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From: Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov>
Newsgroups: comp.lang.ada
Subject: Re: Announce: Grace project site operational
Date: 15 May 2002 14:15:43 -0400
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"Martin Dowie" <martin.dowie@nospam.baesystems.com> writes:

> "Hyman Rosen" <hyrosen@mail.com> wrote in message
> news:3CE15D0A.3050100@mail.com...
> [snip]
> > I first saw this idiom in _Scientific and Engineering C++_
> > by Bartion & Nackman.
> >
> > template<typename RepType, int Mass, int Distance, int Time>
> > struct Unit
> > {
> > RepType value;
> > explicit Unit(value) : value(value) { }
> > Unit operator+(Unit other)
> > { return Unit(value + other.value); }
> > Unit operator-(Unit other)
> > { return Unit(value - other.value); }
> > };
> [snip]
> 
> Didn't someone post an Ada units package similar to
> this a while back (> 1 year)?..

Yes, there are several similar Ada packages. But, all the Ada packages
either have run-time overhead (the Mass, Distance, Time values are
stored in the type, and computed and checked at run-time), or suffer
from combinatoric explosion of types.

The C++ template solution apparently computes and checks the Mass,
Distance, and Time values at _compile_ time, so there is no runtime
overhead. There is, in effect, the same combinatoric explosion of
types (actually operators in this case), but it is handled
automatically by the compiler, thru implicit instantiation (is that
the right term?). And only those operators that are actually used are
ever declared, and the _user_ only has to write the templates, not
the instantiations.

This is pretty neat. One area where C++ templates are superior to Ada
generics (as others have been saying all along :). 

-- 
-- Stephe