Re: How to put 200.0 (is float variable of 32 bits) into an integer sub-type of 10 bits (complete program included)

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

ChristopherL wrote:
> How can I be getting the below error message with
> "Unchecked_Conversion".
> Isn't Unchecked_conversion just suppose to copy bits?

Perhaps because your Ada compiler applies the Ada 95 rules, in 
which a call of (an instance of) Unchecked_Conversion is 
"erroneous" if the result is a scalar with an "invalid 
representation" (RM95 13.9.1(12)). In your program, the assignment 
statement

    Result2 := Get_Bits (Result1);

applies Get_Bits to Result1, which has the value 201, so Get_Bits 
tries to return a Short_integer2 with the value 201, which is 
outside the range of Short_integer2 and so is an invalid 
representation. Your compiler may signal this error by raising the 
exception (but "erroneous" execution can have any result, in 
principle).

Under the Ada 2005 rules, you can get away with this assignment -- 
it is not "erroneous" -- but the only thing you can then do with 
Result2 is to check if it is 'Valid (RM05 13.9.1(12/2)); all other 
uses of Result2 are "erroneous" (until it is assigned some valid 
value).

One way to avoid this error is to remove the range constraints on 
Short_integer2, or widen them to include 201. However, even after 
reading all of this thread I don't understand what you want to 
achieve, so I don't know what to suggest -- except to ask your boss 
for a clearer statement of the problem.

> 
> Chris L.
> with Unchecked_Conversion;
> 
> procedure test is
>   subtype Short_integer1 is Natural range 0.. ((2 ** 10) - 1);
>   subtype Short_integer2 is Integer range -(2 ** 7)..((2 ** 7) - 1);
> 
>   subtype A_Float is Float;
> 
>   subtype A_Natural is Natural range 0..((2 ** 8) - 1);
>   -- The next line (if used) gives compilation error message:
> A_NATURAL does not denote a first subtype
>   --for A_Natural'Size use Short_integer1'Size;
> 
>   Size_Of_Float : integer := Float'Size;                  -- 32
> 
>   Size_Of_short_integer: integer := Short_integer1'Size;  -- 10 bits
> long
>   Size_Of_short_int: integer := Short_integer2'Size;      -- 10 bits
> long
> 
>   Size_Of_Natural: integer := A_Natural'Size;             -- 8
> 
>   Arg     : A_Float;
>   Result2 : Short_integer2;
>   Result1 : Short_integer1;
> 
>   function Get_Bits is new Unchecked_Conversion (Source =>
> Short_integer1, Target => Short_integer2);
> 
> Begin
> 
> Arg := 200.0;
> 
> Result1 := Short_integer1(Arg + 0.5);  -- Result1 becomes 201
> Result2 := Get_Bits (Result1);
> 
> End test;
> 
> -- Error Message
> +++ Program  started at 01/15/09 12:14:02
> 
> Unhandled exception:
> 
> CONSTRAINT_ERROR raised in MAIN
> 
> Range_Check_Failed
> 
> Exception raised at code address: 0x413516
> +++ Program completed with exit code 1 at 01/15/09 12:14:02
> +++
> --------------------------------------------------------------------------------------------------------------

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .