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

12 Jul 2012

      ...
-----Original Message-----
From: boost-users-bounces@lists.boost.org [mailto:boost-users-bounces@lists.boost.org] On Behalf
Of
John Maddock
Sent: Thursday, July 12, 2012 10:32 AM
To: boost-users@lists.boost.org
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::type
autoprefix_norm(const boost::units::measurement<Y> & arg)
{
  return autoprefix_norm_impl::call(arg);
}

and that does the trick :-))

  quantity 
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 : mpl::false_ {};

So would I need

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

but 

template<>
struct is_non_narrowing_conversion : 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,
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