[multibody] Deprecate FrameBase for removal (RobotLocomotion#21891)

Author: jwnimmer-tri

multibody/tree/BUILD.bazel

@@ -91,6 +91,7 @@ drake_cc_library(
         "fixed_offset_frame.cc",
         "force_element.cc",
         "frame.cc",
+        "frame_base.cc",
         "joint.cc",
         "joint_actuator.cc",
         "linear_bushing_roll_pitch_yaw.cc",
@@ -134,6 +135,7 @@ drake_cc_library(
         "fixed_offset_frame.h",
         "force_element.h",
         "frame.h",
+        "frame_base.h",
         "joint.h",
         "joint_actuator.h",
         "linear_bushing_roll_pitch_yaw.h",

multibody/tree/frame.h

@@ -3,10 +3,10 @@
 #include <memory>
 #include <optional>
 #include <string>
-#include <vector>
 
 #include "drake/common/autodiff.h"
 #include "drake/common/nice_type_name.h"
+#include "drake/multibody/tree/frame_base.h"
 #include "drake/multibody/tree/frame_body_pose_cache.h"
 #include "drake/multibody/tree/multibody_element.h"
 #include "drake/multibody/tree/multibody_tree_indexes.h"
@@ -50,15 +50,12 @@ template<typename T> class RigidBody;
 ///
 /// @tparam_default_scalar
 template <typename T>
-class Frame : public MultibodyElement<T> {
+class Frame : public FrameBase<T> {
  public:
   DRAKE_NO_COPY_NO_MOVE_NO_ASSIGN(Frame);
 
   ~Frame() override;
 
-  /// Returns this %Frame's unique index.
-  FrameIndex index() const { return this->template index_impl<FrameIndex>(); }
-
   /// Returns a const reference to the body associated to this %Frame.
   const RigidBody<T>& body() const {
     return body_;
@@ -566,7 +563,7 @@ class Frame : public MultibodyElement<T> {
   explicit Frame(
       const std::string& name, const RigidBody<T>& body,
       std::optional<ModelInstanceIndex> model_instance = {})
-      : MultibodyElement<T>(model_instance.value_or(body.model_instance())),
+      : FrameBase<T>(model_instance.value_or(body.model_instance())),
         name_(internal::DeprecateWhenEmptyName(name, "Frame")),
         body_(body) {}
 

multibody/tree/frame_base.cc

@@ -0,0 +1,15 @@
+#include "drake/multibody/tree/frame_base.h"
+
+#include "drake/common/default_scalars.h"
+
+namespace drake {
+namespace multibody {
+
+template <typename T>
+FrameBase<T>::~FrameBase() = default;
+
+}  // namespace multibody
+}  // namespace drake
+
+DRAKE_DEFINE_CLASS_TEMPLATE_INSTANTIATIONS_ON_DEFAULT_SCALARS(
+    class drake::multibody::FrameBase);

multibody/tree/frame_base.h

@@ -0,0 +1,26 @@
+#pragma once
+
+#include "drake/multibody/tree/multibody_element.h"
+#include "drake/multibody/tree/multibody_tree_indexes.h"
+
+namespace drake {
+namespace multibody {
+
+/// %FrameBase is deprecated and will be removed on or after 2025-01-01.
+/// @tparam_default_scalar
+template <typename T>
+class FrameBase : public MultibodyElement<T> {
+ public:
+  // TODO(jwnimmer-tri) On 2025-01-01 move this into `class Frame`.
+  /// Returns this element's unique index.
+  FrameIndex index() const { return this->template index_impl<FrameIndex>(); }
+
+  ~FrameBase() override;
+
+ protected:
+  explicit FrameBase(ModelInstanceIndex model_instance)
+      : MultibodyElement<T>(model_instance) {}
+};
+
+}  // namespace multibody
+}  // namespace drake