Re: [boost] Boost Units library preview

From: Andy Little (andy_at_[hidden])
Date: 2006-08-21 18:48:23

"Deane Yang" <deane_yang_at_[hidden]> wrote in message 
news:ecdc0m$n5l$1_at_sea.gmane.org...
> 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.

Not really. It is much more difficult to *somehow* enable implicit unit 
conversion for those (like me ) that want it, than it is to provide a means to 
disable it, locally, for those that don't want it.

regards
Andy Little