diff --git a/src/EXTRA-DUMP/dump_yaml.cpp b/src/EXTRA-DUMP/dump_yaml.cpp
index 029415164f..ec7d26af31 100644
--- a/src/EXTRA-DUMP/dump_yaml.cpp
+++ b/src/EXTRA-DUMP/dump_yaml.cpp
@@ -64,10 +64,10 @@ void DumpYAML::write_header(bigint ndump)
     if (update->ntimestep == th->get_timestep()) {
 
       thermo_data += "thermo:\n  - keywords: [ ";
-      for (auto key : th->get_keywords()) thermo_data += fmt::format("{}, ", key);
+      for (const auto &key : th->get_keywords()) thermo_data += fmt::format("{}, ", key);
       thermo_data += "]\n  - data: [ ";
 
-      for (auto val : th->get_fields()) {
+      for (const auto &val : th->get_fields()) {
         if (val.type == multitype::DOUBLE)
           thermo_data += fmt::format("{}, ", val.data.d);
         else if (val.type == multitype::INT)
@@ -90,7 +90,7 @@ void DumpYAML::write_header(bigint ndump)
 
     fmt::print(fp, "natoms: {}\n", ndump);
     fputs("boundary: [ ", fp);
-    for (const auto bflag : boundary) {
+    for (const auto &bflag : boundary) {
       if (bflag == ' ') continue;
       fmt::print(fp, "{}, ", bflag);
     }
diff --git a/src/NETCDF/dump_netcdf.cpp b/src/NETCDF/dump_netcdf.cpp
index cb6aea16cf..8c99ff1f70 100644
--- a/src/NETCDF/dump_netcdf.cpp
+++ b/src/NETCDF/dump_netcdf.cpp
@@ -320,7 +320,7 @@ void DumpNetCDF::openfile()
 
       // perframe variables
       if (thermo) {
-        auto keywords = output->thermo->get_keywords();
+        const auto &keywords = output->thermo->get_keywords();
         int nfield = keywords.size();
         for (int i = 0; i < nfield; i++) {
           NCERRX( nc_inq_varid(ncid, keywords[i].c_str(), &thermovar[i]), keywords[i].c_str() );
@@ -433,8 +433,8 @@ void DumpNetCDF::openfile()
 
       // perframe variables
       if (thermo) {
-        auto fields = output->thermo->get_fields();
-        auto keywords = output->thermo->get_keywords();
+        const auto &fields = output->thermo->get_fields();
+        const auto &keywords = output->thermo->get_keywords();
         int nfield = fields.size();
         for (int i = 0; i < nfield; i++) {
           if (fields[i].type == multitype::DOUBLE) {
@@ -600,8 +600,8 @@ void DumpNetCDF::write()
   start[1] = 0;
 
   if (thermo) {
-    auto keywords = output->thermo->get_keywords();
-    auto fields = output->thermo->get_fields();
+    const auto &keywords = output->thermo->get_keywords();
+    const auto &fields = output->thermo->get_fields();
     int nfield = fields.size();
     for (int i = 0; i < nfield; i++) {
       if (filewriter) {
diff --git a/src/NETCDF/dump_netcdf_mpiio.cpp b/src/NETCDF/dump_netcdf_mpiio.cpp
index fdcd03470e..3aec34dd40 100644
--- a/src/NETCDF/dump_netcdf_mpiio.cpp
+++ b/src/NETCDF/dump_netcdf_mpiio.cpp
@@ -318,7 +318,7 @@ void DumpNetCDFMPIIO::openfile()
 
     // perframe variables
     if (thermo) {
-      auto keywords = output->thermo->get_keywords();
+      const auto &keywords = output->thermo->get_keywords();
       int nfield = keywords.size();
       for (int i = 0; i < nfield; i++) {
         NCERRX( ncmpi_inq_varid(ncid, keywords[i].c_str(), &thermovar[i]), keywords[i].c_str() );
@@ -423,8 +423,8 @@ void DumpNetCDFMPIIO::openfile()
 
     // perframe variables
     if (thermo) {
-      auto fields = output->thermo->get_fields();
-      auto keywords = output->thermo->get_keywords();
+      const auto &fields = output->thermo->get_fields();
+      const auto &keywords = output->thermo->get_keywords();
       int nfield = fields.size();
       for (int i = 0; i < nfield; i++) {
         if (fields[i].type == multitype::DOUBLE) {
@@ -593,8 +593,8 @@ void DumpNetCDFMPIIO::write()
   NCERR( ncmpi_begin_indep_data(ncid) );
 
   if (thermo) {
-    auto keywords = output->thermo->get_keywords();
-    auto fields = output->thermo->get_fields();
+    const auto &keywords = output->thermo->get_keywords();
+    const auto &fields = output->thermo->get_fields();
     int nfield = fields.size();
     for (int i = 0; i < nfield; i++) {
       if (filewriter) {