Re: [Boost-users] [units]

Subject: Re: [Boost-users] [units]
From: Michael Powell (mwpowellnm_at_[hidden])
Date: 2011-08-30 08:31:29

On Tue, Aug 30, 2011 at 6:29 AM, Michael Powell <mwpowellnm_at_[hidden]>wrote:

>
>
> On Tue, Aug 30, 2011 at 2:04 AM, Thomas Taylor <thomas.taylor_at_[hidden]
> > wrote:
>
>> Could you please review the following patch for inclusion in boost::units?
>>
>> *WHY*: Obviously this only defines a conversion for boost::units::one to a
>> numerical 1. When trying to convert at runtime (aiming for a
>> runtime/dynamic
>> unit instead of the compiletime/static unit provided) I use the following
>> function:
>>
>> template <typename U1, typename U2>
>> double unit_dispatch2t(U1 const& u1, U2 const& u2) {
>>    return bu::conversion_factor(u1,u2);
>> }
>>
>> where U1 and U2 are boost::unit::unit derivatives.
>>
>> Without a conversion operator the compiler chickens out by claiming to not
>> be able to convert boost::units::one to (in my case) double. The added
>> conversion operator allows successfull compilation, thus allowing actual
>> runtime units (my application needs to handle calculations where some
>> units
>> are known at compiletime and some only at runtime (e.g. the user wants the
>> calculation to be done in meters or millimeters)).
>>
>
> Oh, the runtime question. Here's how we've decided to approach this.
>
> I'll just tell you, I find the compile-time safety attractive. For runtime,
> we've decided that calculations should be done in a base unit.
>
> For anything else, we *convert to* a view unit, for display or report or
> what not.
>

I should clarify too, in this type of use case, we still know what the
compile-time units should necessarily be. The runtime I like to think of is
the user selecting: system, and possibly also unit. Which is what your
aiming for here?

Use case: calculations on volume, pressure, etc, done in base SI units. View
> may be in SI, US (English), or what have you.
>
>  HTH,
>> Thomas
>>
>>
>>
>> --- one.hpp     2011-08-30 09:50:13.000000000 +0200
>> +++ one2.hpp    2011-08-30 09:50:07.000000000 +0200
>> @@ -19,3 +19,3 @@
>>
>> -struct one { one() {} };
>> +struct one { one() {} operator int() { return 1; } };
>>
>>
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>
>