Re: [boost] units review

From: Steven Watanabe (steven_at_[hidden])
Date: 2007-03-29 21:03:52

AMDG

Noah Roberts <roberts.noah <at> gmail.com> writes:

> 
> Steven Watanabe wrote:
> > Do you think you can provide me with a
> > concrete case that I cannot solve easily using the
> > current library?
> 
> Since any problem can be solved under almost any circumstance, no.

Key word: easily.
Never mind. What you say below answers my question

> <snip>
> 
> There is one use case that we have in our products that is not covered 
> by that.  User defined units.  These are specified as some conversion to 
> an arbitrary base we set up.

Ah.  So to do such conversions you need:

template<class Quantity>
class unit_converter {
  typedef typename get_unit<Quantity>::type Unit;
  typedef typename get_dimensions<Unit>::type Dimensions;
  typedef typename Quantity::value_type value_type;
public:
  typedef Quantity result_type;
  typedef const value_type& argument_type;
  typedef result_type function_type(const value_type&);
  template<class System>
  unit_converter(const unit<Dimensions, System>&) : 
    converter(static_cast<function_type*>(&convert_impl<OtherSystem>)) {}
  unit_converter(const value_type& factor) :
    converter(lambda::bind(from_value(), lambda::_1 * factor)) {}
  unit_converter(const value_type& factor, const value_type& offset) :
    converter(lambda::bind(from_value(), lambda::_1 * factor + offset)) {}
  result_type operator()(const value_type& v) const {
    return(converter(v));
  }
private:
  boost::function<function_type> converter;
  template<class OtherSystem>
  static result_type convert_impl(const value_type& v) {
    return(result_type(v * unit<Dimensions, OtherSystem>()));
  }
  struct from_value {
    template<class T>
    struct sig { typedef result_type type; };
    result_type operator()(const value_type& v) const {
      return(result_type::from_value(v));
    }
  }
};

Adding the reverse conversion ~doubles the size.  Again
if this doesn't fit you needs it may take even more code.

> 
> Your idea of a allowing the unit override might solve may of my 
> objections.  I would have to review that feature as it came.  When I was 
> looking at this library it did not seem at all straight forward to do so.

Allowing it is pretty straight forward.  I admit
that it probably doesn't look like it unless you
know the internals well.

In Christ,
Steven Watanabe