Re: User defined type conversion

[Adam Beneschan <adam@irvine.com>]

On Thursday, May 9, 2013 7:07:33 AM UTC-7, Vinícius Franchini wrote:
> Hi Folks,
> 
>  I have a problem with a type conversion. Hereby is what I'm trying to do:
> 
> type AB is tagged
>   record
>     A : Unsigned_16;
>     B : Unsigned_16;
>   end record;
> 
> type abcde is tagged
>   record
>     a: Unsigned_8;
>     b: Unsigned_8;
>     c: Unsigned_8;
>     d: Unsigned_8; 
>   end record; 
> 
> X : AB;
> Y : abcd;
> 
> X := Y;
> 
> I'd like to do a direct conversion without creating a method for it.

Yes, I'd often like the program to do what I want done without having to write any code.  Unfortunately, Ada is not expected to have mind-reading capabilities until at least Ada 2030.

I don't know what you mean by "direct conversion".  Whatever you're doing, it's not all that direct.  Obviously, using Unchecked_Conversion on the entire record type is a very bad idea since these are tagged records.  There's going to be at least one piece of data in Y that is not the same as any data in X, i.e whatever data the compiler sets aside for "tag" information.  So you'll have to define your own function

    function To_AB (Source : ABCD) return AB is ...

How you write it depends on what you're trying to do.  If the Unsigned_* types are the ones defined in Interfaces, and you're trying to combine Source.a and Source.b into an A field, and Source.c and Source.d into a B field, I'd probably just use the "or" operator and Shift_Left to produce the results, something like

    begin
        return (A => Shift_Left(Unsigned_16(Source.a), 8) or 
                     Unsigned_16(Source.b),
                B => Shift_Left(Unsigned_16(Source.c), 8) or 
                     Unsigned_16(Source.d));
    end;

or something similar depending on exactly what you're trying to do.  I haven't tested the above.

                        -- Adam