diff --git a/JsonGeneratorTests/JsonArray.h b/JsonGeneratorTests/JsonArray.h
index 9d612684..cf1bea97 100644
--- a/JsonGeneratorTests/JsonArray.h
+++ b/JsonGeneratorTests/JsonArray.h
@@ -37,7 +37,7 @@ private:
     JsonValue items[N];
     int itemCount;
 
-    virtual size_t writeTo(Print& p)
+    virtual size_t writeTo(Print& p) const
     {
         size_t n = 0;
         
diff --git a/JsonGeneratorTests/JsonGeneratorTests.vcxproj b/JsonGeneratorTests/JsonGeneratorTests.vcxproj
index 3de0894e..67807102 100644
--- a/JsonGeneratorTests/JsonGeneratorTests.vcxproj
+++ b/JsonGeneratorTests/JsonGeneratorTests.vcxproj
@@ -94,6 +94,7 @@
     <ClInclude Include="JsonObjectBase.h" />
     <ClInclude Include="Print.h" />
     <ClInclude Include="JsonValue.h" />
+    <ClInclude Include="Printable.h" />
     <ClInclude Include="StringBuilder.h" />
   </ItemGroup>
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
diff --git a/JsonGeneratorTests/JsonGeneratorTests.vcxproj.filters b/JsonGeneratorTests/JsonGeneratorTests.vcxproj.filters
index db203657..0ac6b8d9 100644
--- a/JsonGeneratorTests/JsonGeneratorTests.vcxproj.filters
+++ b/JsonGeneratorTests/JsonGeneratorTests.vcxproj.filters
@@ -50,5 +50,8 @@
     <ClInclude Include="Print.h">
       <Filter>Header Files</Filter>
     </ClInclude>
+    <ClInclude Include="Printable.h">
+      <Filter>Header Files</Filter>
+    </ClInclude>
   </ItemGroup>
 </Project>
\ No newline at end of file
diff --git a/JsonGeneratorTests/JsonHashTable.h b/JsonGeneratorTests/JsonHashTable.h
index aca50bf7..f2894d73 100644
--- a/JsonGeneratorTests/JsonHashTable.h
+++ b/JsonGeneratorTests/JsonHashTable.h
@@ -45,7 +45,7 @@ private:
     KeyValuePair items[N];
     int itemCount;
 
-    virtual size_t writeTo(Print& p)
+    virtual size_t writeTo(Print& p) const
     {
         size_t n = 0;
 
diff --git a/JsonGeneratorTests/JsonObjectBase.h b/JsonGeneratorTests/JsonObjectBase.h
index f8ffd3f9..7daad792 100644
--- a/JsonGeneratorTests/JsonObjectBase.h
+++ b/JsonGeneratorTests/JsonObjectBase.h
@@ -7,8 +7,9 @@
 
 #include "JsonValue.h"
 #include "Print.h"
+#include "Printable.h"
 
-class JsonObjectBase
+class JsonObjectBase : public Printable
 {
 public:
 
@@ -18,6 +19,6 @@ public:
         return writeTo(sb);
     }
 
-    virtual size_t writeTo(Print& p) = 0;
+    virtual size_t writeTo(Print& p) const = 0;
 };
 
diff --git a/JsonGeneratorTests/JsonValue.cpp b/JsonGeneratorTests/JsonValue.cpp
index 284fef47..5a23e9b0 100644
--- a/JsonGeneratorTests/JsonValue.cpp
+++ b/JsonGeneratorTests/JsonValue.cpp
@@ -8,12 +8,12 @@
 #include <cstdio>
 #include <cstring>
 
-size_t JsonValue::writeBooleanTo(Print& p)
+size_t JsonValue::writeBooleanTo(Print& p) const
 {
     return p.write(content.boolean ? "true" : "false");
 }
 
-size_t JsonValue::writeNumberTo(Print& p)
+size_t JsonValue::writeNumberTo(Print& p) const
 {
     char tmp[16];
 
@@ -22,7 +22,7 @@ size_t JsonValue::writeNumberTo(Print& p)
     return p.write(tmp);
 }
 
-size_t JsonValue::writeObjectTo(Print& p)
+size_t JsonValue::writeObjectTo(Print& p) const
 {
     if (content.object)
         return ((JsonObjectBase*)content.object)->writeTo(p);
@@ -30,7 +30,7 @@ size_t JsonValue::writeObjectTo(Print& p)
         return p.write("null");
 }
 
-size_t JsonValue::writeStringTo(Print& p)
+size_t JsonValue::writeStringTo(Print& p) const
 {
     auto s = content.string;
 
diff --git a/JsonGeneratorTests/JsonValue.h b/JsonGeneratorTests/JsonValue.h
index 9f98c4dd..5fb506b6 100644
--- a/JsonGeneratorTests/JsonValue.h
+++ b/JsonGeneratorTests/JsonValue.h
@@ -5,11 +5,10 @@
 
 #pragma once
 
+#include "Printable.h"
 #include "StringBuilder.h"
 
-class JsonObjectBase;
-
-class JsonValue
+class JsonValue : public Printable
 {    
 public:
 
@@ -35,13 +34,13 @@ public:
         content.boolean = value;
     }
 
-    JsonValue(JsonObjectBase& value)
+    JsonValue(Printable& value)
         : implementation(&JsonValue::writeObjectTo)
     {
         content.object = &value;
     }
 
-    size_t writeTo(Print& p)
+    virtual size_t writeTo(Print& p) const
     {
         // handmade polymorphism
         return (this->*implementation)(p);
@@ -51,18 +50,18 @@ private:
     
     union Content
     {
-        bool            boolean;
-        double          number;
-        JsonObjectBase* object;
-        const char*     string;
+        bool        boolean;
+        double      number;
+        Printable*  object;
+        const char* string;
     };
 
     Content content;
 
-    size_t(JsonValue::*implementation)(Print& p);
+    size_t(JsonValue::*implementation)(Print& p)const;
 
-    size_t writeBooleanTo(Print& p);
-    size_t writeNumberTo(Print& p);
-    size_t writeObjectTo(Print& p);
-    size_t writeStringTo(Print& p);
+    size_t writeBooleanTo(Print& p) const;
+    size_t writeNumberTo(Print& p) const;
+    size_t writeObjectTo(Print& p) const;
+    size_t writeStringTo(Print& p) const;
 };
\ No newline at end of file
diff --git a/JsonGeneratorTests/Printable.h b/JsonGeneratorTests/Printable.h
new file mode 100644
index 00000000..136aac7d
--- /dev/null
+++ b/JsonGeneratorTests/Printable.h
@@ -0,0 +1,16 @@
+/*
+ * Arduino JSON library
+ * Benoit Blanchon 2014 - MIT License
+ */
+
+#pragma once
+
+class Print;
+
+class Printable
+{
+public:
+
+    virtual size_t writeTo(Print& p) const = 0;
+};
+