multiply inherited C++ classes

[dellaert]

Title: pybind wrapper needs safe handling for inherited methods on multiply inherited C++ classes

Summary

We found a pybind wrapper bug affecting inherited non-virtual base methods when the wrapped C++ class has multiple inheritance but the generated binding only lists one base and does not use py::multiple_inheritance().

Concrete case from GTSAM legged estimators:

• gtsam::LeggedEstimator declares non-virtual methods:
  • void turnHeightPriorOn(double terrainHeight);
  • void turnHeightPriorOff();
• gtsam::LeggedInvariantEKF inherits from:
  • LeftLinearEKF<ExtendedPose3d>
  • LeggedEstimator

Observed behavior

When turnHeightPriorOn/Off() were exposed only on the abstract base LeggedEstimator in the .i file, Python calls on concrete LeggedInvariantEKF instances had no behavioral effect.

Workaround that fixed it immediately:

• redeclare the same methods on each concrete wrapped class in the .i file

That strongly suggests the issue is wrapper inheritance / base-pointer adjustment, not estimator logic.

pybind11 docs say this is undefined behavior unless:

• all C++ bases are listed, or
• py::multiple_inheritance() is supplied

Minimal reproduction pattern

• abstract base class with non-virtual method
• derived class with multiple C++ bases
• wrapper lists only one base
• base method is only bound on the base class
• calling that method from Python on a concrete object does not affect behavior correctly
• rebinding the same method directly on the concrete class works around the problem

The smallest fix that appears correct is:

• emit py::multiple_inheritance() for classes with a base class in generated pybind bindings

Possible better long-term fixes:

• model multiple bases explicitly in the interface language
• validate .i inheritance against the real C++ inheritance
• emit py::multiple_inheritance() only when truly needed, if that can be detected reliably

Regression test suggestion

Add a small pybind integration test with:

• base class method
• multiply inherited derived class
• binding lists one base
• verify that inherited method on derived object behaves correctly only when py::multiple_inheritance() is present

Why this matters

Without this fix, downstream projects have to duplicate inherited methods on concrete classes in .i files as a workaround, which is brittle and obscures the real wrapper problem.

[ProfFan]

yeah I think I had a PR that adds multiple_inheritance for everything, but it's lost in time...