Re: [boost] Re: Re: quantity libraries

From: George A. Heintzelman (georgeh_at_[hidden])
Date: 2001-08-30 12:11:00

> Simple first reason, what is the conversion of kilograms to pounds? 2.2
> pounds to the kilogram right? WRONG! There is no conversion between the two,
> kilograms measure mass ( a constant ) and pounds measure weight  ( a
> relative measure of mass * gravity ). If the quantity library allows both
> systems some compiler manufacturer will implement that wrong conversion and
> it will inevitably sneek into some calculation where it does matter.

A correct quantity library will of course supply conversions between 
kilograms and slugs (the English unit of mass; a mass of 1 slug weighs 
~32 pounds at nominal Earth-gravity) and Newtons and pounds 
respectively. On the other hand, since pounds are frequently conflated 
with mass in colloquial usage, it would probably not be inaccurate to 
supply the constant pound_mass = 1/32 slug (= 1/2.2 kg).

Of course, a correct quantity library will quickly detect an error in 
useage, as any attempt to use something measured in the force unit of 
'pounds' where a mass was required will trigger a compile-time error.

Any quantity library desiring to be useful to US engineers, craftsmen, 
and builders will *at least* have to supply instances of the constants 
'pound' and 'inch' and so forth. Thus this error is potentially present 
in either case. Most likely a library will also have to supply a value 
for 'g', the earth's acceleration due to gravity, in order to be useful 
for earth-bound systems anyway.

I don't think that this complaint is a reason to restrict users to SI 
units.

Regards,

George Heintzelman
georgeh_at_[hidden]