Re: Access Types

[Robert A Duff <bobduff@world.std.com>]

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.