diff --git a/src/ArduinoJson/Json/JsonSerializer.hpp b/src/ArduinoJson/Json/JsonSerializer.hpp
index c47f339a..a4894bbe 100644
--- a/src/ArduinoJson/Json/JsonSerializer.hpp
+++ b/src/ArduinoJson/Json/JsonSerializer.hpp
@@ -6,7 +6,8 @@
 
 #include "../Serialization/measure.hpp"
 #include "../Serialization/serialize.hpp"
-#include "./JsonWriter.hpp"
+#include "../Visitable.hpp"
+#include "JsonWriter.hpp"
 
 namespace ARDUINOJSON_NAMESPACE {
 
@@ -103,32 +104,9 @@ size_t measureJson(const TSource &source) {
 }
 
 #if ARDUINOJSON_ENABLE_STD_STREAM
-inline std::ostream &operator<<(std::ostream &os, const JsonArray &source) {
-  serializeJson(source, os);
-  return os;
-}
-inline std::ostream &operator<<(std::ostream &os, const JsonObject &source) {
-  serializeJson(source, os);
-  return os;
-}
-inline std::ostream &operator<<(std::ostream &os, const JsonVariant &source) {
-  serializeJson(source, os);
-  return os;
-}
-inline std::ostream &operator<<(std::ostream &os, JsonVariantConst source) {
-  os << serializeJson(source, os);
-  return os;
-}
-
-inline std::ostream &operator<<(std::ostream &os,
-                                const JsonArraySubscript &source) {
-  serializeJson(source, os);
-  return os;
-}
-
-template <typename TKey>
-inline std::ostream &operator<<(std::ostream &os,
-                                const JsonObjectSubscript<TKey> &source) {
+template <typename T>
+inline typename enable_if<IsVisitable<T>::value, std::ostream &>::type
+operator<<(std::ostream &os, const T &source) {
   serializeJson(source, os);
   return os;
 }
diff --git a/src/ArduinoJson/JsonArray.hpp b/src/ArduinoJson/JsonArray.hpp
index f3a85e73..a4b73ea8 100644
--- a/src/ArduinoJson/JsonArray.hpp
+++ b/src/ArduinoJson/JsonArray.hpp
@@ -38,13 +38,22 @@ class JsonArrayProxy {
   TData* _data;
 };
 
-class JsonArrayConst : public JsonArrayProxy<const JsonArrayData> {
+class JsonArrayConst : public JsonArrayProxy<const JsonArrayData>,
+                       public Visitable {
   friend class JsonArray;
   typedef JsonArrayProxy<const JsonArrayData> proxy_type;
 
  public:
   typedef JsonArrayConstIterator iterator;
 
+  template <typename Visitor>
+  FORCE_INLINE void accept(Visitor& visitor) const {
+    if (_data)
+      visitor.visitArray(*this);
+    else
+      visitor.visitNull();
+  }
+
   FORCE_INLINE iterator begin() const {
     if (!_data) return iterator();
     return iterator(_data->head);
@@ -62,7 +71,7 @@ class JsonArrayConst : public JsonArrayProxy<const JsonArrayData> {
   }
 };
 
-class JsonArray : public JsonArrayProxy<JsonArrayData> {
+class JsonArray : public JsonArrayProxy<JsonArrayData>, public Visitable {
   typedef JsonArrayProxy<JsonArrayData> proxy_type;
 
  public:
@@ -224,10 +233,7 @@ class JsonArray : public JsonArrayProxy<JsonArrayData> {
 
   template <typename Visitor>
   FORCE_INLINE void accept(Visitor& visitor) const {
-    if (_data)
-      visitor.visitArray(*this);
-    else
-      visitor.visitNull();
+    JsonArrayConst(_data).accept(visitor);
   }
 
  private:
diff --git a/src/ArduinoJson/JsonArraySubscript.hpp b/src/ArduinoJson/JsonArraySubscript.hpp
index a44c0202..1ea37e76 100644
--- a/src/ArduinoJson/JsonArraySubscript.hpp
+++ b/src/ArduinoJson/JsonArraySubscript.hpp
@@ -13,7 +13,8 @@
 #endif
 
 namespace ARDUINOJSON_NAMESPACE {
-class JsonArraySubscript : public JsonVariantBase<JsonArraySubscript> {
+class JsonArraySubscript : public JsonVariantBase<JsonArraySubscript>,
+                           public Visitable {
  public:
   FORCE_INLINE JsonArraySubscript(JsonArray array, size_t index)
       : _array(array), _index(index) {}
diff --git a/src/ArduinoJson/JsonDocument.hpp b/src/ArduinoJson/JsonDocument.hpp
index b5478372..57ce63a5 100644
--- a/src/ArduinoJson/JsonDocument.hpp
+++ b/src/ArduinoJson/JsonDocument.hpp
@@ -12,7 +12,7 @@
 namespace ARDUINOJSON_NAMESPACE {
 
