Re: [boost] Units: brief review

From: Steven Watanabe (steven_at_[hidden])
Date: 2007-03-26 17:20:14

AMDG

Phil Endecott <spam_from_boost_dev <at> chezphil.org> writes:

> 
> 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.
> 
> <snip>
> 
> 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;

 typedef unit<
   composite_dimension<
     mass_tag,1,
     time_tag,-3,
     temperature_tag,-1
   >::type,
   SI::system
 > heat_transfer_coefficient_t;

or

 typedef divide_typeof_helper<
   divide_typeof_helper<
     SI::power,
     SI::temperature
   >::type,
   SI::area
 >::type 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
> 
> <snip>
> 
> 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.

It depends on how many different units you are using.

> 2. Is there any increase in object file size?  I know there shouldn't 
> be, but it would be interesting to know for sure.

suppose you have a function

  template<class T>
  T f(const T&);

Now, if you use float will only be instantiated once.
If, on the other hand you use quantity f will be instantiated
for every unit.

> 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).

The main problem is that the types store a tremendous amount
of information and therefore have very long names.

> 
> 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.

Uh. right.

> 
> - 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.

They are.

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

http://www.boost.org/more/lib_guide.htm#Design_and_Programming

In Christ,
Steven Watanabe