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From: progers@acm.org
Subject: Re: Type conversion between access types (was: Free'ing extended
 types)
Date: 1996/05/23
Message-ID: <4o22no$9ej@uuneo.neosoft.com>#1/1
X-Deja-AN: 156331576
references: <3183AC75.335C@ehs.ericsson.se> <leschkes.832628057@ferret>
 <DrqLG3.1r0@world.std.com> <leschkes.832803959@ferret>
 <Drv666.92J@world.std.com>
organization: NeoSoft Internet Services   +1 713 968 5800
reply-to: progers@acm.org
newsgroups: comp.lang.ada
Date: 1996-05-23T00:00:00+00:00
List-Id: <comp.lang.ada>


In <Drv666.92J@world.std.com>, bobduff@world.std.com (Robert A Duff) writes:
>In article <leschkes.832803959@ferret>,
>Scott Leschke <leschkes@ferret.cig.mot.com> wrote:
>>So what about 'all' causes the conversion to be allowed?  I looked at
>>section 4.6 of the LRM but got a tad confused?
>
>If you write "all", then it's called a "general access type".  4.6(13)
>allows conversion from an access type (general or pool-specific) to a
>general access type.  4.6(21) says that if the target type doesn't fall
>into one the cases allowed previously, then you can only convert within
>the same tree of derived types.  (I suspect that derived access types
>are unusual, so that would probably not apply.)

Derived access type conversion is unusual, at least in my experience,
but it sure is handy when you need it (in this case to prevent infinite
recursion on "="):

package Dynamic_Strings is 

  type Text is limited private;
 
  function "="( Left, Right : Text ) return Boolean;

  ...

private

  type Internal is access String;

  type Text is new Internal;

end Dynamic_Strings;



package body Dynamic_Strings is


  function "="( Left, Right : Text ) return Boolean is
  begin
    if ( Internal(Left) = null ) and ( Internal(Right) = null ) then
      return True;
    elsif ( Internal(Left) = null ) xor ( Internal(Right) = null ) then
      return False; -- since one points to an object & the other to nada
    else -- both refer to objects...
      return ( Left.all = Right.all );
    end if;
  end "=";

  ...

end Dynamic_Strings;




pat
---------------
Patrick Rogers
progers@acm.org