 template <typename TMemoryPool>
-class JsonDocument {
+class JsonDocument : public Visitable {
  public:
   uint8_t nestingLimit;
 
diff --git a/src/ArduinoJson/JsonObject.hpp b/src/ArduinoJson/JsonObject.hpp
index 964e02b2..a7f3fa0d 100644
--- a/src/ArduinoJson/JsonObject.hpp
+++ b/src/ArduinoJson/JsonObject.hpp
@@ -33,6 +33,10 @@ class JsonObjectProxy {
     return objectContainsKey(_data, makeString(key));
   }
 
+  FORCE_INLINE bool isNull() const {
+    return _data == 0;
+  }
+
   FORCE_INLINE size_t size() const {
     return objectSize(_data);
   }
@@ -42,7 +46,8 @@ class JsonObjectProxy {
   TData* _data;
 };
 
-class JsonObjectConst : public JsonObjectProxy<const JsonObjectData> {
+class JsonObjectConst : public JsonObjectProxy<const JsonObjectData>,
+                        public Visitable {
   friend class JsonObject;
   typedef JsonObjectProxy<const JsonObjectData> proxy_type;
 
@@ -52,6 +57,14 @@ class JsonObjectConst : public JsonObjectProxy<const JsonObjectData> {
   JsonObjectConst() : proxy_type(0) {}
   JsonObjectConst(const JsonObjectData* data) : proxy_type(data) {}
 
+  template <typename Visitor>
+  FORCE_INLINE void accept(Visitor& visitor) const {
+    if (_data)
+      visitor.visitObject(*this);
+    else
+      visitor.visitNull();
+  }
+
   FORCE_INLINE iterator begin() const {
     if (!_data) return iterator();
     return iterator(_data->head);
@@ -110,7 +123,7 @@ class JsonObjectConst : public JsonObjectProxy<const JsonObjectData> {
   }
 };
 
-class JsonObject : public JsonObjectProxy<JsonObjectData> {
+class JsonObject : public JsonObjectProxy<JsonObjectData>, public Visitable {
   typedef JsonObjectProxy<JsonObjectData> proxy_type;
 
  public:
@@ -304,16 +317,9 @@ class JsonObject : public JsonObjectProxy<JsonObjectData> {
     return set_impl(key);
   }
 
-  FORCE_INLINE bool isNull() const {
-    return _data == 0;
-  }
-
   template <typename Visitor>
   FORCE_INLINE void accept(Visitor& visitor) const {
-    if (_data)
-      visitor.visitObject(JsonObjectConst(_data));
-    else
-      visitor.visitNull();
+    JsonObjectConst(_data).accept(visitor);
   }
 
  private:
diff --git a/src/ArduinoJson/JsonObjectSubscript.hpp b/src/ArduinoJson/JsonObjectSubscript.hpp
index f75df9eb..bcba2c48 100644
--- a/src/ArduinoJson/JsonObjectSubscript.hpp
+++ b/src/ArduinoJson/JsonObjectSubscript.hpp
@@ -17,7 +17,8 @@ namespace ARDUINOJSON_NAMESPACE {
 
 template <typename TStringRef>
 class JsonObjectSubscript
-    : public JsonVariantBase<JsonObjectSubscript<TStringRef> > {
+    : public JsonVariantBase<JsonObjectSubscript<TStringRef> >,
+      public Visitable {
   typedef JsonObjectSubscript<TStringRef> this_type;
 
  public:
diff --git a/src/ArduinoJson/JsonVariant.hpp b/src/ArduinoJson/JsonVariant.hpp
index 4562ffab..e611ea52 100644
--- a/src/ArduinoJson/JsonVariant.hpp
+++ b/src/ArduinoJson/JsonVariant.hpp
@@ -16,6 +16,7 @@
 #include "Numbers/parseFloat.hpp"
 #include "Numbers/parseInteger.hpp"
 #include "Polyfills/type_traits.hpp"
+#include "Visitable.hpp"
 
 namespace ARDUINOJSON_NAMESPACE {
 
@@ -117,7 +118,8 @@ class JsonVariantProxy {
 // - a string (const char*)
 // - a reference to a JsonArray or JsonObject
 class JsonVariant : public JsonVariantProxy<JsonVariantData>,
-                    public JsonVariantBase<JsonVariant> {
+                    public JsonVariantBase<JsonVariant>,
+                    public Visitable {
   typedef JsonVariantProxy<JsonVariantData> proxy_type;
   friend class JsonVariantConst;
 
