Output of a UDT with autoprefix requires specialization of auto_prefix_norm

[Paul A. Bristow]

Output of a UDT with autoprefix requires specialization of auto_prefix_norm for the scaling to take place. For example, to get expected scaling to kilometers from

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

This can be achieved by adding this to measurement.hpp (where Y is the type of UDT)

// Specialization of autoprefix_norm for UDTboost::units::measurement<Y>
// See io.hpp.
// This specialization 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);
}

However this assumes that the class Y is convertible to double. autoprefix_norm_impl<T, true>::type appears to be always double. I am unclear if this is checked or is the responsibility or the user?