Re: [Boost-users] How to make is_arithmetic true for a UDT

Subject: Re: [Boost-users] How to make is_arithmetic true for a UDT
From: Paul A. Bristow (pbristow_at_[hidden])
Date: 2012-07-12 11:07:40

> -----Original Message-----
> From: boost-users-bounces_at_[hidden] [mailto:boost-users-bounces_at_[hidden]] On Behalf
Of
> John Maddock
> Sent: Thursday, July 12, 2012 10:32 AM
> To: boost-users_at_[hidden]
> Subject: Re: [Boost-users] How to make is_arithmetic true for a UDT
> 
> >I am facing the need to get my User Defined Type to return true from
> >
> >  boost::is_arithmetic
> 
> You don't, a UDT is not an arithmetic type (according to the type system which is what type_traits
honors)
> it's a class type.
> 
> > (This is to meet a check for autoprefixing in Boost.Units)
> 
> Looks like there are two places this is used in Units - and both are implementation details - for
auto-
> prefixing looks like you need to overload autoprefix_norm for your type.  The other use is in
> is_non_narrowing_conversion, which you would [partially-] specialize for your type.

Well I finally managed to string enough neurons together to write

// Specialization of autoprefix_norm for boost::units::measurement<Y>
// See io.hpp
// This is required to get autoprefix to work with this class.
template<class Y>
typename autoprefix_norm_impl<boost::units::measurement<Y>, true>::type
autoprefix_norm(const boost::units::measurement<Y> & arg)
{
  return autoprefix_norm_impl<Y, true>::call(arg);
}

and that does the trick :-))

  quantity<length, measurement<double> > 
biglen(measurement<double>(12345.0,123.0)*meters); autoscaled = 12.345(+/-0.123) km

so many thanks.

But I'm still not clear when I could need the non-narrowing conversion in Boost.units quantity.hpp.

I can see the need for this

template<>
struct is_non_narrowing_conversion<long double, double> : mpl::false_ {};

So would I need

template<>
struct is_non_narrowing_conversion<measurement< double>, double> : mpl::true_ {};

but 

template<>
struct is_non_narrowing_conversion<measurement<long double>, double> : mpl::false_ {};

(though the existence of more than one FP type in the class, means that one needs to call
measurement value() function to get a double, so the answer always false?).

Paul

---
Paul A. Bristow,
Prizet Farmhouse, Kendal LA8 8AB  UK
[phone]  07714330204
pbristow_at_[hidden]