aliased variables

aliased variables

[Richard Beare]

Hi,
I am having a problem trying to get a program with an aliased variable
to compile. I'm sure I'm missing something obvious 

-------------------------------------------------
procedure Al_Exp is

   type Mt is array(Natural range <>) of Integer;

   type H_Mt is access all Mt;

   K : aliased Mt(1 .. 10);
   L : H_Mt;

begin
    L := K'Access;
end;
-------------------------------------------------
The error message is

gcc -c -gnatv al_exp.adb

GNAT 3.10p (970814) Copyright 1992-1997 Free Software Foundation, Inc.

Compiling: al_exp.adb (source file time stamp: 1998-08-13 03:55:58)

    23.     L := K'Access;
                 |
        >>> object subtype must statically match designated subtype



What's the problem here - I haven't found anything in the RM saying I
can't have general access types pointing to unconstrained arrays - the
Rationale says that arrays of pointers to strings can be created without
dynamic allocation this way, but I haven't been able to get that to work
- what am I doing wrong?

-- 
Richard Beare
Richard.Beare@cmis.csiro.au

Re: aliased variables

[Tucker Taft]

Richard Beare (Richard.Beare@cmis.csiro.au) wrote:

: I am having a problem trying to get a program with an aliased variable
: to compile. I'm sure I'm missing something obvious 

: -------------------------------------------------
: procedure Al_Exp is

:    type Mt is array(Natural range <>) of Integer;

:    type H_Mt is access all Mt;

:    K : aliased Mt(1 .. 10);
:    L : H_Mt;

: begin
:     L := K'Access;
: end;
: -------------------------------------------------
: The error message is

: gcc -c -gnatv al_exp.adb

: GNAT 3.10p (970814) Copyright 1992-1997 Free Software Foundation, Inc.

: Compiling: al_exp.adb (source file time stamp: 1998-08-13 03:55:58)

:     23.     L := K'Access;
:                  |
:         >>> object subtype must statically match designated subtype



: What's the problem here - I haven't found anything in the RM saying I
: can't have general access types pointing to unconstrained arrays - the
: Rationale says that arrays of pointers to strings can be created without
: dynamic allocation this way, but I haven't been able to get that to work
: - what am I doing wrong?

This is a nasty corner associated with 'Access on arrays.  The subtypes
have to match statically.  This means you have to define your array object
as (nominally) unconstrained to point to it with an access-to-unconstrained
access value.

But if it is nominally unconstrained, then it needs to be initialized.
Hence, try this:

    K : aliased Mt := (1..10 => 0);
  ...
    ... K'Access ...

Aliased discriminated types work more intuitively than do aliased 
arrays, for what it is worth...

: Richard Beare
: Richard.Beare@cmis.csiro.au

--
-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
Intermetrics, Inc.  Burlington, MA  USA

Re: aliased variables

[David C. Hoos, Sr.]

Richard Beare wrote in message <35D264A8.FC025A5F@cmis.csiro.au>...
>Hi,
>I am having a problem trying to get a program with an aliased variable
>to compile. I'm sure I'm missing something obvious

The problem is that your program does not conform to the rule expresssed in
the following sentence of RM 3.10 (9):
If the view defined by an object_declaration is aliased, and the type of the
object has discriminants, then the object is constrained; if its nominal
subtype is unconstrained, then the object is constrained by its initial
value.

Since K's nominal subtype is unconstrained, the object must be constrained
not by explicit bounds, but by its initial value.

The solution is to declare K as follows:
   K : aliased Mt := (1 .. 10 => 0);

David C. Hoos, Sr.

Re: aliased variables

[Robert Dewar]

Richard may like better the error messages that come out of the latest
version of
GNAT:

     1. procedure Al_Exp is
     2.
     3.    type Mt is array(Natural range <>) of Integer;
     4.
     5.    type H_Mt is access all Mt;
     6.
     7.    K : aliased Mt(1 .. 10);
           |
        >>> warning: aliased object has explicit bounds
        >>> warning: declare with explicit initialization
        >>> warning: for use with unconstrained access

     8.    L : H_Mt;
     9.
    10. begin
    11.     L := K'Access;
                 |
        >>> object subtype must statically match designated subtype

    12. end;