Re: 'size works for SunAda but not GNAT

[Stephen Leake <Stephen.Leake@gsfc.nasa.gov>]

David Haslam wrote:
> 
> I have a lot of code like this, which compiles fine
> with SunAda 1.1:
> 
>         subtype V is integer range 0..63;
>         type A is array (1..9) of V;
>         for A'size use 9 * 8;
> 
> GNAT complains "size for A must be at least 288".
> GNAT wants V to be 32 bits, even though it can fit in 8.
> 
> If GNAT is correct in rejecting this, then it implies that the
> code we have is non-portable (and relied on a Verdix implementation
> dependent feature).

Well, non-portable between Ada83 and Ada95, at least. Although, I
suspect there are other Ada83 compilers that reject this also.

> If so, how do I write portable rep clauses?
> There are two ways of fixing the code for GNAT:
> 1. use a type instead of a subtype
> 2. add the line: "for A'component_size use 8;"
> 
These are certainly good choices; 2 is not Ada83, but it is standard
Ada95 (note that not all attributes GNAT supports are standard Ada95).

> Which of these is better?

If you want Ada83 portability, you have to pick 1. If you can use Ada95,
1 is better if ALL uses of type V must be 8 bits; 2 is better if only
THIS use of V needs 8 bits.
> 
> --
> David Haslam                                 Work: David.Haslam@gecm.com
> GEC-Marconi S3I Ltd                         Home: dch@sirius.demon.co.uk
> Simulation and Training Division

-- 
- Stephe