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From: "Nick Roberts" <nickroberts@blueyonder.co.uk>
Newsgroups: comp.lang.ada
Subject: Re: Elegant 'realloc'?
Date: Thu, 31 Jul 2003 17:35:07 +0100
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Date: 2003-07-31T17:35:07+01:00
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"Lutz Donnerhacke" <lutz@iks-jena.de> wrote in message
news:slrnbihq0e.2j7.lutz@taranis.iks-jena.de...

> When dealing with dynamically allocated variable length
> arrays, the allocated space might change. Is there a
> common idiom to simulate an 'realloc' (especially
> shrinking) other than, allocate, copy, free?

Nope.

>    declare
>       procedure Free is new Unchecked_Deallocation(T_Array,
T_Array_Access);
>       oldp : constant T_Array_Access := current.field;
>    begin
>       current.field := new T_Array'(oldp(oldp'First .. current.last));
>       Free(oldp);
>    end;

The above isn't quite right.

Supposing we have the declarations:

   type Location_Descriptor is ...
   Unknown_Location: constant Location_Descriptor;
   type Location_Array is array (Integer range <>) of Location_Descriptor;
   type Location_History is access Location_Array;
   Track: Location_History;
   procedure Free is new Unchecked_Deallocation( Location_Array,
Location_History );

At a point where we wanted to change the range of Track to, let's say,
New_First..New_Last, we could write:

   declare
      subtype NI is Nonpositive_Integer; -- convenient renaming
      Mid_First: constant NI := NI'Max( Track'First, New_First );
      Mid_Last:  constant NI := NI'Min( Track'Last,  New_Last );
      Temp: constant Location_History := new Location_Array( New_First ..
New_Last );
   begin
      Temp( Mid_First   .. Mid_Last    ) := Track( Mid_First .. Mid_Last );
      Free( Track );
      Temp( New_First   .. Mid_First-1 ) := (others => Unknown_Location);
      Temp( Mid_Last+1 .. New_Last    ) := (others => Unknown_Location);
      Track := Temp;
   end;

I haven't tested this, but I think it's right.

--
Nick Roberts
Jabber: debater@charente.de [ICQ: 159718630]