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:
const Meters si1(300.0*nautical::miles); 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( imperial::length_base_unit, nautical::length_base_unit, double, boost::units::conversion_factor(imperial::foot, si::meter) * boost::units::conversion_factor(si::meter, nautical::mile));
In Christ, Steven Watanabe