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From: Joerg.Ozimek@t-online.de (Joerg Ozimek)
Subject: Re: overload ":=" ???
Date: 1996/07/19
Message-ID: <4snunj$a5d@news00.btx.dtag.de>#1/1
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references: <31ED3F5F.1135B4EA@jinx.sckans.edu>
x-sender: 0823133156-0001@t-online.de (Joerg Ozimek)
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Date: 1996-07-19T00:00:00+00:00
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David Morton <dmorton@jinx.sckans.edu> wrote:
>> Is there a way to overload the assignment operator ":=" ?


The language (-> LRM 4.5) defines only the following six categories of
operators. The corresponding operator_symbols, and only those, can be
used as designators in declarations of functions for user-defined
operators. 

  logical_operator            ::=  and | or  | xor
  relational_operator         ::=  =   | /=  | <   | <= | > | >=
  binary_adding_operator      ::=  +   | -   | &
  unary_adding_operator       ::=  +   | -
  multiplying_operator        ::=  *   | /   | mod | rem
  highest_precedence_operator ::=  **


Therefore the assignment_statement ":=" is not an operator and 
can't be overloaded! :-((


But you have the ability to control the effects of an assignement by
inheriting your own "controlled type" from the private tagged type
Controlled. You find it in the predefined package Ada.Finalization 
(-> LRM 7.6).

----------------------------------------------------
with Ada.Finalization;
use  Ada.Finalization;
package MyText is
   type Text is new Controlled with private;
   procedure Adjust (Object : in out Text);
private
   type Text is new Controlled with
      record
         Buffer : String(1..50);
         Length : Natural := 0;
      end record;
end MyText;
----------------------------------------------------
with Ada.Text_IO;
use  Ada.Text_IO;
package body MyText is
   procedure Adjust (Object : in out Text) is
   begin
      Put_Line ("Here is the text : " & 
                Object.Buffer(1 .. Object.Length));
      -- here you can place some usefull control code,
      -- which is executed after an assignement ...
   end Adjust;
end MyText;
----------------------------------------------------



Joerg Ozimek