Re: private types

["Randy Brukardt" <randy@rrsoftware.com>]

"Justin Gombos" <rpbkbq.xax.gld@uluv.kbq> wrote in message
news:FJHSf.1898$TK2.497@trnddc07...
...
> BTW- I sometimes have a use for creating an abnormal object, and find
>      Ada lacking in not providing such a mechanism.  I'm sometimes
>      forced to either include abnormal values in the set of values
>      declared for a type, or to have a seperate flag.  Both of these
>      are messy.  Why not have a 'invalid function added to Ada to
>      enable simple and explicit initialization of abnormal objects?

You're confusing an "invalid" object with an "abnormal" object. Accessing an
abnormal one is erroneous; surely you don't want to intentionally put that
into your programs. (Remember, "erroneous" is Ada-speak for "anything at all
can happen".)

The reason there is no way to construct invalid values is simple: there is
no requirement that an implementation provide any such values. Some types
(like Integer) don't have any invalid values; what would 'Invalid return for
them. (This is the reason that Normalize_Scalars doesn't require
initialization to invalid values.) Moreover, "invalid" only applies to
scalar objects; there is no similar concept for composite types. In any
case, this is a very complex area, and I don't think anyone wants to tread
here.

I'm dubious about your problem anyway; the values you are talking about are
clearly part of the value set of a type. Claiming that they are not and
trying to avoid including them is just confusing. "Null", for instance, is
just another normal value (there are no valid or invalid values for access
types, as they are not scalar) for an access type. Sure, it means "no
object", and that's a bit unusual, but it's clearly an expected value.

                                     Randy.