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

From: Steven Watanabe (steven_at_[hidden])
Date: 2007-02-12 18:35:37

AMDG

Matthias Schabel <boost <at> schabel-family.org> writes:

> As Steven pointed out, this is not a detail - how would you write a  
> template
> specialization of mpl::times<> that works for an arbitrary mpl::list?  

Well, it is possible, but not a good idea.

template<>
struct mpl::times_impl<mpl::list0<>::tag, mpl::list0<>::tag> {
    template<class X, class Y>
    struct apply {
        //...
    };
};

> 
> > quantity<SI::length> cent(cents);
> > cent = 2 * meters;
> > assert(cent.value() == 200);
> 
> I have no idea what this means. Cents == centimeters? If so,  
> centimeter is not an SI unit of length. And with no
> value, the first line doesn't give you a quantity anyway...

I see.  What Noah is asking for is

template<class Unit, class T>
class quantity {
//...
private:
    T val_;
    T multiplier;
};

IMO, separating val_ into two factors
is not generally useful.

> > quantity<SI::length> unknown(user_unit);
> > unknown = 5 * meters;
> > quantity<SI::length> m(meters);
> > m = unknown;
> > assert(m.value() == 5);
> >
> > void f_cgs(quantity<CGS::length> l);
> > f_cgs(quantity_cast<CGS>(cent));
> >
> > The syntax is not necessarily what mcs does or would do but the  
> > concepts
> > are what is important.
> >
> > It's been argued that this adds unnecessary complexity or would cause
> > inefficiencies.  I don't agree.
> 

It does add overhead -- about 2x.  This may be
fine for many uses, but for some it would
be unacceptable.

In Christ,
Steven Watanabe