Re: [boost] units: review

From: Matthias Schabel (boost_at_[hidden])
Date: 2007-03-29 16:50:02

> Hmm.  I still don't understand the rationale for the extra
> multiplication
> operation.  How is this any different from:
>
> quantity<SI::meter> q(2);

It's a faux operation - the multiplication of a scalar times a unit  
(a class with no data members at all) decorates the scalar to produce  
a quantity of the appropriate unit and value type. The problem with  
using a raw value type for construction can be demonstrated here :

using namespace SI;

/// this is two meters
quantity<length>	q(2);

...some months later...

using namespace CGS;

/// uh oh - this is 2 centimeters now
quantity<length>	q(2);

The way it's currently implemented, you incur a conversion in the  
constructor, but the code remains correct if the units are  
convertible and gives a compile time error if they are not...

Matthias