From 4708d7ad0e502e62cc88c38f1c1049c52dad1c35 Mon Sep 17 00:00:00 2001
From: Fan Jiang <i@fanjiang.me>
Date: Mon, 17 May 2021 14:40:17 -0400
Subject: [PATCH] Add comment on functor signature

---
 gtsam/nonlinear/CustomFactor.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/gtsam/nonlinear/CustomFactor.h b/gtsam/nonlinear/CustomFactor.h
index 9d9db9eda..34cb5ad51 100644
--- a/gtsam/nonlinear/CustomFactor.h
+++ b/gtsam/nonlinear/CustomFactor.h
@@ -27,6 +27,14 @@ typedef std::vector<Matrix> JacobianVector;
 
 class CustomFactor;
 
+/*
+ * NOTE
+ * ==========
+ * pybind11 will invoke a copy if this is `JacobianVector &`, and modifications in Python will not be reflected.
+ *
+ * This is safe because this is passing a const pointer, and pybind11 will maintain the `std::vector` memory layout.
+ * Thus the pointer will never be invalidated.
+ */
 typedef std::function<Vector(const CustomFactor&, const Values&, const JacobianVector*)> CustomErrorFunction;
 
 /**