Re: win32ada design question/problem

[Mats Weber <Mats.Weber@elca-matrix.ch>]

Tom Moran wrote:

>   Why exactly is X'Address preferable to X'Unchecked_Access?
>   Having spent a fair amount of time in the past debugging/correcting
> supposedly portable code that incorrectly assumed an access type was a
> 'Address, I'm rather sensitive to this issue.

X'Address works in all cases, including arrays (Strings) for example:

   S : String := "xyz" & ascii.nul;

   function C_Function (X : in System.Address);

   C_Function(S'Address);

makes it clear that you are passing the whole of S.

On the other hand,

   type String_Access is access String;

   function C_Function (X : in String_Access);

   C_Function(S'Unchecked_Access);

will sometimes not work because some compilers (and that includes GNAT)
use fat pointers for access to unconstrained arrays (I am not saying
that this is bad).

so you have to do this instead:

   C_Function(S(S'first)'Unchecked_Access);

which does not make clear that S is passed as a whole.


The second problem is that people see a compiler message like "object
has accessibility level deeper than access type" and instead of using
'Unchecked_Access, they move the declaration to the library level and
create non task safe code. I am not making this up, see the samples
directory that comes with the win32ada binding.