Re: [boost] [Units] Unique ordinal values (was Re: Review)

From: Michael Fawcett (michael.fawcett_at_[hidden])
Date: 2007-04-05 14:17:29

On 4/5/07, Matthias Schabel <boost_at_[hidden]> wrote:
> > http://thread.gmane.org/gmane.comp.lib.boost.devel/101946/focus=102070
> >
> > Would that be enough?  Usage would look like:
> >
> > #include BOOST_UNITS_ORDINAL_GENERATOR()
> > struct my_unit : ordinal<BOOST_UNITS_NEXT_ORDINAL>
> > {
> > };
>
> I believe the problem with this is that it doesn't guarantee
> uniqueness across translation units; Steven understands these issues
> better than I do, though.

I think that problem only arises if you actually define units in a
source file instead of a header file.  That shortcoming could just be
documented, since it seems pretty rare that someone would define their
custom unit in a source file instead of a header.  If they really
wanted to do that, they could still just use the current method.

A quick test with VC8.0 just now shows that even __COUNTER__ has that
problem when used in source files, but in headers Paul M.'s method
works just fine.

--Michael Fawcett