diff --git a/blobstamper/stamp_enumerator.cpp b/blobstamper/stamp_enumerator.cpp
index 8e1146c..ea12be6 100644
--- a/blobstamper/stamp_enumerator.cpp
+++ b/blobstamper/stamp_enumerator.cpp
@@ -26,10 +26,10 @@
 
 std::string StampStrEnumerator::ExtractStr(std::shared_ptr<Blob> blob)
 {
-  std::vector<std::string> data = ExtractStrVector(blob);
+  const std::vector<std::string> &data = ExtractStrVector(blob);
   std::string res = "";
 
-  for (std::string s : data)
+  for (const std::string &s : data)
   {
     if (!res.empty())
     {
diff --git a/blobstamper/stamp_json.cpp b/blobstamper/stamp_json.cpp
index 0fb5d2d..ec86029 100644
--- a/blobstamper/stamp_json.cpp
+++ b/blobstamper/stamp_json.cpp
@@ -27,7 +27,7 @@
 PoolPickerStamp::PoolPickerStamp(std::vector<std::shared_ptr<StampBaseStr>> new_pool)
         : pool{new_pool}
 {
-  for (auto stamp : pool)
+  for (const auto &stamp : pool)
   {
      std::weak_ptr<StampBaseStr> wp = stamp;
      weak_pool.push_back(wp);
@@ -40,7 +40,7 @@ PoolPickerStamp::isRecursive()
   if (is_recursive || is_in_recursion)
     return true;
   is_in_recursion = true;
-  for (auto stamp : weak_pool)
+  for (const auto &stamp : weak_pool)
   {
     if (stamp.lock()->isRecursive())
     {
@@ -62,7 +62,7 @@ PoolPickerStamp::ExtractStr(std::shared_ptr<Blob> blob)
   std::vector<std::weak_ptr<StampBaseStr>> target_pool;
   std::vector<std::weak_ptr<StampBaseStr>> unbounded_pool;
 
-  for (auto stamp_w : weak_pool)
+  for (const auto &stamp_w : weak_pool)
   {
     auto stamp = stamp_w.lock();
     if (stamp->minSize() <= blob->Size())
@@ -91,7 +91,7 @@ PoolPickerStamp::minSize()
   if (is_in_recursion)
     return res;
   is_in_recursion = true; /* Do not use isRecursive() inside as it uses same flag*/
-  for(auto stamp : weak_pool)
+  for (const auto &stamp : weak_pool)
   {
     int candidat = stamp.lock()->minSize();
     if (res > candidat)
@@ -111,7 +111,7 @@ PoolPickerStamp::maxSize()
   if (is_recursive || is_in_recursion)
     return -1;
   is_in_recursion = true; /* Do not use isRecursive() inside as it uses same flag*/
-  for (auto stamp : weak_pool)
+  for (const auto &stamp : weak_pool)
   {
     int candidat = stamp.lock()->maxSize();
     if (candidat == -1)
@@ -144,8 +144,8 @@ StampJSONString::ExtractStr(std::shared_ptr<Blob> blob)
 std::string
 StampJSONHashEl::ExtractStr(std::shared_ptr<Blob> blob)
 {
-   std::string n = stamp_name->ExtractStr(blob);
-   std::string v = stamp_value->ExtractStr(blob);
+   const std::string &n = stamp_name->ExtractStr(blob);
+   const std::string &v = stamp_value->ExtractStr(blob);
    return n + ": " + v;
 }
 
diff --git a/blobstamper/stamp_math_op.cpp b/blobstamper/stamp_math_op.cpp
index 50ef4a6..7987f22 100644
--- a/blobstamper/stamp_math_op.cpp
+++ b/blobstamper/stamp_math_op.cpp
@@ -29,7 +29,7 @@ StampMathUnaryOp::ExtractStr(std::shared_ptr<Blob> blob)
 std::string
 StampMathBinaryOp::ExtractStr(std::shared_ptr<Blob> blob)
 {
-  std::vector<std::shared_ptr<Blob>> blobs = extract_internal(blob);
+  const std::vector<std::shared_ptr<Blob>> &blobs = extract_internal(blob);
   return (std::string)"(" +  stamp1->ExtractStr(blobs[0]) + " " +  op_name + " " + stamp2->ExtractStr(blobs[1]) + ")";
 }
 
diff --git a/blobstamper/stamp_text.cpp b/blobstamper/stamp_text.cpp
index cadddfd..040b370 100644
--- a/blobstamper/stamp_text.cpp
+++ b/blobstamper/stamp_text.cpp
@@ -38,10 +38,10 @@ StampTextPulp::ExtractStr(std::shared_ptr<Blob> blob)
 
 std::string StampTextPulpWords::ExtractStr(std::shared_ptr<Blob> blob)
 {
-  std::vector<std::string> data = ExtractStrVector(blob);
+  const std::vector<std::string> &data = ExtractStrVector(blob);
   std::string res = "";
 
-  for(std::string s : data)
+  for (const std::string &s : data)
   {
     if (!res.empty())
     {
@@ -54,10 +54,10 @@ std::string StampTextPulpWords::ExtractStr(std::shared_ptr<Blob> blob)
 
 std::string StampTextDictWords::ExtractStr(std::shared_ptr<Blob> blob)
 {
-  std::vector<std::string> data = ExtractStrVector(blob);
+  const std::vector<std::string> &data = ExtractStrVector(blob);
   std::string res = "";
 
-  for(std::string s : data)
+  for (const std::string &s : data)
   {
     if (!res.empty())
     {