@@ -279,7 +281,8 @@ class JsonVariant : public JsonVariantProxy<JsonVariantData>,
 };
 
 class JsonVariantConst : public JsonVariantProxy<const JsonVariantData>,
-                         public JsonVariantBase<JsonVariantConst> {
+                         public JsonVariantBase<JsonVariantConst>,
+                         public Visitable {
   typedef JsonVariantProxy<const JsonVariantData> proxy_type;
 
  public:
@@ -318,15 +321,4 @@ class JsonVariantConst : public JsonVariantProxy<const JsonVariantData>,
     return JsonVariantConst(objectGet(variantAsObject(_data), makeString(key)));
   }
 };
-
-class JsonVariantLocal : public JsonVariant {
- public:
-  explicit JsonVariantLocal(MemoryPool *memoryPool)
-      : JsonVariant(memoryPool, &_localData) {
-    _localData.type = JSON_NULL;
-  }
-
- private:
-  JsonVariantData _localData;
-};
 }  // namespace ARDUINOJSON_NAMESPACE
diff --git a/src/ArduinoJson/MsgPack/MsgPackDeserializer.hpp b/src/ArduinoJson/MsgPack/MsgPackDeserializer.hpp
index 92e87e29..936c3075 100644
--- a/src/ArduinoJson/MsgPack/MsgPackDeserializer.hpp
+++ b/src/ArduinoJson/MsgPack/MsgPackDeserializer.hpp
@@ -278,7 +278,8 @@ class MsgPackDeserializer {
     if (_nestingLimit == 0) return DeserializationError::TooDeep;
     --_nestingLimit;
     for (; n; --n) {
-      JsonVariantLocal key(_memoryPool);
+      JsonVariantData keyData;
+      JsonVariant key(_memoryPool, &keyData);
       DeserializationError err = parse(key);
       if (err) return err;
       if (!key.is<char *>()) return DeserializationError::NotSupported;
diff --git a/src/ArduinoJson/Polyfills/type_traits/is_base_of.hpp b/src/ArduinoJson/Polyfills/type_traits/is_base_of.hpp
index dc3750ee..bb4896db 100644
--- a/src/ArduinoJson/Polyfills/type_traits/is_base_of.hpp
+++ b/src/ArduinoJson/Polyfills/type_traits/is_base_of.hpp
@@ -18,8 +18,7 @@ class is_base_of {
   static No &probe(...);
 
  public:
-  enum {
-    value = sizeof(probe(reinterpret_cast<TDerived *>(0))) == sizeof(Yes)
-  };
+  static const bool value =
+      sizeof(probe(reinterpret_cast<TDerived *>(0))) == sizeof(Yes);
 };
 }  // namespace ARDUINOJSON_NAMESPACE
diff --git a/src/ArduinoJson/Visitable.hpp b/src/ArduinoJson/Visitable.hpp
new file mode 100644
index 00000000..f7a47686
--- /dev/null
+++ b/src/ArduinoJson/Visitable.hpp
@@ -0,0 +1,18 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2018
+// MIT License
+
+#pragma once
+
+#include "Polyfills/type_traits.hpp"
+
+namespace ARDUINOJSON_NAMESPACE {
+
+struct Visitable {
+  // template<Visitor>
+  // void accept(Visitor&) const;
+};
+
+template <typename T>
+struct IsVisitable : is_base_of<Visitable, T> {};
+}  // namespace ARDUINOJSON_NAMESPACE
diff --git a/test/JsonArray/isNull.cpp b/test/JsonArray/isNull.cpp
index b2bf2456..5ceb55ac 100644
--- a/test/JsonArray/isNull.cpp
+++ b/test/JsonArray/isNull.cpp
@@ -6,39 +6,29 @@
 #include <catch.hpp>
 
 TEST_CASE("JsonArray::isNull()") {
-  SECTION("returns true for undefined JsonArray") {
-    JsonArray array;
-    REQUIRE(array.isNull() == true);
+  DynamicJsonDocument doc;
+
+  SECTION("returns true") {
+    JsonArray arr;
+    REQUIRE(arr.isNull() == true);
   }
 
