diff --git a/iguana/dynamic.hpp b/iguana/dynamic.hpp
index cfb5b116..4b479b0c 100644
--- a/iguana/dynamic.hpp
+++ b/iguana/dynamic.hpp
@@ -17,9 +17,9 @@ constexpr inline uint8_t ENABLE_ALL = 0x0F;
 
 template <typename T, uint8_t ENABLE_FLAG = ENABLE_PB>
 struct base_impl : public base {
-  void to_pb(std::string& str) override {
+  void to_pb(std::string& str) const override {
     if constexpr ((ENABLE_FLAG & ENABLE_PB) != 0) {
-      to_pb_adl((iguana_adl_t*)nullptr, *(static_cast<T*>(this)), str);
+      to_pb_adl((iguana_adl_t*)nullptr, *(static_cast<T const*>(this)), str);
     }
     else {
       throw std::runtime_error("Protobuf Disabled");
@@ -35,9 +35,9 @@ struct base_impl : public base {
     }
   }
 
-  void to_json(std::string& str) override {
+  void to_json(std::string& str) const override {
     if constexpr ((ENABLE_FLAG & ENABLE_JSON) != 0) {
-      to_json_adl((iguana_adl_t*)nullptr, *(static_cast<T*>(this)), str);
+      to_json_adl((iguana_adl_t*)nullptr, *(static_cast<T const*>(this)), str);
     }
     else {
       throw std::runtime_error("Json Disabled");
@@ -53,9 +53,9 @@ struct base_impl : public base {
     }
   }
 
-  void to_xml(std::string& str) override {
+  void to_xml(std::string& str) const override {
     if constexpr ((ENABLE_FLAG & ENABLE_XML) != 0) {
-      to_xml_adl((iguana_adl_t*)nullptr, *(static_cast<T*>(this)), str);
+      to_xml_adl((iguana_adl_t*)nullptr, *(static_cast<T const*>(this)), str);
     }
     else {
       throw std::runtime_error("Xml Disabled");
@@ -71,9 +71,9 @@ struct base_impl : public base {
     }
   }
 
-  void to_yaml(std::string& str) override {
+  void to_yaml(std::string& str) const override {
     if constexpr ((ENABLE_FLAG & ENABLE_YAML) != 0) {
-      to_yaml_adl((iguana_adl_t*)nullptr, *(static_cast<T*>(this)), str);
+      to_yaml_adl((iguana_adl_t*)nullptr, *(static_cast<T const*>(this)), str);
     }
     else {
       throw std::runtime_error("Yaml Disabled");
@@ -89,18 +89,20 @@ struct base_impl : public base {
     }
   }
 
-  iguana::detail::field_info get_field_info(std::string_view name) override {
+  iguana::detail::field_info get_field_info(
+      std::string_view name) const override {
     static constexpr auto map = iguana::get_members<T>();
     iguana::detail::field_info info{};
-    for (auto& [no, field] : map) {
+    for (auto const& [no, field] : map) {
       if (info.offset > 0) {
         break;
       }
       std::visit(
-          [&](auto val) {
+          [&](auto const& val) {
             if (val.field_name == name) {
               info.offset = member_offset((T*)this, val.member_ptr);
-              using value_type = typename decltype(val)::value_type;
+              using value_type =
+                  typename std::remove_reference_t<decltype(val)>::value_type;
 #if defined(__clang__) || defined(_MSC_VER) || \
     (defined(__GNUC__) && __GNUC__ > 8)
               info.type_name = type_string<value_type>();
@@ -136,17 +138,18 @@ struct base_impl : public base {
     static constexpr auto map = iguana::get_members<T>();
     std::any result;
 
-    for (auto [no, field] : map) {
+    for (auto const& [no, field] : map) {
       if (result.has_value()) {
         break;
       }
       std::visit(
-          [&](auto val) {
+          [&](auto const& val) {
             if (val.field_name == name) {
-              using value_type = typename decltype(val)::value_type;
+              using value_type =
+                  typename std::remove_reference_t<decltype(val)>::value_type;
               auto const offset = member_offset((T*)this, val.member_ptr);
-              auto ptr = (((char*)this) + offset);
-              result = {*((value_type*)ptr)};
+              auto ptr = (char*)this + offset;
+              result = *((value_type*)ptr);
             }
           },
           field);
@@ -157,7 +160,7 @@ struct base_impl : public base {
 
   virtual ~base_impl() {}
 
-  size_t cache_size = 0;
+  mutable size_t cache_size = 0;
 };
 
