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From: Noah Roberts (roberts.noah_at_[hidden])
Date: 2007-02-14 12:44:02

Steven Watanabe wrote:
> Noah Roberts <roberts.noah <at>> writes:
>> "Converting the second to match the first" doesn't make sense to me. I
>> guess you might be talking about operands of '+'? Yeah, not necessary.
>> The answer is pretty simple. All arithmetic operators work in the base
>> system. The result is converted on assignment if necessary.
> Ok. What happens here:
> std::cout << (x + y) << std::endl;

Nothing good would probably come of it the way I see things; but since
mcs does magic unit label stuff it could just output that. Without,
allowing for the user to override certain behavior this could be more
well behaved:

std::cout << ((x + y) * unit) << std::endl;

You're probably not too worried about the moderate overhead of the extra
work there when I/O is going to be bottlenecking anyway.

Except in debugging, the developer is going to have a pretty strong idea
of what units need labels and create them. Other units and dimensions
are of no consequence.

> The former is exactly what I said. It stores
> two doubles instead of one. Further,
> temp = a + b;
> c = temp * d;
> is not equivalent to
> c = d * (a + b);
> Every operation incurs an extra multiplication
> per operand. This is a far cry from zero overhead.

But you don't have to pay this cost unless you need to. Imagine:

class qty_withunit;
class bare_qty;

template < typename T1, typename T2 >
bare_qty operator * ( T1 const& l, T2 const& r) { return bare_qty(...); }

greatly simplified for posting purposes...the return type alone is a few
lines of code.

quantities have assignment ops that react to anything that is_quantity<>
as do all operators.

You then write your functions like so:

bare_qty f(bare_qty) { some calcs and a return; }

and use them like so:

qty_withunit x(unit);
x = f(x);

and possibly:
qty_withunit y = f(x) * unit;

Yes, there is an overhead to the unit but it is localized to the places
where conversion is necessary. The library user does have to use some
care, and I haven't thought of a way to allow them to use this stuff
willy nilly in something needing extreem performance, but I don't think
that is too much to expect.

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