Re: [boost] Re: new compile-time dimensional analysis and unitlibrary

From: Walter Landry (wlandry_at_[hidden])
Date: 2003-12-15 20:37:37

Matthias Schabel <boost_at_[hidden]> wrote:
> > | It is not uncommon to have expressions like sin(theta + theta^2).  It
> > | sure doesn't look clear to me that theta and theta^2 should have
> > | different units.  They should certainly be allowed to mix.
> > |
> > This might be what you want, but it may conflict with what dimensional
> > analysis/checking system is possible in C++.
> >
> > To achieve this in practice, could you reasonably require the user to  
> > cast to
> > some POD?
> >
> > sin(static_cast<float>(theta) + static_cast<float>(theta^2) );
> 
> There are a couple of options for those who don't feel that degrees and  
> radians
> should be classified as units :
> 
> 1) continue to use the <cmath> functions and go on your merry way.

That seems pointless

> 2) extract the value from the unit like this :  
> std::sin(theta.value()+(theta*theta).value())

Ugly

> 3) convert the units to dimensionless like this :  
> std::sin(theta/_radians + (theta/_radians)^2)

Ugly

> 4) write your own dimensional analysis/unit library and prohibit the
> use of angle tags somehow.

Unhelpful

> I personally feel like this discussion is largely tangential

Yes

> (and am under the distinct impression that Mr. Landry couldn't be
> bothered to give the SI web site I referenced his time)

No.  It doesn't address the theta + theta^2 issue either.

> Whether or not your personal religion admits degrees, radians,
> bunches of grapes, dozens, or any other random existing or future
> tag is largely irrelevant.  There are people for whom it will be
> helpful in decreasing the error potential of their code to be able
> to use dimensional analysis, and those people are the users I'd like
> to target with this library.  If you don't like the fact that
> radians appear in the SI unit system, there is no compulsion to use
> them nor any consequence to not doing so.

I have no problems with making degrees a unit.  Those shouldn't be
mixed with anything else without conversions.  But radians are a
different matter.  If I take arctan(1.0), I expect a number, not
something with units.

Regards,
Walter Landry
wlandry_at_[hidden]