diff --git a/cpp/include/utils/data_conversion.hpp b/cpp/include/utils/data_conversion.hpp
index e9ca7067..86e78309 100644
--- a/cpp/include/utils/data_conversion.hpp
+++ b/cpp/include/utils/data_conversion.hpp
@@ -20,14 +20,14 @@
 #define UPPER_CASE_BIT_MASK 0b11011111
 #define LOWER_CASE_BIT_MASK 0b00100000
 
-bool is_letter(const char& c) {
+bool is_letter(char c) {
     char lower_cased_char = (char)(c | LOWER_CASE_BIT_MASK);
     return lower_cased_char >= 'a' && lower_cased_char <= 'z';
 }
 
 std::string make_upper_case(const std::string& input) {
     std::string result;
-    for (auto& c: input) {
+    for (auto c: input) {
 
         if (is_letter(c)) {
             result += c & UPPER_CASE_BIT_MASK;
@@ -42,7 +42,7 @@ std::string make_upper_case(const std::string& input) {
 
 std::string make_lower_case(const std::string& input) {
     std::string result;
-    for (auto& current_char: input) {
+    for (auto current_char: input) {
 
         if (is_letter(current_char)) {
             result += current_char | LOWER_CASE_BIT_MASK;