Re: [boost] mcs::units informal review request

From: Steven Watanabe (steven_at_[hidden])
Date: 2007-02-19 14:14:11

AMDG

Noah Roberts <roberts.noah <at> gmail.com> writes:

> > Then why go to the trouble of providing
> > all the arithmetic functions?
> 
> Because they are occasionally useful.
> 
> dp = inlet - outlet;
> 
> vs.
> 
> dp = (inlet.quantity() - outlet.quantity()) * dp.unit();

Well, ok.  I personally prefer the latter, But
I can see why some might not want to type more.
I would definitely want

dp = (inlet - outlet) * dp.unit();

> 
> >> quantity f() { static quantity const C = 9.43 * psi; ... equation 
> >> written with psi constant ... }
> > 
> > I don't understand what is wrong with
> > 
> > quantity<psi_type> f() {
> >     static quantity<psi_type> const C = 9.43 * psi;
> >     ... equation written with psi constant ...
> > }
> 
> Well, if we are talking about in terms of the current proposal I believe 
> that would result in a lot of conversions from psi to pascals (assuming 
> si base).  Because now C is in some psi system whereas the rest of the 
> function uses the SI system.  Incredibly inefficient.

How was I supposed to know that the rest of
the function was in SI?  Even so,

quantity<SI::pressure> f() {
    static quantity<SI::pressure> const C =
        quantity_cast<SI::system>(9.43 * psi);
    ... equation written with psi constant ...
}

OR

quantity<psi_type> f() {
    static quantity<SI::pressure> const C =
        quantity_cast<SI::system>(9.43 * psi);
    ... equation written with psi constant ...
}

> 
> > 
> >>> quantity<pressure> p1(psi);
> >>> quantity<pressure> p2(pascals);
> >>> p1 = whatever;
> >>> p2 = p1;
> >>>
> >>> Now, what should the unit of p2 be?
> >> pascals.  The rule is conversion on assignment.  This does cause some 
> >> need to be careful with such things as std containers that work through 
> >> assignment.
> >>
> > 
> > Right.  I can't see a way to make a std container
> > that holds quantities of different units safe at all.
> 
> Most commonly you would not want them to convert to a different unit on 
> assignment.  You would want the elements in your container to stay a 
> certain unit while converting whatever value is being assigned to that 
> unit.  When that is not what is wanted a wrapper is needed.
> 

typedef runtime_quantity<SI::length> length_t;
std::vector<runtime_quantity<length_t> > v;
//fill v
v.insert(v.begin(), ...);
//the units of the elements are now unspecified.

A type that is almost a value type but isn't
quite is asking for trouble.

In Christ,
Steven Watanabe