diff --git a/include/json.hpp b/include/json.hpp
index 259370e..9cbee67 100644
--- a/include/json.hpp
+++ b/include/json.hpp
@@ -2905,9 +2905,8 @@ struct next_is_optional_t
 struct va_arg_end
 {};
 
-class dumper
+struct dumper
 {
-public:
     template <typename var_t, typename... rest_t>
     json::value _to_json(const char* key, const var_t& var, rest_t&&... rest) const
     {
@@ -2923,68 +2922,66 @@ class dumper
     json::value _to_json(va_arg_end) const { return {}; }
 };
 
-class checker
+struct checker
 {
-public:
     template <typename var_t, typename... rest_t>
-    bool _check_json(const json::value& in, std::string& error_key, const char* key, var_t&, rest_t&&... rest)
+    bool _check_json(const json::value& in, std::string& error_key, const char* key, const var_t&,
+                     rest_t&&... rest) const
     {
-        if (auto opt = in.find(key)) {
-            if (!opt->is<var_t>()) {
-                error_key = key;
-                return false;
-            }
-            _next_is_optional = false;
-        }
-        else if (!_next_is_optional) {
+        auto opt = in.find(key);
+        if (!opt || !opt->is<var_t>()) {
             error_key = key;
             return false;
         }
         return _check_json(in, error_key, std::forward<rest_t>(rest)...);
     }
-    template <typename... rest_t>
-    bool _check_json(const json::value& in, std::string& error_key, const char*, next_is_optional_t, rest_t&&... rest)
+    template <typename var_t, typename... rest_t>
+    bool _check_json(const json::value& in, std::string& error_key, const char*, next_is_optional_t, const char* key,
+                     const var_t&, rest_t&&... rest) const
     {
-        _next_is_optional = true;
+        auto opt = in.find(key);
+        if (opt) {
+            if (!opt->is<var_t>()) {
+                error_key = key;
+                return false;
+            }
+        } // next_is_optional_t, ignore key not found
+
         return _check_json(in, error_key, std::forward<rest_t>(rest)...);
     }
-    bool _check_json(const json::value&, std::string&, va_arg_end) { return true; }
-
-private:
-    bool _next_is_optional = false;
+    bool _check_json(const json::value&, std::string&, va_arg_end) const { return true; }
 };
 
-class loader
+struct loader
 {
-public:
     template <typename var_t, typename... rest_t>
-    bool _from_json(const json::value& in, std::string& error_key, const char* key, var_t& var, rest_t&&... rest)
+    bool _from_json(const json::value& in, std::string& error_key, const char* key, var_t& var, rest_t&&... rest) const
     {
-        if (auto opt = in.find(key)) {
-            if (!opt->is<var_t>()) {
-                auto test = opt->dumps();
-                error_key = key;
-                return false;
-            }
-            var = std::move(opt)->as<var_t>();
-            _next_is_optional = false;
-        }
-        else if (!_next_is_optional) {
+        auto opt = in.find(key);
+        if (!opt || !opt->is<var_t>()) {
             error_key = key;
             return false;
         }
+        var = std::move(opt)->as<var_t>();
+
         return _from_json(in, error_key, std::forward<rest_t>(rest)...);
     }
-    template <typename... rest_t>
-    bool _from_json(const json::value& in, std::string& error_key, const char*, next_is_optional_t, rest_t&&... rest)
+    template <typename var_t, typename... rest_t>
+    bool _from_json(const json::value& in, std::string& error_key, const char*, next_is_optional_t, const char* key,
+                    var_t& var, rest_t&&... rest) const
     {
-        _next_is_optional = true;
+        auto opt = in.find(key);
+        if (opt) {
+            if (!opt->is<var_t>()) {
+                error_key = key;
+                return false;
+            }
+            var = std::move(opt)->as<var_t>();
+        } // next_is_optional_t, ignore key not found
+
         return _from_json(in, error_key, std::forward<rest_t>(rest)...);
     }
-    bool _from_json(const json::value&, std::string&, va_arg_end) { return true; }
-
-private:
-    bool _next_is_optional = false;
+    bool _from_json(const json::value&, std::string&, va_arg_end) const { return true; }
 };
 } // namespace json::_jsonization_helper
 
diff --git a/sample/sample.cpp b/sample/sample.cpp
index 34245cd..e4169dc 100644
--- a/sample/sample.cpp
+++ b/sample/sample.cpp
@@ -308,7 +308,7 @@ json::value to_json(const TypeFromOtherLibrary& t)
 {
     return t.a_i;
 }
-bool check_json(const json::value& j, TypeFromOtherLibrary)
+bool check_json(const json::value& j, const TypeFromOtherLibrary&)
 {
     return j.is_number();
 }
@@ -335,7 +335,7 @@ void test_jsonization()
         int b_i = 10;
         double b_d = 0.5;
 
-        AAA b_aaa;
+        std::vector<AAA> b_aaa;
 
         MEO_JSONIZATION(b_i, MEO_OPT b_d, b_aaa);
     };