Re: Null Record is not always Null

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Colin_Paul_Gloster@ACM.org (Colin Paul Gloster) writes:

> Peter Richtmyer posted different behavior on three compilers and said:
> "Don't know if there is a "right" result, or is this one of 
> those gray areas. Anybody?"
> 
> Well, though 13.12 re pragmas has:
> 
> "[..]
> 
> An implementation may place limitations on the values of the expression
> that are supported, and limitations on the supported combinations of
> restrictions. The
> consequences of violating such limitations are implementation defined.

13.12 is not talking about pragmas in general.  It is talking about
pragma Restrictions, so this quote is irrelevant.

> a pragma should never stop a compilation.

Right -- compilers should never crash.

> As for the rest, clearly you were given suboptimal results for your
> possibly wacky null record but compilers are allowed to ignore your
> requests:
> 
> "[..]
> 
> [..] A representation or operational item that is not supported by the
> implementation is illegal, [..]
> 
> An implementation may interpret aspects of representation in an
> implementation-defined manner. An implementation may place
> implementation-defined restrictions
> on representation items. [..]" (13.1).

Yes, but you also have to look at all the "Recommended Levels of
Support" (which are *requirements* if the compiler supports the SP
annex), and see if it talks about 'Size of null records.  It doesn't,
but it does talk about 'Size of things like "range 0..-1".

> In 13.3 is "nonnegative" chosen to denote >0 or >=0?

Zero is not negative.
But it's pretty close.  ;-)

By the way, it would be helpful if you specified paragraph numbers when
referring to the RM, as in 13.3(48) for this paragraph:

> "[..]
> 
> Size may be specified for stand-alone objects via an
> attribute_definition_clause; the expression of such a clause shall be
> static and its value nonnegative.
> 
> [..]"

So this means that a compiler cannot support "for T'Size use -1;",
but it *can* support "for T'Size use 0;".  You have to look at the
Recommended Levels of Support to see whether it's *required* to support
"... use 0".

- Bob