 IGUANA_INLINE std::shared_ptr<base> create_instance(std::string_view name) {
diff --git a/iguana/pb_util.hpp b/iguana/pb_util.hpp
index 5fa7365c..27bc9a89 100644
--- a/iguana/pb_util.hpp
+++ b/iguana/pb_util.hpp
@@ -419,6 +419,7 @@ IGUANA_INLINE size_t pb_oneof_size(Type&& t, Arr& size_arr) {
   int len = 0;
   std::visit(
       [&len, &size_arr](auto&& value) IGUANA__INLINE_LAMBDA {
+        using raw_value_type = decltype(value);
         using value_type =
             std::remove_const_t<std::remove_reference_t<decltype(value)>>;
         constexpr auto offset =
@@ -427,7 +428,7 @@ IGUANA_INLINE size_t pb_oneof_size(Type&& t, Arr& size_arr) {
             ((field_no + offset) << 3) |
             static_cast<uint32_t>(get_wire_type<value_type>());
         len = pb_key_value_size<variant_uint32_size_constexpr(key), false>(
-            std::forward<value_type>(value), size_arr);
+            std::forward<raw_value_type>(value), size_arr);
       },
       std::forward<Type>(t));
   return len;
@@ -454,7 +455,7 @@ IGUANA_INLINE size_t pb_key_value_size(Type&& t, Arr& size_arr) {
                                    std::decay_t<decltype(tuple)>>;
           constexpr auto value = std::get<decltype(i)::value>(tuple);
           using U = typename field_type::value_type;
-          auto& val = value.value(t);
+          auto const& val = value.value(t);
           if constexpr (variant_v<U>) {
             constexpr auto offset =
                 get_variant_index<U, typename field_type::sub_type,
diff --git a/iguana/pb_writer.hpp b/iguana/pb_writer.hpp
index 4c39305a..219650fb 100644
--- a/iguana/pb_writer.hpp
+++ b/iguana/pb_writer.hpp
@@ -83,6 +83,7 @@ IGUANA_INLINE void to_pb_oneof(Type&& t, It&& it, uint32_t*& sz_ptr) {
   using T = std::decay_t<Type>;
   std::visit(
       [&it, &sz_ptr](auto&& value) IGUANA__INLINE_LAMBDA {
+        using raw_value_type = decltype(value);
         using value_type =
             std::remove_const_t<std::remove_reference_t<decltype(value)>>;
         constexpr auto offset =
@@ -90,7 +91,7 @@ IGUANA_INLINE void to_pb_oneof(Type&& t, It&& it, uint32_t*& sz_ptr) {
         constexpr uint32_t key =
             ((field_no + offset) << 3) |
             static_cast<uint32_t>(get_wire_type<value_type>());
-        to_pb_impl<key, false>(std::forward<value_type>(value), it, sz_ptr);
+        to_pb_impl<key, false>(std::forward<raw_value_type>(value), it, sz_ptr);
       },
       std::forward<Type>(t));
 }
@@ -449,7 +450,7 @@ IGUANA_INLINE void build_sub_proto(Map& map, std::string_view str_type,
 }  // namespace detail
 
 template <typename T, typename Stream>
-IGUANA_INLINE void to_pb(T& t, Stream& out) {
+IGUANA_INLINE void to_pb(T const& t, Stream& out) {
   std::vector<uint32_t> size_arr;
   auto byte_len = detail::pb_key_value_size<0>(t, size_arr);
   detail::resize(out, byte_len);
@@ -491,7 +492,7 @@ IGUANA_INLINE void to_proto_file(Stream& stream, std::string_view ns = "") {
 #endif
 
 template <typename T, typename Stream>
-IGUANA_INLINE void to_pb_adl(iguana_adl_t* p, T& t, Stream& out) {
+IGUANA_INLINE void to_pb_adl(iguana_adl_t* p, T const& t, Stream& out) {
   to_pb(t, out);
 }
 
diff --git a/iguana/reflection.hpp b/iguana/reflection.hpp
index ceb605a4..6bad499c 100644
--- a/iguana/reflection.hpp
+++ b/iguana/reflection.hpp
@@ -562,17 +562,18 @@ struct field_info {
 };
 
 struct base {
-  virtual void to_pb(std::string &str) {}
+  virtual void to_pb(std::string &str) const {}
   virtual void from_pb(std::string_view str) {}
-  virtual void to_xml(std::string &str) {}
+  virtual void to_xml(std::string &str) const {}
   virtual void from_xml(std::string_view str) {}
-  virtual void to_json(std::string &str) {}
+  virtual void to_json(std::string &str) const {}
   virtual void from_json(std::string_view str) {}
-  virtual void to_yaml(std::string &str) {}
+  virtual void to_yaml(std::string &str) const {}
   virtual void from_yaml(std::string_view str) {}
   virtual std::vector<std::string_view> get_fields_name() const { return {}; }
   virtual std::any get_field_any(std::string_view name) const { return {}; }
-  virtual iguana::detail::field_info get_field_info(std::string_view name) {
+  virtual iguana::detail::field_info get_field_info(
+      std::string_view name) const {
     return {};
   }
 
@@ -802,6 +803,7 @@ struct field_t {
   uint32_t field_no;
 
   auto &value(owner_type &value) const { return value.*member_ptr; }
+  auto const &value(owner_type const &value) const { return value.*member_ptr; }
 };
 
 template <typename T>
diff --git a/test/test_pb.cpp b/test/test_pb.cpp
index 85e27988..7e85aa1b 100644
--- a/test/test_pb.cpp
+++ b/test/test_pb.cpp
@@ -242,6 +242,17 @@ struct numer_st PUBLIC(numer_st) {
 };
 REFLECTION(numer_st, a, b, c);
 
+struct MyPerson : public iguana::base_impl<MyPerson, iguana::ENABLE_JSON> {
+  MyPerson() = default;
+  MyPerson(std::string s, int d) : name(s), age(d) {}
+  std::string name;
+  int64_t age;
+  bool operator==(const MyPerson &other) const {
+    return name == other.name && age == other.age;
+  }
+};
+
+REFLECTION(MyPerson, name, age);
 struct person PUBLIC(person) {
   person() = default;
   person(int32_t a, std::string b, int c, double d)
@@ -386,6 +397,19 @@ TEST_CASE("test reflection") {
         std::any_cast<std::variant<int, double>>(mvariant_any);
     assert(mvariant == temp_variant);
   }
+  {
+    // to_json is an const member_function now
+    MyPerson const p1{"xiaoming", 10};
+    std::string str;
+    p1.to_json(str);
+
+    // p1.to_pb(str); // compile failed
+
+    MyPerson p2;
+    p2.from_json(str);
+
+    assert(p1 == p2);
+  }
   {
     auto t = iguana::create_instance("pair_t");
     t->set_field_value("x", 12);