Added JsonPrintable::measureLength() (issue #75)

include/ArduinoJson/Internals/DummyPrint.hpp

@@ -0,0 +1,20 @@
+// Copyright Benoit Blanchon 2014-2015
+// MIT License
+//
+// Arduino JSON library
+// https://github.com/bblanchon/ArduinoJson
+
+#pragma once
+
+#include "../Arduino/Print.hpp"
+
+namespace ArduinoJson {
+namespace Internals {
+
+// A dummy Print implementation used in JsonPrintable::measureLength()
+class DummyPrint : public Print {
+ public:
+  virtual size_t write(uint8_t) { return 1; }
+};
+}
+}

include/ArduinoJson/Internals/JsonPrintable.hpp

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include "DummyPrint.hpp"
 #include "IndentedPrint.hpp"
 #include "JsonWriter.hpp"
 #include "Prettyfier.hpp"
@@ -47,6 +48,16 @@ class JsonPrintable {
     return prettyPrintTo(indentedPrint);
   }
 
+  size_t measureLength() const {
+    DummyPrint dp;
+    return printTo(dp);
+  }
+
+  size_t measurePrettyLength() const {
+    DummyPrint dp;
+    return prettyPrintTo(dp);
+  }
+
  private:
   const T &downcast() const { return *static_cast<const T *>(this); }
 };