access type problems

access type problems

[Ramon Costa Castelló]

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Hi,

If I try to compile this system, composed
by file buffer.ads and main.adb:

buffer.ads:

package buffer is
   type Cell is
   record
      A:Integer;
   end record;
   type Ptr_Cell is access all Cell;
end buffer;

main.adb :

with buffer; use buffer;
procedure Main is
   A: aliased Cell;
   B: Ptr_Cell;
begin
   B:=A'access;
end Main;

I get the following message (from gnat.3.10p under windows'NT) :

main.adb:9:07: object has deeper accessibility level than access type

Differently if I try to compile the system composed by files
buffer2.ads and main2.adb:

buffer2.ads:

package buffer2 is
   type Cell is
   record
      A:Integer;
   end record;
end buffer2;

main2.adb:

with buffer2; use buffer2;
procedure main2 is
   type Ptr_Cell is access all Cell;
   A: aliased Cell;
   B: Ptr_Cell;
begin
   B:=A'access;
end main2;

it compiles perfectly without an error or
warning.

Can Anyone explain me the difference between both codes ?
How can I solve the problem appearing in the first one ?


Best Regards,

Ramon Costa-Castello

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Re: access type problems

[Mats Weber]

Ramon Costa Castell� wrote:

> package buffer is
>    type Cell is
>    record
>       A:Integer;
>    end record;
>    type Ptr_Cell is access all Cell;

This access type is at the library level (the most global level).

> end buffer;
> 
> main.adb :
> 
> with buffer; use buffer;
> procedure Main is

The fact that this is a main program is not relevant. Ada rules treat it
just as a normal subprogram.

>    A: aliased Cell;
>    B: Ptr_Cell;
> begin
>    B:=A'access;

As A is in a subprogram, its access level is one deeper than the library
level. The subprogram Main could be called from elsewhere, and thus the
call would return before the program terminates. So A could possibly
cease to exist before the access type Buffer.Ptr_Cell, and access values
are not allowed to outlive the object they point to.

> end Main;

> package buffer2 is
>    type Cell is
>    record
>       A:Integer;
>    end record;
> end buffer2;


> with buffer2; use buffer2;
> procedure main2 is
>    type Ptr_Cell is access all Cell;
>    A: aliased Cell;
>    B: Ptr_Cell;

In this case, the accessibility level of the object and that of the
access type are the same, so there is no problem.

> begin
>    B:=A'access;
> end main2;
> 
> it compiles perfectly without an error or
> warning.

This is normal.