Re: [boost] Boost Units library preview

From: Andy Little (andy_at_[hidden])
Date: 2006-08-29 16:48:59

"Jeff Garland" <jeff_at_[hidden]> wrote in message 
news:44F4A262.4070607_at_crystalclearsoftware.com...
> Ben Strasser wrote:
>> Jeff Garland <jeff <at> crystalclearsoftware.com> writes:
>>
>>>> typedef unit<length, "m"> meter;
>>> This isn't valid code...can't give char strings to templates.
>> I was talking about an ideal world. ;)
>
> Ok, well we don't have that world...
>
>>> The problem is hardcoding the "m" anywhere -- this shouldn't be done.  You
>>> need an I/O facet that has the defaults and can be overridden by the user at
>>> runtime.  This allows different output for the same value in the same 
>>> process.
>>>   Your view of how to solve this is on the wrong course in my view...
>>
>> I depends on the situation. Some units are unique to the application. Here we
>> probably wont need any internationalization so hard coding "m" will do the 
>> job
>> just fine.
>
> Boost is an international organization with an international audience so it's
> a different context to do something only for yourself.
>
>> There are however also other units. For example length units. It might be 
>> useful
>> to let the programmer use meters in his program and have it output feet based 
>> on
>> the local in the stream.
>>
>> What I mean is something like:
>>
>> cout.imbue(new unit_out_facet<length, feet>("", "ft"));
>> wcout.imbue(new unit_wout_facet<length, feet>(L"", L"ft"));
>> //or
>> cout.imbue(new unit_out_facet<length, meter>("", "m"));
>> wcout.imbue(new unit_wout_facet<length, meter>(L"", L"m"));
>> //...
>> cout<<meter(4); // will print meter or feet based on the current local
>>
>> This seems as a good idea to me and this is possbile to implement.
>
> On the right track, but this scares me a bit.  I'm not sure I want conversions
> in the facet code.  I think I'd rather the user do that explicitly.
>
>> Input would be a bit more tricky.
>
> Input is much harder...because you only have access to an input iterator.  You
> can't backtrack.  So suppose you want to handle input like this:
>   10 m
>   10 meters
>   10 microseconds
>
> The first 2 are a valid length -- the last isn't.  Anyway, it's sticky.

FWIW I think that the problems of I/O of quantities( at least in SI Units ) are 
orthogonal to the main bulk of a quantities library. IOW its probably not that 
difficult to work on this type of functionality independent ( as far as 
possible) of a particular library.

regards
Andy Little