Re: Ada Alternatives to Unrestricted_Access

[Jere <jhb.chat@gmail.com>]

On Wednesday, March 7, 2018 at 10:18:47 PM UTC-5, gautier wrote:
> I had the same problem for GL[U[T]] bindings (work on both GNAT and ObjectAda, and perhaps other Ada systems) [1]. A quick search with AZip (shameless plug :-) [2] leads to 
>   package A2A_double is new System.Address_To_Access_Conversions(Double);
> ...
>   procedure Vertex (v: Double_Vector_3D) is
>   begin
>     Vertex3dv(A2A_double.To_Pointer(v(0)'Address));
>     -- This method is functionally identical
>     -- to using GNAT's 'Unrestricted_Access
>   end Vertex;
> 
> Other places use an Unchecked_Conversion.
> The GNAT doc says:
> "The Unrestricted_Access attribute is similar to Access except that all accessibility and aliased view checks are omitted. This is a user-beware attribute. It is similar to Address, for which it is a desirable replacement where the value desired is an access type. In other words, its effect is identical to first applying the Address attribute and then doing an unchecked conversion to a desired access type."
> 
> ___
> 
> [1] https://globe3d.sourceforge.io/
> [2] https://azip.sourceforge.io/

Yep, that's basically the same conclusion I came to.  My doubts center
around whether it is legit in portable Ada to do so to bypass the "in"
mode.  I don't know if the language allows for a compiler to use
different representations for a constant variable and a mutable variable.
If so, what I am doing might possibly be erroneous behavior.  It would
work in GNAT based on the section you quoted, but I was curious if
it would run afoul in other compilers possibly.