Provide access to owning MBPlant from any MBElement. (#12380)

multibody/plant/test/multibody_plant_test.cc

@@ -1566,6 +1566,27 @@ TEST_F(SplitPendulum, MassMatrix) {
   EXPECT_NEAR(M(0, 0), Io, 1.0e-6);
 }
 
+// Verify that we can obtain the owning MultibodyPlant from one of its
+// MultibodyElements, and that we get a proper error message if we try
+// this for an element that isn't owned by a MultibodyPlant.
+TEST_F(SplitPendulum, GetMultibodyPlantFromElement) {
+  const MultibodyPlant<double>& pins_plant = pin_->GetParentPlant();
+  EXPECT_EQ(&pins_plant, &plant_);
+
+  // Create an element-owning MBTreeSystem that _is not_ an MBPlant.
+  struct MyMBSystem : public internal::MultibodyTreeSystem<double> {
+    MyMBSystem() {
+      rigid_body = &mutable_tree().AddBody<RigidBody>(SpatialInertia<double>());
+      Finalize();
+    }
+    const RigidBody<double>* rigid_body{};
+  } mb_system;
+
+  DRAKE_EXPECT_THROWS_MESSAGE(
+      mb_system.rigid_body->GetParentPlant(), std::logic_error,
+      ".*multibody element.*not owned by.*MultibodyPlant.*");
+}
+
 // Verifies we can parse link collision geometries and surface friction.
 GTEST_TEST(MultibodyPlantTest, ScalarConversionConstructor) {
   const std::string full_name = drake::FindResourceOrThrow(

multibody/tree/multibody_element.h

@@ -9,6 +9,9 @@
 namespace drake {
 namespace multibody {
 
+template <typename T>
+class MultibodyPlant;
+
 namespace internal {
 // This is a class used by MultibodyTree internals to create the implementation
 // for a particular joint object.
@@ -58,6 +61,29 @@ class MultibodyElement {
   /// element belongs.
   ModelInstanceIndex model_instance() const { return model_instance_;}
 
+  /// Returns the MultibodyPlant that owns this %MultibodyElement.
+  ///
+  /// @note You can only invoke this method if you have a definition of
+  /// %MultibodyPlant available. That is, you must include
+  /// `drake/multibody/plant/multibody_plant.h` in the translation unit that
+  /// invokes this method; multibody_element.h cannot do that for you.
+  ///
+  /// @throws std::logic_error if there is no %MultibodyPlant owner.
+  template <typename MultibodyPlantDeferred = MultibodyPlant<T>>
+  const MultibodyPlantDeferred& GetParentPlant() const {
+    HasParentTreeOrThrow();
+
+    const auto plant = dynamic_cast<const MultibodyPlantDeferred*>(
+        &get_parent_tree().tree_system());
+
+    if (plant == nullptr) {
+      throw std::logic_error(
+          "This multibody element was not owned by a MultibodyPlant.");
+    }
+
+    return *plant;
+  }
+
  protected:
   /// Default constructor made protected so that sub-classes can still declare
   /// their default constructors if they need to.

multibody/tree/multibody_tree.h

@@ -2331,6 +2331,13 @@ class MultibodyTree {
   }
   //@}
 
+  /// Returns the MultibodyTreeSystem that owns this MultibodyTree.
+  /// @pre There is an owning MultibodyTreeSystem.
+  const MultibodyTreeSystem<T>& tree_system() const {
+    DRAKE_DEMAND(tree_system_ != nullptr);
+    return *tree_system_;
+  }
+
   /// (Internal use only) Informs the MultibodyTree how to access its resources
   /// within a Context.
   void set_tree_system(MultibodyTreeSystem<T>* tree_system) {