Re: [Boost-users] Boost.Units Example: Convert between nautical mile and imperial foot

Subject: Re: [Boost-users] Boost.Units Example: Convert between nautical mile and imperial foot
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2018-09-27 01:56:40

AMDG

On 09/26/2018 07:30 PM, Gavin Lambert via Boost-users wrote:
> On 27/09/2018 00:57, Carel Combrink wrote:
>> I have implemented the example code for the "Radar Beam Height" [1]
>> section but trying to convert between nautical mile and imperial foot
>> result in a compiler error:
> [...]
>> The first few works, converting between meters and the new units, but
>> converting between the two does not work.
>>
>> To see the issue see the code on Compiler Explorer here:
>> https://gcc.godbolt.org/z/MpvuMy
> 
> You've defined a conversion between imperial feet and meters, and a
> conversion between nautical miles and meters.
> 
> But you haven't defined a direct conversion between imperial feet and
> nautical miles.  It's apparently not smart enough to do the conversion
> through an unspecified intermediate unit.  (This might be intentional,
> since it could be ambiguous or lossy.)
> 

The library has no way to know what the correct
intermediate unit is.  You can use
BOOST_UNITS_DEFAULT_CONVERSION to specify.
(Yes, I know this is kind of clunky).

> You can either write your code to first convert the value into meters
> explicitly before converting it to the other unit:
> 
> Â Â Â  const Meters si1(300.0*nautical::miles);
> Â Â Â  const NMiles mile4(si1);
> 
> Or you can explicitly define the conversion you want (using the
> previously defined conversions as a base to avoid duplicating the factors):
> 
> BOOST_UNITS_DEFINE_CONVERSION_FACTOR(
> Â Â Â  imperial::length_base_unit,
> Â Â Â  nautical::length_base_unit,
> Â Â Â  double,
> Â Â Â  boost::units::conversion_factor(imperial::foot, si::meter) *
> Â Â Â Â Â Â Â  boost::units::conversion_factor(si::meter, nautical::mile));
> 

In Christ,
Steven Watanabe