Trait DefaultDistance 

pub trait DefaultDistance<Other> {
    type Strategy: Default;
}

“Which distance strategy do we pick by default for a pair of coordinate-system families Self (for the first geometry’s CS family) and Other (for the second)?”

Mirrors boost::geometry::strategy::distance::services:: default_strategy<Tag1, Tag2, Point1, Point2, CsTag1, CsTag2> from boost/geometry/strategies/distance/services.hpp, together with the per-CS specialisations in strategies/{cartesian,spherical,geographic}/distance.hpp and the umbrella strategies/default_strategy.hpp.

In Boost the dispatch keys on the coordinate-system tag (cs_tag<Point>::type). The Rust analogue is the CoordinateSystem::Family associated type defined in geometry-cs — every concrete CS (Cartesian, Spherical<Degree>, Spherical<Radian>, Geographic<Degree>, …) reports one of the family marker types (CartesianFamily, SphericalFamily, GeographicFamily, PolarFamily).

Implementations land in geometry-strategy alongside each concrete strategy:

impl DefaultDistance<CartesianFamily>  for CartesianFamily  { type Strategy = Pythagoras; }
impl DefaultDistance<SphericalFamily>  for SphericalFamily  { type Strategy = Haversine; }
impl DefaultDistance<GeographicFamily> for GeographicFamily { type Strategy = Andoyer; }

The Strategy: Default bound matches Boost’s expectation that services::default_strategy<...>::type is default-constructible (every strategies/.../distance_*.hpp concrete strategy has a no-arg constructor).

Required Associated Types

type Strategy: Default

The strategy chosen for (Self, Other). Must implement Default because the free-function distance(a, b) builds it without arguments — same contract as Boost’s default-constructed services::default_strategy<...>::type.

Dyn Compatibility

This trait is dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety".

Implementations on Foreign Types

impl DefaultDistance<CartesianFamily> for CartesianFamily

Cartesian × Cartesian defaults to Pythagoras.

Mirrors the services::default_strategy<point_tag, point_tag, P1, P2, cartesian_tag, cartesian_tag> specialisation at strategies/cartesian/distance_pythagoras.hpp:276-283.

type Strategy = Pythagoras

impl DefaultDistance<GeographicFamily> for GeographicFamily

Geographic × Geographic defaults to Andoyer.

Mirrors the services::default_strategy<point_tag, point_tag, P1, P2, geographic_tag, geographic_tag> specialisation in strategies/geographic/distance.hpp — Boost picks strategy::distance::geographic<strategy::andoyer, Spheroid> as the geographic default, which is exactly strategy::distance::andoyer<Spheroid>.

type Strategy = Andoyer

impl DefaultDistance<SphericalFamily> for SphericalFamily

Spherical × Spherical defaults to Haversine.

Mirrors the services::default_strategy<point_tag, point_tag, P1, P2, spherical_equatorial_tag, spherical_equatorial_tag> specialisation in strategies/spherical/distance_haversine.hpp (around the services::* block at lines 222-260).

type Strategy = Haversine

Implementors