.All;
             J:= Null;
         End Result;

     End Obj;
End Test_UCA;

Re: Pointer to instance of indefinite array?

[Dmitry A. Kazakov]

On Wed, 13 Aug 2014 09:06:51 -0700 (PDT), NiGHTS wrote:

> Is it possible to have a constrained array of indefinite arrays without
> using access?

Yes, if the semantics is by-value/copy

> In my target program, the "Do_Something_With_Array" function is actually
> an interface to a FIFO queue where groups of indefinite arrays are
> temporarily buffered for use by another task. Of course this will be via
> an entry to a protected object but those details aside how can I construct
> a constrained array of indefinite arrays?

I have several questions to that:

1. Do you really need marshaling? You say you buffer arrays that means the
producer and consumer are decoupled and you could enqueue several request
before servicing one. Is to so? Because in a synchronous schema when
requests are serviced as they appear you don't need to marshal requests.
You simply do a rendezvous from the producer to the consumer and do not
copy anything. This is also possible to implement though a protected object
(but would require an access type). In both cases the producer holds the
array, which is possible due to synchronicity.

2. It is almost always a bad idea to service requests in a protected
operation. Since you are working with arrays, it would involve loops, which
is certainly not good to do within such an operation. Sooner or later you
would like to do some I/O or other blocking stuff.

3. 1-1 FIFO actually does not need locking. You can implement it without
protected objects. One possible implementation is here

http://www.dmitry-kazakov.de/ada/components.htm#Generic_Indefinite_FIFO

It works with indefinite arrays. Arrays will copied in and out (marshaled).

4. For n-1, 1-m, n-m FIFOs, deploying a mutex for interlocking is almost
always better than servicing on the context of a protected action. It might
be slightly slower but much safer. A FIFO end with has 1 task attached can
access FIFO in a lock-free manner. E.g. for n-1, you must interlock only
producers etc.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

Re: Pointer to instance of indefinite array?

[Per Sandberg]

Well,
The compiler uses the most appropriate method of passing parameters (by 
value or by reference) depending on the kind of object, this usually 
means that composite types is passed by reference and scalars is passed 
by value.
However your sample does compile with some small modifications:
------------------------------------------
package Test_Package is
    type Element is
       record
          ID_1 : Positive;
          ID_2 : Positive;
       end record;
    type Element_Array     is array (Positive range <>) of Element;
    type Element_Array_Ptr is access all Element_Array;
    --> Use "all" to be able to reference objects allocated on staticly 
or stack.
    procedure Do_Something_With_Array (In_Array : in Element_Array_Ptr);
end Test_Package; 	
-----------------------------
with Test_Package; use Test_Package;
procedure main is
    Test_Array : aliased Element_Array (1 .. 20);
begin
    Do_Something_With_Array (In_Array => Test_Array'Unrestricted_Access);
    --  Use Unrestricted_Access attribute to disable
    --  accessibility and aliased view checks
end main;
----------------------------
/Per

On 13.08.2014 05:06, NiGHTS wrote:
> On Tuesday, August 12, 2014 10:29:04 PM UTC-4, Shark8 wrote:
>> On 12-Aug-14 20:07, NiGHTS wrote:
>> What are you really trying to do? You can do everything you want in the
>> example w/o touching access types:
>
> On Tuesday, August 12, 2014 10:09:37 PM UTC-4, Jeffrey Carter wrote:
>> Think in Ada and get rid of the access type.
>
> Hmm. You're right. It worked! Compiler must be changing it internally to a pointer though since it would be far too inefficient to copy the entire array.
>

Re: Pointer to instance of indefinite array?

[Georg Bauhaus]

On 13.08.14 06:31, Per Sandberg wrote:
> procedure main is
>     Test_Array : aliased Element_Array (1 .. 20);
> begin
>     Do_Something_With_Array (In_Array => Test_Array'Unrestricted_Access);
>     --  Use Unrestricted_Access attribute to disable
>     --  accessibility and aliased view checks
> end main;

Since 'Unrestricted_Access is not Ada but a GNAT specific pragma,
and since it is not strictly needed, I think that standard Ada's own
'Unchecked_Acceess should be enough?


procedure Main is
    Test_Array : aliased Element_Array := (1 .. 20 => (1, 2));
    -- declared as an unconstrained type to match the pointer type
begin
    Do_Something_With_Array (In_Array => Test_Array'Unchecked_Access);
    --  Use Unchecked_Access attribute since passing
    --  an object that will be inaccessibility at higher levels
    --  (that of the pointer type)
end Main;

If there is need to use a pointer at all, e.g. when using GNAT and
a really big array and the stack of GNAT's environment task is limited
by the operating system, then one may still pass the dereference
   Test_Array_Ptr.all
without any concerns about copying. So Do_Something_With_Array can
declare just an array type in its profile, not a pointer.

