Re: [boost] Interest in dimension/unit-checking library?

From: Matt Calabrese (rivorus_at_[hidden])
Date: 2005-10-26 12:32:45

On 10/26/05, Noah Stein <noah_at_[hidden]> wrote:

> Unless I'm mistaken, this is only a theoretically lossless operation. In
> the real world of precision issues and whatnot, a conversion back and
> forth
> might result in a slightly different answer.

Of course, but such is true for any operations you perform on floating point
types, not just unit conversions. For instance, if you multiply two float
values together, the result does not have perfect precision. a * b / a does
not necessarily yield the same value as a, just like meters_a + feet_b -
meters_a doesn't necessarily result in the same value as meters_a. These
aren't problems with implicit casting of unit types, they are there with
just about all floating point operations other than negation, and they exist
because floating point types simply can't have infinite precision without
taking up an infinitely large amount of memory. Forcing explicit casts on
unit types does not solve this since the floating point operations you'd be
using which would require casts would have limited precision regardless of
whether or not unit conversion takes place.

Aside from all of that, keep in mind that implicit conversions occur only
where you'd be explicitly casting anyway, so really you're just making the
user type more to perform simple operations. If for some reason you want to
mark the conversion explictly with a unit cast, there is nothing stopping
you, just like you can always explictly cast fundamental types in C++,
however I don't see why that should be required. Really, the main place it's
important to explicitly cast would be as the last operation of an entire
unit expression, since you may want the end result to be in a particular
unit type such as for output or to extract that raw value of a given
quantity such that it can be used in more low-level operations.

Other than situations like that, the choice for explicit casting should be
left up to the user. The point of the library is to make working with units
easy as well as potentially more optimized, not unecessarily complicated
without gain. Especially in cases where the unit types of the operands may
or may not be the same depending on template arguments, forcing explicit
casts becomes horribly complex for no reason, and even makes the code harder
to read and difficult to write correctly not to mention efficiently. Making
necessary conversions automatic through non-explicit constructors solves all
of those problems.

--
-Matt Calabrese