Add default json path

cpp/route_guide/helper.cc

@@ -55,6 +55,8 @@ std::string GetDbFileContent(int argc, char** argv) {
         db_path = argv_1.substr(start_position + 1);
       }
     }
+  } else {
+    db_path = "route_guide_db.json";
   }
   std::ifstream db_file(db_path);
   if (!db_file.is_open()) {

cpp/route_guide/route_guide_client.cc

@@ -239,6 +239,7 @@ class RouteGuideClient {
 int main(int argc, char** argv) {
   grpc_init();
 
+  // Expect only arg: --db_path=path/to/route_guide_db.json.
   std::string db = examples::GetDbFileContent(argc, argv);
   RouteGuideClient guide(
       grpc::CreateChannelDeprecated("localhost:50051", ChannelArguments()),

cpp/route_guide/route_guide_server.cc

@@ -199,6 +199,7 @@ void RunServer(const std::string& db_path) {
 int main(int argc, char** argv) {
   grpc_init();
 
+  // Expect only arg: --db_path=path/to/route_guide_db.json.
   std::string db = examples::GetDbFileContent(argc, argv);
   RunServer(db);