[boost] Re: physical quantities (was: runtime unit-modifiable ...)

From: Jan Langer (jan_at_[hidden])
Date: 2004-01-07 14:07:11

Andy Little wrote:
>>unit::Speed s = 2 * unit::m / unit::s;
> 
> My syntax in my physical_quantities type for this is:
> 
> q_velocity::m_div_s  s(2);

i know. but there is imho nothing wrong with both syntaxes. the first 
one constructs a quantity from another one with a non-explicit ctor like:
        quantity (quantity const &q2);

and your syntax uses a ctor like:
        explicit quantity (value_type v);

i really dont see a problem. the above syntax would also allow 
constructions from more complex expressions.

> IOW the units are part of the type declaration.
> The q_ prefix is to prevent name collisions.
> Unless I'm missing something it is also as legible if not more so and
> if operator *()  is actually a runtime multiplication in the top example
>  over which there seems to be some confusion,  better in raw performance
> terms,
> (no runtime multiplication involved in my declaration).
> which I had thought was a critical factor to many.

yes but there is nothing which cannot be optimized away. meter is a 
constant and 2 too. but the whole dimensions lib will not work without 
sufficient compiler optimizations, so i dont think this will mean a problem.

actually its similar to the case of the sine function.

you have to write
        double v = 2 * sin (x);

which is imho very similar to
        length l = 2 * meter;

jan

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