diff --git a/src/core/examples/command-line-example.cc b/src/core/examples/command-line-example.cc
index 61e0f7c6a..7bb4e3fdc 100644
--- a/src/core/examples/command-line-example.cc
+++ b/src/core/examples/command-line-example.cc
@@ -49,7 +49,7 @@ std::string g_cbArg = "cbArg default";
  * \returns \c true.
  */
 bool
-SetCbArg(std::string val)
+SetCbArg(const std::string& val)
 {
     g_cbArg = val;
     return true;
diff --git a/src/core/examples/hash-example.cc b/src/core/examples/hash-example.cc
index b46f4a5da..514e69197 100644
--- a/src/core/examples/hash-example.cc
+++ b/src/core/examples/hash-example.cc
@@ -437,7 +437,7 @@ class DictFiles
      * \param [in] file The word file to add.
      * \return \c true If the file is new to the list.
      */
-    bool Add(const std::string file)
+    bool Add(const std::string& file)
     {
         if (std::find(m_files.begin(), m_files.end(), file) == m_files.end())
         {
diff --git a/src/core/model/command-line.cc b/src/core/model/command-line.cc
index 537f99612..3423bb08e 100644
--- a/src/core/model/command-line.cc
+++ b/src/core/model/command-line.cc
@@ -761,7 +761,7 @@ CommandLine::AddValue(const std::string& name,
 void
 CommandLine::AddValue(const std::string& name,
                       const std::string& help,
-                      ns3::Callback<bool, std::string> callback,
+                      ns3::Callback<bool, const std::string&> callback,
                       const std::string& defaultValue /* = "" */)
 
 {
diff --git a/src/core/model/command-line.h b/src/core/model/command-line.h
index 3cde4a9b0..60d42f1cf 100644
--- a/src/core/model/command-line.h
+++ b/src/core/model/command-line.h
@@ -322,7 +322,7 @@ class CommandLine
      */
     void AddValue(const std::string& name,
                   const std::string& help,
-                  ns3::Callback<bool, std::string> callback,
+                  ns3::Callback<bool, const std::string&> callback,
                   const std::string& defaultValue = "");
 
     /**
@@ -533,7 +533,7 @@ class CommandLine
         bool HasDefault() const override;
         std::string GetDefault() const override;
 
-        ns3::Callback<bool, std::string> m_callback; /**< The Callback */
+        ns3::Callback<bool, const std::string&> m_callback; /**< The Callback */
         std::string m_default; /**< The default value, as a string, if it exists. */
     };                         // class CallbackItem