GTSAM 4 introduces several new features, most notably Expressions and a Python toolbox. It also deprecates some older APIs and incorrectly named methods, but GTSAM_ALLOW_DEPRECATED_SINCE_V4 is enabled by default so most existing code can continue to build during migration.

To build the Python toolbox, you must explicitly disable that flag.

GTSAM 4 also introduces traits, which makes it possible to optimize with non-GTSAM types.

One behavioral change that will not necessarily trigger a compile error is that zero-initializing Point2 and Point3 is deprecated. Code that relied on default construction may therefore need attention during migration.