diff --git a/cartographer/io/proto_stream.h b/cartographer/io/proto_stream.h
index f9d5645..f5c0cbe 100644
--- a/cartographer/io/proto_stream.h
+++ b/cartographer/io/proto_stream.h
@@ -20,6 +20,7 @@
 #include <fstream>
 
 #include "cartographer/common/port.h"
+#include "cartographer/io/proto_stream_interface.h"
 #include "google/protobuf/message.h"
 
 namespace cartographer {
@@ -52,17 +53,16 @@ class ProtoStreamWriter {
 };
 
 // A reader of the format produced by ProtoStreamWriter.
-class ProtoStreamReader {
+class ProtoStreamReader : public ProtoStreamReaderInterface {
  public:
-  ProtoStreamReader(const std::string& filename);
+  explicit ProtoStreamReader(const std::string& filename);
   ~ProtoStreamReader() = default;
 
   ProtoStreamReader(const ProtoStreamReader&) = delete;
   ProtoStreamReader& operator=(const ProtoStreamReader&) = delete;
 
-  bool ReadProto(google::protobuf::Message* proto);
-
-  bool eof() const;
+  bool ReadProto(google::protobuf::Message* proto) override;
+  bool eof() const override;
 
  private:
   bool Read(std::string* decompressed_data);
diff --git a/cartographer/io/proto_stream_interface.h b/cartographer/io/proto_stream_interface.h
new file mode 100644
index 0000000..62b6be1
--- /dev/null
+++ b/cartographer/io/proto_stream_interface.h
@@ -0,0 +1,42 @@
+/*
+ * Copyright 2018 The Cartographer Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef CARTOGRAPHER_IO_PROTO_STREAM_INTERFACE_H_
+#define CARTOGRAPHER_IO_PROTO_STREAM_INTERFACE_H_
+
+#include "cartographer/common/port.h"
+#include "google/protobuf/message.h"
+
+namespace cartographer {
+namespace io {
+
+// A reader of the format produced by ProtoStreamWriter.
+class ProtoStreamReaderInterface {
+ public:
+  ProtoStreamReaderInterface() = default;
+  virtual ~ProtoStreamReaderInterface(){};
+
+  // Deserialize compressed proto from the pb stream.
+  virtual bool ReadProto(google::protobuf::Message* proto) = 0;
+
+  // 'End-of-file' marker for the pb stream.
+  virtual bool eof() const = 0;
+};
+
+}  // namespace io
+}  // namespace cartographer
+
+#endif  // CARTOGRAPHER_IO_PROTO_STREAM_INTERFACE_H_
diff --git a/cartographer/mapping/map_builder.cc b/cartographer/mapping/map_builder.cc
index aa56626..c5c1d42 100644
--- a/cartographer/mapping/map_builder.cc
+++ b/cartographer/mapping/map_builder.cc
@@ -244,7 +244,7 @@ void MapBuilder::SerializeState(io::ProtoStreamWriter* const writer) {
   }
 }
 
-void MapBuilder::LoadMap(io::ProtoStreamReader* const reader) {
+void MapBuilder::LoadMap(io::ProtoStreamReaderInterface* const reader) {
   proto::PoseGraph pose_graph;
   CHECK(reader->ReadProto(&pose_graph));
 
diff --git a/cartographer/mapping/map_builder.h b/cartographer/mapping/map_builder.h
index 3393cf1..3d717f4 100644
--- a/cartographer/mapping/map_builder.h
+++ b/cartographer/mapping/map_builder.h
@@ -62,7 +62,7 @@ class MapBuilder : public MapBuilderInterface {
 
   void SerializeState(io::ProtoStreamWriter* writer) override;
 
-  void LoadMap(io::ProtoStreamReader* reader) override;
+  void LoadMap(io::ProtoStreamReaderInterface* reader) override;
 
   int num_trajectory_builders() const override;
 
diff --git a/cartographer/mapping/map_builder_interface.h b/cartographer/mapping/map_builder_interface.h
index 90de96e..e59a19f 100644
--- a/cartographer/mapping/map_builder_interface.h
+++ b/cartographer/mapping/map_builder_interface.h
@@ -77,7 +77,7 @@ class MapBuilderInterface {
   virtual void SerializeState(io::ProtoStreamWriter* writer) = 0;
 
   // Loads submaps from a proto stream into a new frozen trajectory.
-  virtual void LoadMap(io::ProtoStreamReader* reader) = 0;
+  virtual void LoadMap(io::ProtoStreamReaderInterface* reader) = 0;
 
   virtual int num_trajectory_builders() const = 0;
 
diff --git a/cartographer_grpc/client_server_test.cc b/cartographer_grpc/client_server_test.cc
index 6e59171..2271038 100644
--- a/cartographer_grpc/client_server_test.cc
+++ b/cartographer_grpc/client_server_test.cc
@@ -57,7 +57,7 @@ class MockMapBuilder : public cartographer::mapping::MapBuilderInterface {
       std::string(const cartographer::mapping::SubmapId&,
                   cartographer::mapping::proto::SubmapQuery::Response*));
   MOCK_METHOD1(SerializeState, void(cartographer::io::ProtoStreamWriter*));
-  MOCK_METHOD1(LoadMap, void(cartographer::io::ProtoStreamReader*));
+  MOCK_METHOD1(LoadMap, void(cartographer::io::ProtoStreamReaderInterface*));
   MOCK_CONST_METHOD0(num_trajectory_builders, int());
   MOCK_METHOD0(pose_graph, PoseGraphInterface*());
 };
diff --git a/cartographer_grpc/mapping/map_builder_stub.cc b/cartographer_grpc/mapping/map_builder_stub.cc
index 140df68..4c07563 100644
--- a/cartographer_grpc/mapping/map_builder_stub.cc
+++ b/cartographer_grpc/mapping/map_builder_stub.cc
@@ -92,7 +92,8 @@ void MapBuilderStub::SerializeState(
   LOG(FATAL) << "Not implemented";
 }
 
-void MapBuilderStub::LoadMap(cartographer::io::ProtoStreamReader* reader) {
+void MapBuilderStub::LoadMap(
+    cartographer::io::ProtoStreamReaderInterface* reader) {
   LOG(FATAL) << "Not implemented";
 }
 
diff --git a/cartographer_grpc/mapping/map_builder_stub.h b/cartographer_grpc/mapping/map_builder_stub.h
index bdddca8..5b06ab5 100644
--- a/cartographer_grpc/mapping/map_builder_stub.h
+++ b/cartographer_grpc/mapping/map_builder_stub.h
@@ -48,7 +48,7 @@ class MapBuilderStub : public cartographer::mapping::MapBuilderInterface {
       const cartographer::mapping::SubmapId& submap_id,
       cartographer::mapping::proto::SubmapQuery::Response* response) override;
   void SerializeState(cartographer::io::ProtoStreamWriter* writer) override;
-  void LoadMap(cartographer::io::ProtoStreamReader* reader) override;
+  void LoadMap(cartographer::io::ProtoStreamReaderInterface* reader) override;
   int num_trajectory_builders() const override;
   cartographer::mapping::PoseGraphInterface* pose_graph() override;