-  SECTION("returns false when allocation succeeds") {
-    StaticJsonDocument<JSON_ARRAY_SIZE(0)> doc;
-    JsonArray array = doc.to<JsonArray>();
-    REQUIRE(array.isNull() == false);
+  SECTION("returns false") {
+    JsonArray arr = doc.to<JsonArray>();
+    REQUIRE(arr.isNull() == false);
   }
-
-  /*  SECTION("returns true when allocation fails") {
-      StaticJsonDocument<1> doc;
-      JsonArray array = doc.to<JsonArray>();
-      REQUIRE(array.isNull() == true);
-    }*/
 }
 
 TEST_CASE("JsonArrayConst::isNull()") {
-  SECTION("returns true for undefined JsonArray") {
-    JsonArrayConst array;
-    REQUIRE(array.isNull() == true);
+  DynamicJsonDocument doc;
+
+  SECTION("returns true") {
+    JsonArrayConst arr;
+    REQUIRE(arr.isNull() == true);
   }
 
-  SECTION("returns false when allocation succeeds") {
-    StaticJsonDocument<JSON_ARRAY_SIZE(0)> doc;
-    JsonArrayConst array = doc.to<JsonArray>();
-    REQUIRE(array.isNull() == false);
+  SECTION("returns false") {
+    JsonArrayConst arr = doc.to<JsonArray>();
+    REQUIRE(arr.isNull() == false);
   }
-
-  /*  SECTION("returns true when allocation fails") {
-      StaticJsonDocument<1> doc;
-      JsonArray array = doc.to<JsonArray>();
-      REQUIRE(array.isNull() == true);
-    }*/
 }
diff --git a/test/JsonObject/isNull.cpp b/test/JsonObject/isNull.cpp
index 176b9dae..6c7e0a9a 100644
--- a/test/JsonObject/isNull.cpp
+++ b/test/JsonObject/isNull.cpp
@@ -6,20 +6,29 @@
 #include <catch.hpp>
 
 TEST_CASE("JsonObject::isNull()") {
-  SECTION("returns true for undefined JsonObject") {
-    JsonObject array;
-    REQUIRE(array.isNull() == true);
+  DynamicJsonDocument doc;
+
+  SECTION("returns true") {
+    JsonObject obj;
+    REQUIRE(obj.isNull() == true);
   }
 
-  SECTION("returns false when allocation succeeds") {
-    StaticJsonDocument<JSON_OBJECT_SIZE(0)> doc;
-    JsonObject array = doc.to<JsonObject>();
-    REQUIRE(array.isNull() == false);
+  SECTION("returns false") {
+    JsonObject obj = doc.to<JsonObject>();
+    REQUIRE(obj.isNull() == false);
+  }
+}
+
+TEST_CASE("JsonObjectConst::isNull()") {
+  DynamicJsonDocument doc;
+
+  SECTION("returns true") {
+    JsonObjectConst obj;
+    REQUIRE(obj.isNull() == true);
+  }
+
+  SECTION("returns false") {
+    JsonObjectConst obj = doc.to<JsonObject>();
+    REQUIRE(obj.isNull() == false);
   }
-
-  /*  SECTION("returns true when allocation fails") {
-      StaticJsonDocument<1> doc;
-      JsonObject array = doc.to<JsonObject>();
-      REQUIRE(array.isNull() == true);
-    }*/
 }
diff --git a/test/Misc/TypeTraits.cpp b/test/Misc/TypeTraits.cpp
index 884f2f9e..5bd84c84 100644
--- a/test/Misc/TypeTraits.cpp
+++ b/test/Misc/TypeTraits.cpp
@@ -50,4 +50,19 @@ TEST_CASE("Polyfills/type_traits") {
     CHECK(is_unsigned<float>::value == false);
     CHECK(is_unsigned<double>::value == false);
   }
+
+  SECTION("IsVisitable") {
+    CHECK(IsVisitable<DeserializationError>::value == false);
+    CHECK(IsVisitable<JsonPair>::value == false);
+    CHECK(IsVisitable<JsonVariant>::value == true);
+    CHECK(IsVisitable<JsonVariantConst>::value == true);
+    CHECK(IsVisitable<JsonArray>::value == true);
+    CHECK(IsVisitable<JsonArraySubscript>::value == true);
+    CHECK(IsVisitable<JsonArrayConst>::value == true);
+    CHECK(IsVisitable<JsonObject>::value == true);
+    CHECK(IsVisitable<JsonObjectSubscript<const char*> >::value == true);
+    CHECK(IsVisitable<JsonObjectConst>::value == true);
+    CHECK(IsVisitable<DynamicJsonDocument>::value == true);
+    CHECK(IsVisitable<StaticJsonDocument<10> >::value == true);
+  }
 }