Re: Valid Attribute and Unchecked Conversion

[kst@thomsoft.com (Keith Thompson)]

In <dewar.844696412@schonberg> dewar@schonberg.cs.nyu.edu (Robert Dewar) writes:
> Keith says
> 
> "For example, the language could have defined two additional attributes,
> analagous to 'Pos and 'Val, but operating on the internal representation
> of an enumeration type.  If the argument is an invalid representation,
> simply raise Constraint_Error.  GNAT provides half of this with its
> 'Enum_Rep attribute."
> 
> The intention in Ada 95 is that both operations are achievbable using
> unchecked conversion. It is a bug in the RM if this is not the case.

Ok, but unchecked conversion to what?  Since you can't get at the
underlying representation values as static expressions, there's no
portable way (that I know of) to declare an integer type of the same
size as an arbitrary enumeration type.  If I could write

    type Int_Type is range Enum_Type'Blorch(Enum_Type'First) ..
                           Enum_Type'Blorch(Enum_Type'Last);

(where 'Blorch is the non-existent attribute that returns the internal
code for an enumeration value), I could safely do unchecked conversions
between Int_Type and Enum_Type (if I ignore 13.9.1(12)).

Something like this is *probably* close enough:

    type Enum_Type is (...);
    for Enum_Type use (...);
    type Int_Type is range 0 .. 2**Enum_Type'Size-1;
    -- OR type Int_Type is mod 2**Enum_Type'Size;
    for Int_Type'Size use Enum_Type'Size; -- superfluous?
    function To_Int  is new Ada.Unchecked_Conversion(Enum_Type, Int_Type);
    function To_Enum is new Ada.Unchecked_Conversion(Int_Type, Enum_Type);

but it won't quite work if some of the enumeration codes are negative.

> However, in practice, of coruse this works, no matter what it says in
> the RM, so this point can be left moot without having any pragmatic 
> effect.

Moot in both senses of the word!  8-)}

I don't agree that there's no pragmatic effect.  The effect is not on
implementers, but on users.  Even if all implementations happen to behave
"sensibly" in this area, a careful user trying to write portable code
will avoid anything that the RM says is erroneous.

I think we need a binding interpretatin saying something like "When we
said 'erroneous' in 13.9.1(12), we were only kidding.  Yeah, there's
the ticket!"  8-)}  8-)}

-- 
Keith Thompson (The_Other_Keith) kst@thomsoft.com <*>
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