Re: Signed integer to modular type conversion

[Mark Johnson <mark_h_johnson@raytheon.com>]

Adam Beneschan wrote:
> 
> Supposing Standard.Integer is a 32-bit signed integer type.  I have a
> modular type
> 
>     type ModType is mod 2**32;
> 
>     X : Integer;
>     Y : ModType;
> 
> I'd like to set Y := X mod 2**32 (which should essentially just treat
> X as an unsigned integer without changing the data).
A reasonably efficient way to do this is Unchecked_Conversion. This for
types of the same size (as in this case), it will copy the bytes from
the source to destination. As you noted in the rest of your message, the
direct type conversion doesn't do what you want and the arithmetic is an
inefficient method. There are a lot of reasons not to use
Unchecked_Conversion, but this doesn't appear to be one of them.
  --Mark

PS: I don't generally recommend this solution because many use it to
excess. Try...
  with Unchecked_Conversion;
  function "+" is new Unchecked_Conversion(Integer, Mod_Type);
Then your example assignment will be...
  Y := +X;
The abuse comes from using this method for all sorts of type
conversions, avoiding type safety, etc.