Change some parms to const reference (satisfies cppcheck)

src/operators/operator.cc

@@ -138,7 +138,7 @@ bool Operator::evaluate(Transaction *transaction, const std::string& a) {
     return true;
 }
 
-Operator *Operator::instantiate(std::string op, std::string param_str) {
+Operator *Operator::instantiate(const std::string& op, const std::string& param_str) {
     std::string op_ = utils::string::tolower(op);
     std::unique_ptr<RunTimeString> param(new RunTimeString());
     param->appendText(param_str);

src/operators/operator.h

@@ -101,7 +101,8 @@ class Operator {
         }
 
     virtual ~Operator() { }
-    static Operator *instantiate(std::string opName, std::string param);
+    static Operator *instantiate(const std::string& opName,
+        const std::string& param);
 
     virtual bool init(const std::string &arg, std::string *error) {
         return true;

src/transaction.cc

@@ -278,7 +278,7 @@ Transaction::~Transaction() {
  *
  */
 #ifndef NO_LOGS
-void Transaction::debug(int level, std::string message) const {
+void Transaction::debug(int level, const std::string& message) const {
     if (m_rules == NULL) {
         return;
     }

src/variables/tx.h

@@ -99,7 +99,7 @@ class Tx_DynamicElement : public Variable {
     }
 
     static void storeOrUpdateFirst(Transaction *t, const std::string &var,
-        std::string value) {
+        const std::string &value) {
         t->m_collections.m_tx_collection->storeOrUpdateFirst(var, value);
     }