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From: Richard Riehle <richard@adaworks.com>
Newsgroups: comp.lang.ada
Subject: Re: "access constant" discriminant
Date: Sat, 15 Feb 2003 15:53:32 -0800
Organization: AdaWorks Software Engineering
Message-ID: <3E4ED2FC.3310564A@adaworks.com>
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Date: 2003-02-15T23:45:00+00:00
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Larry Kilgallen wrote:

> Why is it necessary to have the parameter be an access type ?

Good question.  Consider this package,

        package Side_Effect_Experiment is

             type Item_Type is private;
             type Reference is access all Item_Type;
             procedure Create (Data : in out Item_Type);
             function Is_Valid (Data :  in Item_Type) return Boolean;
             function Is_Valid (Data : in Reference)  return Boolean;
             function Is_Valid (Data : access Item_Type) return Boolean;
       private
             type Item_Type is record
                  ID                : Natural := 0;
                  Description : String(1..20) := (others => ' ');
                  Valid           : Boolean := False;
             end record;
      end Side_Effect_Experiment;

with this corresponding package body,

    package body Side_Effect_Experiment is

        procedure Create (Data : in out Item_Type) is
        begin
           null;
       end Create;

       function Is_Valid (Data :  in Item_Type) return Boolean is
       begin
            Data.Valid := True;      -- not valid for in parameter
            return Data.Valid;
       end Is_Valid;

       function Is_Valid (Data : in Reference) return Boolean is
       begin
            Data.Valid := True;
            return Data.Valid;
      end Is_Valid;

      function Is_Valid (Data : access Item_Type) return Boolean is
      begin
           Data.Valid := True;
           return Data.Valid;
     end Is_Valid;

  end Side_Effect_Experiment;

Note that the side-effect cannot happen when we have a normal in
parameter.  However,  it can occur with any variation of an access
type parameter.   By adding syntax to the function such as,

          constant function Is_Valid(Data : access Item_Type) return Boolean;
or
          function Is_Valid (Data : access constant Item_Type) return Boolean;

the compiler would be able to note the attempt at a side-effect and prevent
it.

Richard Riehle