Re: [boost] Boost Units library preview

From: Deane Yang (deane_yang_at_[hidden])
Date: 2006-08-27 00:09:24

Andy Little wrote:
> "Deane Yang" <deane_yang_at_[hidden]> wrote in message
> news:ecpndi$2mb$1_at_sea.gmane.org...
>> Andy Little wrote:
>>
>>>> Can the dimension checking part of the library be used without the SI
>>>> unit system? For most other unit systems I use, I mainly need
>>>> dimension checking and not conversions between different units, such
>>>> as km or m? Thus just a quantity with dimensions would be useful to
>>>> catch programming errors.
>>> Yes.  In the signature fixed_quantity<Unit, ValueType>, the Unit parameter
>>> can
>>> be so modelled that it only allows base units to be used, and in this case
>>> there
>>> is no conversion overhead BTW, a multiply is simply a multiply of the numeric
>>> values for example).  By default Quan wont work across Unit models (IOW the
>>> class template names of units must match), which means that attempting to do
>>> calculations between Units with different class template names will fail at
>>> compile time. By this means you can guarantee that you can prevent any
>>> conversions. (FWIW there is also the option to allow  unit conversions but
>>> make
>>> them explicit, prompted by Deane Yang easrlier in this thread.)
>>>
>> I guess I wasn't particularly clear earlier in this thread, because
>> fixed_quantity appears to be what I want. I'm sorry for not RTFM (I did
>> try to read the documentation once, but my memory is not what it used to
>> be), but I have two quick questions:
>>
>> 1) Does operator*(fixed_quantity<Unit1, ValueType1> x,
>> fixed_quantity<Unit2, ValueType2> y) work and, if so, what is the result
>> type, assuming that operator*(ValueType1 v, ValueType2 w) returns a
>> ValueType3?
> 
> OK. The following relates to the CVS version currently on the two_param-branch,
> but much applies to previous versions of Quan/PQS.
> 
>  Dealing with the ValueTypes first, the result type of ValueType1 * ValueType2
> is encoded in the compile time expression:
> binary_operation<ValueType1,times, ValueType2>::type.
> The implementation provides a specialisation for inbuilt types, and this follows
> the promotion/conversion  rules on the respective calc in the C++ standard.
> We have also been recently testing with boost::numeric::interval and (with the
> appropriate specialisations) this seems to be working now and I believe that
> other numeric UDT ValueTypes will now work OK.

Does this mean that if ValueType1, ValueType2, and ValueType3 are UDT's 
and I do something like:

template <>
struct binary_operation<ValueType1,ValueType2>
{
   typedef ValueType3 type;
};

then everything will work properly?

> 
> The dimension of the result is found by adding the respective dimensions of the
> operands. ...
 > <snip>

Oops. I guess fixed_quantity has to have one of the physical dimensions 
represented in SI units, eh? Never mind.