[boost] Units: brief review

From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2007-03-26 16:51:14

I have spent a couple of hours looking at the units library.  This is 
not a full review as I have not been able to compile my simple example 
in the time that I have available.

I was motivated to write this review having yesterday needed to do some 
rather basic, but unfamiliar, physical calculations.  At that time I 
used a simple awk script to do the arithmetic.  If dimensional analysis 
had been available, it would have been a useful confidence builder that 
the code was doing the right thing.  The problem was this: I have 
measured the temperatures on the surfaces of an electrical device - a 
power supply - and I want to calculate the power dissipated.  The maths 
is simple: power = temperature difference to atmosphere * area * heat 
transfer coefficient.  I tried to write this using the units library.

Power, temperature and area were straightforward:

typedef quantity<power>         power_t;
typedef quantity<area>          area_t;
typedef quantity<temperature>   temp_diff_t;

The units of the heat transfer coefficient are W C^-1 m^-2.  As far as 
I was able to see, I was not able to write anything like

typedef quantity<power/temperature/area> heat_transfer_coefficient_t;

Rather, it was necessary for me to reduce W C^-1 m^-2 to basic 
dimensions manually.  This is not easy for me.  I have to remember 
phyics that I knew in 1988 but have not used much since.  My guess was:

power = energy / time
energy = force * distance
force = mass * acceleration
acceleration = length / time^2

so I concluded that the dimensions of power are mass length time^-3 and 
hence of heat transfer coefficient are mass time^-3 temperature^-1 
length^-1, and wrote this:

typedef composite_dimension<mass_tag,1,time_tag,-3,temperature_tag,-1,length_tag,-1>
   heat_transfer_coefficient_t;

Defining the unit was easier:

const heat_transfer_coefficient_t watts_per_square_meter_per_celcis
   = watts / square_meter / kelvin;

But this fails to compile, with this error:

error: conversion from 
‘boost::units::unit<boost::units::dimension_list<boost::units::dim<boost::units::mass_tag, 
boost::units::static_rational<1l, 1l> >, 
boost::units::dimension_list<boost::units::dim<boost::units::time_tag, 
boost::units::static_rational<-0x00000000000000003l, 1l> >, 
boost::units::dimension_list<boost::units::dim<boost::units::temperature_tag, 
boost::units::static_rational<-0x00000000000000001l, 1l> >, 
boost::units::dimensionless_type> > >, 
boost::units::homogeneous_system<boost::units::SI::system_tag> >’ to 
non-scalar type 
‘boost::units::composite_dimension<boost::units::mass_tag, 1, 
boost::units::time_tag, -0x00000000000000003, 
boost::units::temperature_tag, -0x00000000000000001, 
boost::units::length_tag, -0x00000000000000001, 
boost::units::dimensionless_type, 0, boost::units::dimensionless_type, 
0, boost::units::dimensionless_type, 0, 
boost::units::dimensionless_type, 0>’ requested

Presumably this is because I have got something wrong in my working.  I 
gave up at this point.

For this library to be useful for me, it needs to be quick to learn and 
easy to apply.  Working only with the included units it does look like 
it would work reasonably well, but in the situation I have described 
above it was quickly obvious that it was taking more effort to apply it 
to my program than the benefit that would result.

There are two areas that I was interested in looking in more detail at, 
and I encourage other reviewers who get further than me to have a look 
at them:

1. What is the effect on compile time?  I used #if to switch between 
units and plain floats.
2. Is there any increase in object file size?  I know there shouldn't 
be, but it would be interesting to know for sure.
3. Are the error messages comprehensible?  The one show above is not 
great, but it could have been worse; there are some Boost libraries 
which I find unusable because of the volumne and inpenetrability of the 
error messages that result from a simple typo (though the compiler must 
share the blame for this).

Finally some random notes from my reading of the documentation:

- "io" is a misnomer since it only does output, as far as I can see.

- I tend to refer to temperature differences in Celcius, rather than 
Kelvin.  There is an obvious issue when dealing with absolute 
temperatures though.

- "Meter" vs. "Metre".  My dictionary says "meter" is "chiefly U.S.".  
My physics books say "metre" consistently.  Presumably both can be included.

- Am I the only person who uses capital letters for units named after 
people?  Hmm, maybe I am.

- Please don't call the units used in the U.S. "English" units.  Here 
in England, we use the S.I. system for everything except pints of beer 
and miles on roadsigns.  The units that we did use here until about 50 
years ago were not the same as the ones that the Americans use.

I hope this helps.  I will not express an opinion about whether the 
library should be included; I leave that to people who have made more 
progress with it.

Phil.