Re: [boost] Re: rational interface

From: Andras Erdei (aerdei_at_[hidden])
Date: 2005-03-10 15:55:58

Jonathan Turkanis wrote:

>>i think we need two parameters: one for specifying the precision,
>>and one for specifying the variant to use
>
> But this leaves out the int_type; users must be able to write
>
>          rational<big_int>
>
> or similarly for any UDT which supports the euclidean algorithm.

[personally i don't like the name big_int, i always have to translate
it mentally from meaning "arbitrary/big, but limited precision" like a
modular arithmetic based integer type, to "unlimited (dynamic) precision"]

i'm not entirely convinced that the unlimited precision rational should
be treated (at the interface level) as a special case of the fixed
precision rational type: rounding/checking does not make sense with it,
it may turn out to require a (sadly) slightly different interface (e.g. it may
be better off with allowing expression templates to be used -- although that
could be true for limited precision as well), and the implementation will
surely not share code with that of limited precision

keeping it separate may also solve our backward compatyibility problem
(having no rounding by default): if rational<> is kept for unlimited (but
allow instantiation with any type), then we'll still have the legacy
behaviour, and we can choose a new name for limited rational, e.g. rational
being an alias for unlimited::rational, and having separate limited::rational
and unlimited::rational

> I believe precision should be specified by supplying an appropriate type as the
> int type, e.g., restricted_int<1000>.

the point of specifying the limit would be to free the user from having to
portably figure out which is the fastest type on the given platform whose
precision exceeds the limit given (not having to specify a type at all)

>>2. i don't know how can we implement the selection of UDTs: let's say
>>i have implemented a modular arithmetic bigint type, with up to a few
>>hundred bits of precision -- how can i tell rational that it can
>>select this as the underlying type?
>
> It has to be specified as a template parameter. Even if we specified a whole
> range of types, including big_int, to serve as the underlying type depending on
> the specified precision, an arbitrary user-supplied type would never be
> selected.

i was hoping for some solution that e.g. allows UDTs to be somehow registered
(at compile time) and then selected by rational automagically, but again, i'm
not sure it is good what i hope for -- maybe it's simpler and more natural to
say rational<modular> than register(modular) and then rational<128>

> On top of this could be built a restricted precision rational class.

if you mean rational<10000,round> will translate to something like
rational<int,int,...,round>, and the latter would also be directly available
to users who need it, then of course, i just wouldn't make it the top-level
interface, rather something for advanced users who know what they are doing

br,
andras