[boost] Re: physical-quantities

From: Dan W. (danw_at_[hidden])
Date: 2004-01-09 14:04:28

Matthias Schabel 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.

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