Help: Type not accessible

Help: Type not accessible

[Mattimus]

Ok guys here's the jist of what I have.

package Navs is

    type Nav is tagged
        record
            Position : Integer;
        end record;

    type Navpointer is access all Nav;

    function Getposition (Anav : in Nav) return Integer;
    procedure Setposition (Anav : in out Nav; Anint : in Integer);
end Navs;

with Navs;
use Navs;

with Navinterfaces;
use Navinterfaces;

with Displays;
use Displays;

with Ada.Text_Io;
use Ada.Text_Io;


procedure Main is

    Mynav : aliased Nav;

    Pointer_To_Nav : Navpointer;

 begin

    --create a pointer to the nav to give to the nav interface
    Pointer_To_Nav := Mynav'Access;  <--- THIS LINE WILL NOT COMPILE


It tells me that 'Mynav is not accessible from  access all nav'.
Can someone please help. I have no idea what to do here, or what is causing
it.

Thanks a lot guys.

Re: Help: Type not accessible

[Stephen Leake]

"Mattimus" <mattisgrat@yahoo.ca> writes:

> Ok guys here's the jist of what I have.
> 
> package Navs is
> 
>     type Nav is tagged
>         record
>             Position : Integer;
>         end record;
> 
>     type Navpointer is access all Nav;
> 
>     function Getposition (Anav : in Nav) return Integer;
>     procedure Setposition (Anav : in out Nav; Anint : in Integer);
> end Navs;

This package declares an access type at "library level". This means
that there _can_ be variables of type Navpointer that _never_ go out
of scope.

><snip>
> 
> procedure Main is
> 
>     Mynav : aliased Nav;
> 
>     Pointer_To_Nav : Navpointer;
> 
>  begin
> 
>     --create a pointer to the nav to give to the nav interface
>     Pointer_To_Nav := Mynav'Access;  <--- THIS LINE WILL NOT COMPILE
> 

Mynav is in a procedure, which is a lower level than "library"; it
_can_ go out of scope. Sure, in this case it's the "main" procedure,
but the compiler doesn't know that (remember, the name "main" is _not_
special in Ada). So the compiler says "saving a pointer to Mynav is
bad, since it may go out of scope, and then you'd have a dangling
pointer". 

> It tells me that 'Mynav is not accessible from access all nav'. Can
> someone please help. I have no idea what to do here, or what is
> causing it.

The solution is to declare another package, say Main_Package, and
declare the pointer there. Packages are at library level, they don't
go out of scope.

In general, you will find that you need to declare most of your
application at library level, and very little in the actual main
routine. 

-- 
-- Stephe

Re: Help: Type not accessible

[Christoph Grein]

> procedure Main is
> 
>     Mynav : aliased Nav;
> 
>     Pointer_To_Nav : Navpointer;
> 
>  begin
> 
>     --create a pointer to the nav to give to the nav interface
>     Pointer_To_Nav := Mynav'Access;  <--- THIS LINE WILL NOT COMPILE
> 
> 
> It tells me that 'Mynav is not accessible from  access all nav'.
> Can someone please help. I have no idea what to do here, or what is causing
> it.


You've fallen victim to the accessibility level problem. Your Navpointer has a 
longer lifetime than your variable Mynav. You could copy Pointer_To_Nav to 
another pointer with longer lifetime than Main, i.e. it would point to an object 
that no longer exists.

That you don't do this here is irrelevant, what matters is lifetime or in RM 
speak accessibility levels.

Use 'Unchecked_Access instead, and you're on yourself to prevent dangling 
pointers.

Re: Help: Type not accessible

[Stephen Leake]

Christoph Grein <christoph.grein@eurocopter.com> writes:

> Use 'Unchecked_Access instead, and you're on yourself to prevent dangling 
> pointers.

Please don't introduce hacks to newbies; show them the right way
first! This is Ada, not C!

-- 
-- Stephe

Re: Help: Type not accessible

[Christoph Grein]

From: Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov>
> Christoph Grein <christoph.grein@eurocopter.com> writes:
> 
> > Use 'Unchecked_Access instead, and you're on yourself to prevent dangling 
> > pointers.
> 
> Please don't introduce hacks to newbies; show them the right way
> first! This is Ada, not C!
> 
> -- 
> -- Stephe

This is as little a hack as Unchecked_Deallocation. You simply need it in some 
cases... If 'Unchecked_Access is a hack, also 'Access is a hack. I rarely feel 
the need to use aliases.

Note that I gave complete information why 'Access does not work (omitted in your 
quote).