Re: Ada and string literals

[Adam Beneschan <adam@irvine.com>]

On Wednesday, January 30, 2013 6:23:08 PM UTC-8, Robert A Duff wrote:
> Jeffrey Carter writes:

> '[Unchecked_]Access is about object identity.
> On the other hand 'Address is not -- two objects can have
> the same address without being the same object.

However, if two objects have the same 'Address, it's hard to imagine them not having the same 'Access as well, in practice.  There aren't many cases where this is an issue, but:

   procedure P (Addr : in System.Address) is

      X : aliased T;
      for X'Address use Addr;
      pragma Import (Ada, X);

      Y : aliased T;
      for Y'Address use Addr;
      pragma Import (Ada, Y);

      XA : T_Acc := X'Access;
      YA : T_Acc := Y'Access;

      ...

      if XA = YA then ...

   end Proc;

Assuming T is a simple record type, for instance, I can't imagine XA=YA being false in any implementation, even though I believe X and Y are distinct objects by the RM's definitions.  The additional logic and/or storage required to make sure XA /= YA would be unacceptable distributed overhead.

(This may or may not apply to unconstrained String subtypes such as in the example; if an implementation already uses an implicit level of indirection somewhere to implement those, then it may be possible that 'Address would be the same and 'Access different without any additional overhead.)

                               -- Adam