node_grpc_main.cc中的关键一句:

auto map_builder = absl::make_unique<::cartographer::cloud::MapBuilderStub>(
    FLAGS_server_address, FLAGS_client_id);

接着看map_builder_stub.cc的构造函数

MapBuilderStub::MapBuilderStub(const std::string& server_address,
                               const std::string& client_id)
    : client_channel_(::grpc::CreateChannel(
          server_address, ::grpc::InsecureChannelCredentials())),
      pose_graph_stub_(make_unique<PoseGraphStub>(client_channel_, client_id)),
      client_id_(client_id)
{
  LOG(INFO) << "Connecting to SLAM process at " << server_address
            << " with client_id " << client_id;
  std::chrono::system_clock::time_point deadline(
      std::chrono::system_clock::now() +
      std::chrono::seconds(kChannelTimeoutSeconds));
  if (!client_channel_->WaitForConnected(deadline)) {
    LOG(FATAL) << "Failed to connect to " << server_address;
  }
}