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