Re: [Boost-users] [Units] help with type-safe unit-specific conversions

Subject: Re: [Boost-users] [Units] help with type-safe unit-specific conversions
From: Matthias Schabel (boost_at_[hidden])
Date: 2009-07-22 16:57:02

Presumably these external libraries expect their input to be in a  
specific unit. For maximal safety, you should use explicit  
construction of that unit in a wrapper. So, for example, you have some  
external function (not tested) :

void doSomethingWithLength(double length /* expects meters */);

create a wrapper function that expects a quantity (you could improve  
this by only overloading on units with length dimensionality)

namespace wrap {

template<Unit>
void doSomethingWithLength(const quantity<Unit>& q)
{
        return doSomethingWithLength((quantity<si::length>(q)).value());
}

} // namespace wrap

Then you can call wrap::doSomethingWithLength with any unit having  
length dimension and the argument will be converted to meters and  
"demarshalled"...

Matthias

> I have been playing with Boost.Units as part of a work project  
> recently, and I am not sure what the best approach to  
> "demarshalling" values in a specific unit.  For example, I would  
> like to do something like:
>
> using namespace boost::units;
>
> struct A {
>    enum { feet, kilometers };
>
>    quantity<si::length> some_length;
>
>    double foo(int type) {
>       switch(type) {
>       case feet:
>           return quantity<feet>(some_length).value();
>       case kilometers:
>           return quantity<kilometers>(some_length).value();
>       }
>    }
> };
>
> This approach is necessary for passing values to external libraries  
> which do not (and probably will never) use Boost.Units.  Is  
> conversion_factor the best approach, or is there a way to achieve  
> this implicitly using the type system?
>
> Thanks
> Justin
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