Prefix to 'ACCESS must either statically match... But why?

Prefix to 'ACCESS must either statically match... But why?

[Wojtek Narczynski]

Hello,

Could somebody please explain me why is this rule present? I find it
very limting. Is this because AR2 bounds are not stored with the
object?

Type Stream_Element_Array_access is access all Stream_Element_Array;

AR1 : aliased Stream_Element_Array := ( 2, 4, 5 );
AR2 : aliased Stream_Element_Array ( 1 .. 3 ) := ( 2, 4, 5 );

AR1A : Stream_Element_Array_access := AR1'access;

-- Illegal
AR2A : Stream_Element_Array_access := AR2'access;


LRM:3.10.2(27), The nominal subtype of the prefix to 'ACCESS or
'UNCHECKED_ACCESS must either statically match the designated subtype
of the expected type or the designated subtype must be discriminated
and unconstrained, Continuing


Thanks,
Wojtek

Re: Prefix to 'ACCESS must either statically match... But why?

[Martin Krischik]

On Sat, 25 Jan 2003 14:31:17 +0000, Wojtek Narczynski wrote:

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> From: wojtek@power.com.pl (Wojtek Narczynski)
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> Subject: Prefix to 'ACCESS must either statically match... But why?
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> 
> Hello,
> 
> Could somebody please explain me why is this rule present? I find it
> very limting. Is this because AR2 bounds are not stored with the
> object?
> 
> Type Stream_Element_Array_access is access all Stream_Element_Array;
> 
> AR1 : aliased Stream_Element_Array := ( 2, 4, 5 );
> AR2 : aliased Stream_Element_Array ( 1 .. 3 ) := ( 2, 4, 5 );
> 
> AR1A : Stream_Element_Array_access := AR1'access;
> 
> -- Illegal
> AR2A : Stream_Element_Array_access := AR2'access;
> 
> 
> LRM:3.10.2(27), The nominal subtype of the prefix to 'ACCESS or
> 'UNCHECKED_ACCESS must either statically match the designated subtype
> of the expected type or the designated subtype must be discriminated
> and unconstrained, Continuing

Well I am a Ada-beginner myself so I might be wrong, but as far as I
understand strictly typed languages in general "Stream_Element_Array ( 1 .. 3 )" is a
new anonymous type which ist not a Stream_Element_Array anymore. A bit like typing

Type Stream_Element_Array_AR2 is new Stream_Element_Array ( 1 .. 3 );
AR2 : aliased Stream_Element_Array_AR2 := ( 2, 4, 5 );

Hope it helps

Martin

-- 
Martin Krischik
mailto://Martin@krischik.com
http://www.ada.krischik.com

Re: Prefix to 'ACCESS must either statically match... But why?

[Vadim Godunko]

Wojtek Narczynski wrote:
> 
> Could somebody please explain me why is this rule present? I find it
> very limting. Is this because AR2 bounds are not stored with the
> object?
> 
> Type Stream_Element_Array_access is access all Stream_Element_Array;
> 

RM 3.3.1(8): The _subtype_indication_ or full type definition of an 
_object_declaration_ defines the nominal subtype of the object. The 
_object_declaration_ declares an object of the type of the nominal subtype.

> AR1 : aliased Stream_Element_Array := ( 2, 4, 5 );

AR1 nominal subtype is Stream_Element_Array (<>) of type 
Stream_Element_Array (<>)

> AR2 : aliased Stream_Element_Array ( 1 .. 3 ) := ( 2, 4, 5 );

AR2 nominal subtype is Stream_Element_Array (1 .. 3) of type 
Stream_Element_Array (<>)

> 
> AR1A : Stream_Element_Array_access := AR1'access;
> 
> -- Illegal
> AR2A : Stream_Element_Array_access := AR2'access;
> 
> 
RM/TC1 3.10.2 (24): ... The view denoted by the prefix X shall satisfy 
the following additional requirements, presuming the expected type for 
the X'Access is the general access type A with designated type D:

RM/TC1 3.10.2 (27/1): ...; if D is untagged, then the type view shall be 
D, and A's designated subtype either statically match the nominal 
subtype of the view or be discriminanted and unconstrained.

A = Stream_Element_Array_Access
D = Stream_Element_Array (<>)

D - untagged; type of both view is Stream_Element_Array (<>) = D; and 
A's designated subtype (Stream_Element_Array (<>)) is discriminanted and 
unconstrained.


What's wrong?


Vadim Godunko

GNAT / Aonix compiler incompatibility (Was: Prefix to 'ACCESS must either statically match... But why?)

[Wojtek Narczynski]

Vadim Godunko <vgodunko@vipmail.ru> wrote in message news:<mailman.1.1043696103.30820.comp.lang.ada@ada.eu.org>...
> 
> What's wrong?
> 

Well, I don't know... 

Could you analyze this source for compliance with RM also, please?

Regards,
Wojtek Narczynski


with ada.strings.Fixed; use ada.strings.Fixed;
with Ada.Text_IO; use Ada.Text_IO;

procedure tiny is
-- Is this code valid ?!

	type string_access is access all string;

	subtype Len_Positive is Positive range 1 .. 20;
	subtype Len_String is String ( len_Positive );
	subtype Len_String_Access is String_Access( Len_Positive );

	LS : aliased Len_String := Len_String'Length * "*";


	-- \/ This compiles with GNAT, but not with Aonix
	LSA : Len_String_Access := LS'access;


begin

	-- \/ This compiles with GNAT, but not with Aonix
	Put_Line( LSA.all );

end tiny;