The C++11 containers unordered_set and unordered_map only work with hashable types, a concept that was itself added in C++11. Most afw value types were implemented before we adopted C++11, and therefore do not have a specialization of std::hash. Implementing std::hash will also make it easier to implement DM-7621, as the C++11 requirements for hash are stricter than the Python requirements.

In most cases, hash can be quickly implemented by adopting a modified version of the algorithm from Effective Java, Item 9; the only difference between the C++ requirements and the Java ones is that hash functions are implied to be strictly deterministic before C++14, but the EffJ algorithm meets that criterion if the hash for all member variables does.

The classes covered by this ticket are a subset (possibly taken from DM-9936) of the following types:

• afw::cameraGeom::CameraPoint
• afw::cameraGeom::CameraSys
• afw::cameraGeom::CameraSysPrefix
• afw::cameraGeom::Orientation
• afw::coord::Coord and its subclasses
• afw::coord::Observatory
• afw::coord::Weather
• afw::detection::Threshold
• afw::geom::Angle
• afw::geom::AngleUnit
• afw::geom::Box
• afw::geom::CoordinateBase and its subclasses
• afw::geom::CoordinateExpr
• afw::geom::Span
• afw::geom::SpherePoint
• afw::geom::polygon::Polygon
• afw::image::Calib
• afw::image::Color
• afw::image::DefectBase
• afw::image::Filter
• afw::image::FilterProperty
• afw::math::FitResults
• afw::math::Function and its subclasses
• afw::math::MaskedVector
• afw::math::Statistics
• afw::table::BaseRecord and its subclasses
• afw::table::ConstFunctorKey
• afw::table::FieldBase and its subclasses
• afw::table::InputFunctorKey and its subclasses
• afw::table::KeyBase and its subclasses
• afw::table::Match
• afw::table::OutputFunctorKey and its subclasses
• afw::table::ReferenceFunctorKey
• afw::table::SchemaItem
• afw::table::io::Persistable
• afw::table::io::PersistableFacade