perf: avoid copies in range-based for loops

- take loop variables by const reference where the body does not modify
  them
- clang-tidy performance-for-range-copy
  https://clang.llvm.org/extra/clang-tidy/checks/performance/for-range-copy.html

fairmq/SuboptParser.cxx

@@ -79,7 +79,7 @@ Properties SuboptParser(const vector<string>& channelConfig, const string& devic
 
     ptree channelsArray;
 
-    for (auto token : channelConfig) {
+    for (const auto& token : channelConfig) {
         string channelName;
         ptree channelProperties;
 

fairmq/shmem/Poller.h

@@ -71,7 +71,7 @@ class Poller final : public fair::mq::Poller
         try {
             int offset = 0;
             // calculate offsets and the total size of the poll item set
-            for (std::string channel : channelList) {
+            for (const std::string& channel : channelList) {
                 fOffsetMap[channel] = offset;
                 offset += channelsMap.at(channel).size();
                 fNumItems += channelsMap.at(channel).size();
@@ -80,7 +80,7 @@ class Poller final : public fair::mq::Poller
             fItems = new zmq_pollitem_t[fNumItems];
 
             int index = 0;
-            for (std::string channel : channelList) {
+            for (const std::string& channel : channelList) {
                 for (unsigned int i = 0; i < channelsMap.at(channel).size(); ++i) {
                     index = fOffsetMap[channel] + i;
 

fairmq/zeromq/Poller.h

@@ -86,7 +86,7 @@ class Poller final : public fair::mq::Poller
             fItems = new zmq_pollitem_t[fNumItems];
 
             int index = 0;
-            for (std::string channel : channelList) {
+            for (const std::string& channel : channelList) {
                 for (unsigned int i = 0; i < channelsMap.at(channel).size(); ++i) {
                     index = fOffsetMap[channel] + i;