diff --git a/cartographer/cloud/internal/sensor/serialization.cc b/cartographer/cloud/internal/sensor/serialization.cc
index 8127c4e..39a7acc 100644
--- a/cartographer/cloud/internal/sensor/serialization.cc
+++ b/cartographer/cloud/internal/sensor/serialization.cc
@@ -92,7 +92,7 @@ void CreateSensorDataForLocalSlamResult(
   for (const auto& insertion_submap : insertion_result.insertion_submaps) {
     // We only send the probability grid up if the submap is finished.
     auto* submap = proto->mutable_local_slam_result_data()->add_submaps();
-    insertion_submap->ToProto(submap, insertion_submap->finished());
+    *submap = insertion_submap->ToProto(insertion_submap->finished());
     submap->mutable_submap_id()->set_trajectory_id(trajectory_id);
     submap->mutable_submap_id()->set_submap_index(starting_submap_index);
     ++starting_submap_index;
diff --git a/cartographer/io/internal/mapping_state_serialization.cc b/cartographer/io/internal/mapping_state_serialization.cc
index 4c42e9a..e6dd101 100644
--- a/cartographer/io/internal/mapping_state_serialization.cc
+++ b/cartographer/io/internal/mapping_state_serialization.cc
@@ -84,14 +84,13 @@ void SerializeSubmaps(
   for (const auto& submap_id_data : submap_data) {
     SerializedData proto;
     auto* const submap_proto = proto.mutable_submap();
+    *submap_proto = submap_id_data.data.submap->ToProto(
+        /*include_probability_grid_data=*/submap_id_data.data.submap
+            ->finished());
     submap_proto->mutable_submap_id()->set_trajectory_id(
         submap_id_data.id.trajectory_id);
     submap_proto->mutable_submap_id()->set_submap_index(
         submap_id_data.id.submap_index);
-    submap_id_data.data.submap->ToProto(
-        submap_proto,
-        /*include_probability_grid_data=*/submap_id_data.data.submap
-            ->finished());
     writer->WriteProto(proto);
   }
 }
diff --git a/cartographer/mapping/2d/submap_2d.cc b/cartographer/mapping/2d/submap_2d.cc
index 5fd4cf9..dee0368 100644
--- a/cartographer/mapping/2d/submap_2d.cc
+++ b/cartographer/mapping/2d/submap_2d.cc
@@ -82,9 +82,10 @@ Submap2D::Submap2D(const proto::Submap2D& proto,
   set_finished(proto.finished());
 }
 
-void Submap2D::ToProto(proto::Submap* const proto,
-                       bool include_probability_grid_data) const {
-  auto* const submap_2d = proto->mutable_submap_2d();
+proto::Submap Submap2D::ToProto(
+    const bool include_probability_grid_data) const {
+  proto::Submap proto;
+  auto* const submap_2d = proto.mutable_submap_2d();
   *submap_2d->mutable_local_pose() = transform::ToProto(local_pose());
   submap_2d->set_num_range_data(num_range_data());
   submap_2d->set_finished(finished());
@@ -92,6 +93,7 @@ void Submap2D::ToProto(proto::Submap* const proto,
     CHECK(grid_);
     *submap_2d->mutable_grid() = grid_->ToProto();
   }
+  return proto;
 }
 
 void Submap2D::UpdateFromProto(const proto::Submap& proto) {
diff --git a/cartographer/mapping/2d/submap_2d.h b/cartographer/mapping/2d/submap_2d.h
index 2e97113..6c668d0 100644
--- a/cartographer/mapping/2d/submap_2d.h
+++ b/cartographer/mapping/2d/submap_2d.h
@@ -47,8 +47,7 @@ class Submap2D : public Submap {
   explicit Submap2D(const proto::Submap2D& proto,
                     ValueConversionTables* conversion_tables);
 