I remember something about the ICC compiler supporting an implementation
specific attribute similar to 'Unrestricted_Access. Is there a significant
advantage in using 'Unrestricted_Access in user programs, perhaps suppression
of more checks so that to something runs a lot faster?

Re: Pointer to instance of indefinite array?

[Egil H H]

On Wednesday, August 13, 2014 9:47:40 AM UTC+2, Georg Bauhaus wrote:
> I remember something about the ICC compiler supporting an implementation
> 
> specific attribute similar to 'Unrestricted_Access. Is there a significant
> 
> advantage in using 'Unrestricted_Access in user programs, perhaps suppression
> 
> of more checks so that to something runs a lot faster?


I don't know the performance characteristics of 'Unrestricted_Access, but it enables the effect of 'Unchecked_Access for subprograms.

Re: Pointer to instance of indefinite array?

[Adam Beneschan]

On Wednesday, August 13, 2014 12:47:40 AM UTC-7, Georg Bauhaus wrote:

> Since 'Unrestricted_Access is not Ada but a GNAT specific pragma, 
> and since it is not strictly needed, I think that standard Ada's own 
> 'Unchecked_Acceess should be enough? 

No, because of a somewhat obscure rule that deals with 'Access on an array when the target type is access-to-unconstrained-array.  The rule is in 3.10.2(27-27.2).  The reason for the rule is that access-to-unconstrained-array types need to either carry or point to information about the bounds; and in implementations where the bounds have to be stored together with the data, and the access object points to the bounds, you don't want to force the program to store the bounds for *every* array.

Because of this, even if 'Unrestricted_Access compiles on GNAT in Per's example code, it would have to be tested to make sure it works.  If there's no bound information, then the resulting code will be incorrect.  I think GNAT stores the bounds next to the data, but I'm not sure. 

> I remember something about the ICC compiler supporting an implementation
> specific attribute similar to 'Unrestricted_Access. Is there a significant
> advantage in using 'Unrestricted_Access in user programs, perhaps suppression
> of more checks so that to something runs a lot faster?

'Unrestricted_Access in ICC Ada just allows you to get the 'Access of something that wouldn't otherwise be legal.  I think it's the same in GNAT, but I'm not positive.  Note that 'Unchecked_Access suppresses accessibility checks (allowing an access object to point to something on the stack that could disappear before the access object does), and therefore it allows you to violate the static accessibility rules, but it doesn't allow you to violate other rules.  For example, if you use X'Unchecked_Access, X still has to be aliased, and there are other rules that are still checked.  'Unrestricted_Access suppresses checks like that too.  (In addition (as Egil posted), it can be used on subprograms, for which Ada doesn't allow 'Unchecked_Access.)

So, no, 'Unrestricted_Access doesn't affect performance.  In any case, the only checks performed at run-time are accessibility-level checks, and 'Unchecked_Access suppresses those, so it's hard to come up with any further performance improvements.

                                 -- Adam

Re: Pointer to instance of indefinite array?

[Georg Bauhaus]

On 13.08.14 17:32, Adam Beneschan wrote:
>> Since 'Unrestricted_Access is not Ada but a GNAT specific pragma,
>> >and since it is not strictly needed, I think that standard Ada's own
>> >'Unchecked_Acceess should be enough?
> No, because of a somewhat obscure rule that deals with 'Access on an array when the target type is access-to-unconstrained-array.  The rule is in 3.10.2(27-27.2).

Wow. Also found AI95-363.
This is similar to, but more entertaining than Sudoku.

Re: Pointer to instance of indefinite array?

[Niklas Holsti]

On 14-08-13 07:31 , Per Sandberg wrote:
> Well,
> The compiler uses the most appropriate method of passing parameters (by
> value or by reference) depending on the kind of object, this usually
> means that composite types is passed by reference and scalars is passed
> by value.

Anecdote: Long ago, there was an (in)famous version of GNAT (something
like 2.95p, perhaps?) for which AdaCore had decided to be "more
compatible with C" and therefore to pass record types by copy.

I was developing an application that heavily relied on passing large
records around. We ended up delivering the application with advice to
use a 10 MB stack limit... a large amount, at the time. Luckily the
application was not time-critical.

However, this compiler "feature" also revealed an aliasing bug in the
application, where one of my procedures was reading a record object
passed as a parameter, while modifying it as a global (IIRC). Perhaps
GNAT should have an option to use pass-by-copy wherever it can, to smoke
out such bugs :-).

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
      .      @       .