Re: [boost] Boost Units library preview

From: Deane Yang (deane_yang_at_[hidden])
Date: 2006-08-21 18:27:32

Andy Little wrote:
>>>>> This brings up a good question.  Say you have two objects:
>>>>>
>>>>> meters m = 1;
>>>>> feet f = 2;
>>>>>
>>>>> What is the type (or unit) of the following expression?
>>>>>
>>>> m + f;
>>>>
>>> I apologize for repeating myself, but I want this to fail to compile. I
>>> do not want implicit conversion of units. I believe I am not completely
>>> alone in this view.
>> Personally I love implicit unit conversions. :-)
>> I can add a function to prevent them, but you will have to apply that
>> explicitly:
>>
>> T b = no_cast(a);
>>
>> or some such.
>>
>> How's that?
>>
>> regards
>> Andy Little
> 
> For addition you would need to do this:
> 
> nocast(a) + nocast(b);
> 
> for multiplication
> 
> nocast(a) * nocast(b);
> 
> You could make this a general purpose utility:
> 
> template <typename T>
> struct no_caster{
> T t;
> /*...*/
> };
> 
> template <typename T1, typename T2>
> typename boost:enable_if<
>     boost::is_same<T1, T2>,T1
>> ::type
> 
>  operator Op (nocaster<T1> ,nocaster<T2>);
> 

My main objection to this is that I am pretty sure that a quantity/units 
library that does not do implicit unit conversion is a *lot* easier to 
implement than one that does. And it's hard for me to believe that a 
library that does automatic unit conversion doesn't have inside of it a 
simpler non-automatic unit conversion library. So implementing explicit 
unit conversion on top of an implicit conversion library seems, well, 
backwards.

Deane