-  void ToProto(proto::Submap* proto,
-               bool include_probability_grid_data) const override;
+  proto::Submap ToProto(bool include_probability_grid_data) const override;
   void UpdateFromProto(const proto::Submap& proto) override;
 
   void ToResponseProto(const transform::Rigid3d& global_submap_pose,
diff --git a/cartographer/mapping/2d/submap_2d_test.cc b/cartographer/mapping/2d/submap_2d_test.cc
index 1508971..0f01763 100644
--- a/cartographer/mapping/2d/submap_2d_test.cc
+++ b/cartographer/mapping/2d/submap_2d_test.cc
@@ -103,8 +103,8 @@ TEST(Submap2DTest, ToFromProto) {
                     common::make_unique<ProbabilityGrid>(expected_map_limits,
                                                          &conversion_tables),
                     &conversion_tables);
-  proto::Submap proto;
-  expected.ToProto(&proto, true /* include_probability_grid_data */);
+  const proto::Submap proto =
+      expected.ToProto(true /* include_probability_grid_data */);
   EXPECT_TRUE(proto.has_submap_2d());
   EXPECT_FALSE(proto.has_submap_3d());
   const auto actual = Submap2D(proto.submap_2d(), &conversion_tables);
diff --git a/cartographer/mapping/3d/submap_3d.cc b/cartographer/mapping/3d/submap_3d.cc
index 7f46b48..83b0a07 100644
--- a/cartographer/mapping/3d/submap_3d.cc
+++ b/cartographer/mapping/3d/submap_3d.cc
@@ -213,9 +213,10 @@ Submap3D::Submap3D(const proto::Submap3D& proto)
   set_finished(proto.finished());
 }
 
-void Submap3D::ToProto(proto::Submap* const proto,
-                       bool include_probability_grid_data) const {
-  auto* const submap_3d = proto->mutable_submap_3d();
+proto::Submap Submap3D::ToProto(
+    const bool include_probability_grid_data) const {
+  proto::Submap proto;
+  auto* const submap_3d = proto.mutable_submap_3d();
   *submap_3d->mutable_local_pose() = transform::ToProto(local_pose());
   submap_3d->set_num_range_data(num_range_data());
   submap_3d->set_finished(finished());
@@ -225,6 +226,7 @@ void Submap3D::ToProto(proto::Submap* const proto,
     *submap_3d->mutable_low_resolution_hybrid_grid() =
         low_resolution_hybrid_grid().ToProto();
   }
+  return proto;
 }
 
 void Submap3D::UpdateFromProto(const proto::Submap& proto) {
diff --git a/cartographer/mapping/3d/submap_3d.h b/cartographer/mapping/3d/submap_3d.h
index 9b24020..a770936 100644
--- a/cartographer/mapping/3d/submap_3d.h
+++ b/cartographer/mapping/3d/submap_3d.h
@@ -46,8 +46,7 @@ class Submap3D : public Submap {
            const transform::Rigid3d& local_submap_pose);
   explicit Submap3D(const proto::Submap3D& proto);
 
-  void ToProto(proto::Submap* proto,
-               bool include_probability_grid_data) const override;
+  proto::Submap ToProto(bool include_probability_grid_data) const override;
   void UpdateFromProto(const proto::Submap& proto) override;
 
   void ToResponseProto(const transform::Rigid3d& global_submap_pose,
diff --git a/cartographer/mapping/3d/submap_3d_test.cc b/cartographer/mapping/3d/submap_3d_test.cc
index b16fd41..3868d95 100644
--- a/cartographer/mapping/3d/submap_3d_test.cc
+++ b/cartographer/mapping/3d/submap_3d_test.cc
@@ -28,8 +28,8 @@ TEST(SubmapsTest, ToFromProto) {
       0.05, 0.25,
       transform::Rigid3d(Eigen::Vector3d(1., 2., 0.),
                          Eigen::Quaterniond(0., 0., 0., 1.)));
-  proto::Submap proto;
-  expected.ToProto(&proto, true /* include_probability_grid_data */);
+  const proto::Submap proto =
+      expected.ToProto(true /* include_probability_grid_data */);
   EXPECT_FALSE(proto.has_submap_2d());
   EXPECT_TRUE(proto.has_submap_3d());
   const auto actual = Submap3D(proto.submap_3d());
diff --git a/cartographer/mapping/submaps.h b/cartographer/mapping/submaps.h
index cca356e..d464f78 100644
--- a/cartographer/mapping/submaps.h
+++ b/cartographer/mapping/submaps.h
@@ -62,8 +62,7 @@ class Submap {
       : local_pose_(local_submap_pose) {}
   virtual ~Submap() {}
 
-  virtual void ToProto(proto::Submap* proto,
-                       bool include_probability_grid_data) const = 0;
+  virtual proto::Submap ToProto(bool include_probability_grid_data) const = 0;
   virtual void UpdateFromProto(const proto::Submap& proto) = 0;
 
   // Fills data into the 'response'.