The Aliased Access Mystery

General
1

Hello,

I am working with access types and I am not sure how to resolve a compiler error:

x86_64-linux-gnu-gcc-10 -c test.adb
test.adb:13:29: prefix of "Access" attribute must be aliased
gnatmake: "test.adb" compilation error

package Things is
   type Ref is access all Natural;
   type RefRef is access all Ref;

   type Rec_A is record
      Element : access Natural;
   end record;

   Member   : aliased access Natural;
   Instance : aliased Rec_A;
end Things;

with Things;

procedure Test is
   Target : Things.RefRef;
begin
   -- [Assignment 1] Compiles
   Target := Things.Member'Access;
   -- [Assignment 2] Compilation Error
   Target := Things.Instance.Element'Access;
end Test;

Am I missing an ‘aliased’ somewhere?

Thanks,
N

2

I should note that changing the ‘Element’ element to ‘aliased’ doesn’t seem to help:

package Things is
   type Ref is access all Natural;
   type RefRef is access all Ref;

   type Rec_A is record
      Element : aliased access Natural; -- Add 'aliased'
   end record;

   Member   : aliased access Natural;
   Instance : aliased Rec_A;
end Things;
3

I found the solution!

Changing the ‘access Natural’ to ‘Ref’ (with aliased) works.

This is the final version:

package Things is
   type Ref is access all Natural;
   type RefRef is access all Ref;

   type Rec_A is record
      Element : aliased Ref; -- [Fixed here]
   end record;

   Member   : aliased access Natural;
   Instance : aliased Rec_A;
end Things;

It seems like the original version (Element : aliased access Natural) should work too. Is this a compiler bug?

4

Are you transliterating C code?
I ask because the double-access is highly unusual. (There are some cases where this is necessary, but as one of the Ada gurus says: “To a first-order approximation, accesses are never needed.”)

Note that, by definition, the refrant of an access is aliased.

1 Like
5

I have a binding to a C library which uses pointers to data structures.