Re: [Boost-users] boost::units - converting from one derived_dimension to another across systems (imperial to metric)

16 Nov 2018

      On 11/16/18 4:57 AM, Leon Mlakar via Boost-users wrote:
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On 15.11.2018 17:27, Michael Powell via Boost-users wrote:
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On Thu, Nov 15, 2018 at 11:11 AM Roger Leigh via Boost-users
 wrote:
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On 15/11/2018 15:18, Steven Watanabe via Boost-users wrote:
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AMDG
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I am having trouble converting from one derived_dimension to another across systems (imperial to metric).  I'd like to convert, say, lb/in to, say, kg/m.  I thought I would do the following but get a compiler error #:
It's not going to work.  lb is a unit of
force, but kg is a unit of mass.  They are not
compatible and there is no conversion between
On 11/14/2018 05:04 AM, Matt Vinson via Boost-users wrote:
them.
lb·f (foot-pounds) is force; lb is mass.
No, lb-f is a unit of Torque. lb is a unit of Force.
Hmm, that pound is primarily unit of force is a news to me. The
following is from the UK's Weights and Measures Act 1963:
"The yard https://en.wikipedia.org/wiki/Yard or the metre
https://en.wikipedia.org/wiki/Metre shall be the unit of measurement
of length https://en.wikipedia.org/wiki/Length and the pound or
the kilogram https://en.wikipedia.org/wiki/Kilogram shall be the
unit of measurement of mass by reference to which any measurement
involving a measurement of length or mass
https://en.wikipedia.org/wiki/Mass shall be made in the United
Kingdom; and- (a) the yard shall be 0.9144 metre exactly; (b) the
pound shall be 0.45359237 kilogram exactly."
To my knowledge, the US still adhere to 1894's update to Mendenhall
Order which defined  that there shall be 2.20462234pounds to a kilogram.
On the other hand, lbf is often used to denote "pound force" unit,
with 1 lbf being approximately 4.448N or 0.454 kp.
So Boost.Units should then not be used for cooking recipes as the
conversions from pound based to gram based quantities would only work
on Earth?
Cheers,
Leon
In Common, and legal, usage, the Pound represents both a mass unit and a
Force unit, a pound of food is mass, in a pound/sq-in it is a force. In
scientific uses, one shouldn't use the base pound unless it is totally
obvious which one is being used, but instead you use the units of
pound-force or pound-mass, lbf (not lb-f) vs lbm,

Boost, since it can't know which one is being used, as the context will
never be obvious to the compiler, probably should not have a raw lb
unit, but only lbf and lbm.

-- 
Richard Damon