diff --git a/include/util/string.hpp b/include/util/string.hpp
index c8c0209..24a9b2b 100644
--- a/include/util/string.hpp
+++ b/include/util/string.hpp
@@ -4,12 +4,12 @@
 
 const std::string WHITESPACE = " \n\r\t\f\v";
 
-inline std::string ltrim(const std::string s) {
+inline std::string ltrim(const std::string& s) {
   size_t begin = s.find_first_not_of(WHITESPACE);
   return (begin == std::string::npos) ? "" : s.substr(begin);
 }
 
-inline std::string rtrim(const std::string s) {
+inline std::string rtrim(const std::string& s) {
   size_t end = s.find_last_not_of(WHITESPACE);
   return (end == std::string::npos) ? "" : s.substr(0, end + 1);
 }