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