From mboxrd@z Thu Jan  1 00:00:00 1970
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From: Robert A Duff <bobduff@world.std.com>
Subject: Re: Access Types
Date: 1999/07/21
Message-ID: <wcchfmxsy3y.fsf@world.std.com>#1/1
X-Deja-AN: 503614299
Sender: bobduff@world.std.com (Robert A Duff)
References: <7mkgqq$elo$1@dailyplanet.wam.umd.edu>
Organization: The World Public Access UNIX, Brookline, MA
Newsgroups: comp.lang.ada
Date: 1999-07-21T00:00:00+00:00
List-Id: <comp.lang.ada>

rayoub@wam.umd.edu (Ronald Ayoub) writes:

> For this line of code:
> 
> type File_Access is access constant File_Type;
>
> I understand this to mean that File_Access points to a File_Type variable
> which cannot be changed but the value pointed to can be changed. Is this
> correct?

Um, the thing pointed to *is* the File_Type variable.  Maybe this will help:

    type Pointer_To_Const is access constant Integer;
    type Pointer_To_Var   is access all      Integer;

    X: Pointer_To_Const := ...;
    Y: Pointer_To_Var := ...;

    Z: constant Pointer_To_Const := ...;
    W: constant Pointer_To_Var := ...;

    X.all := 1; -- Illegal.
    X := new Integer'(3); -- OK.
    Y.all := 1; -- OK.
    Y := new Integer'(3); -- OK.
    
    Z.all := 1; -- Illegal;
    Z := new Integer'(3); -- Illegal.
    W.all := 1; -- OK.
    W := new Integer'(3); -- Illegal.
    
    ...

X and Y are constants; Z and W are variables.

The objects Y and W point to are variables.
The objects X and Z point to might be constant or variable,
but they're read-only via X.all and Z.all.

- Bob
-- 
Change robert to bob to get my real email address.  Sorry.