Hello Ben, [snip]
I picture a few reasonable casts: (0) Casting equivalent units (meters -> feet). This is very very safe and so should have its own cast (if it has a cast at all). (Perhaps quantity could just have an explicit constructor so that static_cast works? Then a precompiler definition could toggle that "explicit" for those who want this to be implicit.) (1) static_casting enclosed types (perhaps "quantity_static_cast"?) (2) Adding explicit dimensions to something that's got only general units. That is, quantity<length> -> quantity<meters> but not quantity<length> -> quantity<time>. This seems reasonably safe. (Perhaps that would get quantity_cast?) (2) a. Casting doubles to quantity<D>s. b. Casting back to the enclosed type for use in other libraries. These are both potentially dangerous, but not too bad as long as it allows only conversions to and from enclosed types (e.g., quantity<length> ->> double and double -> quantity<time>) but disallows quantity-to-quantity conversion (e.g., quantity<length> -> quantity<time>). (Perhaps reinterpret_quantity_cast?)
So what I'm proposing is three new casts: (1) quantity_static_cast to static_cast the enclosed type. (This would show up in searches for static_cast.) (2) quantity_cast to add or remove quantity information (length <-> meters). (3) reinterpret_quantity_cast to go to and from, e.g., doubles. (This would also show up in searches for quantity_cast and for reinterpret_.) This wouldn't be necessary for scalars since division and multiplication would work, but would be useful for arrays.
Sorry to interleave your discussion. I'd add (2.5) quantity_cast to translate compatible quantities, e.g. meters <-> miles. I'm also not quite happy with having three casts instead of one. IMO, this would complicate the library usage with almost nothing in return. Although I may imagine some searching profit of having quantity_static_cast, it doesn't worth it. And as for reinterpret_quantity_cast, I can really see no use for it. The value of the quantity may be obtained via a simple method, like "get" or "value" and the opposite conversion may be done with explicit constructor with applying a dimension. I'm not quite sure I followed you in "would be useful for arrays", so I may be missing something. And after all reinterpret_quantity_cast may need a better name if it is to be implemented. The difference in word order in quantity_static_cast and reinterpret_quantity_cast is confusing and I feel getting older while typing "reinterpret_quantity_cast". :) -- Best regards, Andrey mailto:andysem@mail.ru