[boost] Re: physical-quantities

From: Jan Langer (jan_at_[hidden])
Date: 2004-01-09 14:40:38

Dan W. wrote:
>> This absolute vs. relative quantity issues arises pretty much  
>> everywhere.  For example,
>> if I have two points along the x-axis, one at 7 meters and another at  
>> 12 meters, what does
>> it mean to add the two?  Nothing really.  We can add an increment in  
>> distance to either one,
>> to get a new absolute position or we could add two increments to get  
>> another increment, but
>> adding two absolute quantities doesn't really make sense.  In the 
>> same  way, if I write
>>
>> 32*_fahrenheit+32*_fahrenheit = 64*_fahrenheit
>>
>> it makes perfect sense if one or both of the terms on the left is a  
>> relative quantity, but not
>> if both are absolutes...
> 
> 
> That's right. I'd define a Farenheit_diff such that
> Farenheit + Farenheit_diff -> Farenheit; ok
> Farenheit + Farenheit -> ERROR;
> Farenheit_diff + Farenheit_diff -> Farenheit_diff; ok
> 
> Same thing goes for Date and days_diff, point3d and vector3d, etc.

i disagree. i think this issue is not so much related to unit or 
dimensional analysis in the first place. it is possible with each and 
every unit. i think it would lead to a much more complicated design.

most people wont need it. but i think it could be dealt with by means of 
an appropriate value type. this might also be useful in other scenarios.

the same goes imho for non-linear scales like dB and so on.

> So, it would be unwise to allow addition of apples and apples by 
> default... :)

i dont understand this.
jan

-- 
jan langer ... jan_at_[hidden]
"pi ist genau drei"