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doc/Migrating code to v4.md

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+Migrating code written for Arduino JSON v3 to v4
+================================================
+
+Arduino JSON v4 was a major rewrite of the library, and the API change significantly.
+
+## Includes
+
+Arduino JSON v3 had two include files:
+
+    #include <JsonParser.h>
+    #include <JsonGenerator.h>
+
+Arduino JSON v4 only has one:
+
+	#include <ArduinoJson.h>
+
+Node: the header `src/ArduinoJson.h` is intended to be used within the Arduino IDE, if you're in another environment, you may need to include the following headers:
+
+	#include <ArduinoJson/StaticJsonBuffer.hpp>
+	#include <ArduinoJson/JsonObject.hpp>
+	#include <ArduinoJson/JsonArray.hpp>
+
+## Namespaces
+
+Arduino JSON v3 had two namespaces:
+
+	using namespace ArduinoJson::Parser;
+	using namespace ArduinoJson::Generator;
+
+Arduino JSON v4 only has one:
+
+	using namespace ArduinoJson;
+
+If you include the header `ArduinoJson.h` (recommended if in Arduino IDE), the `using` directivei is already done for you, so you don't have to write it.
+
+## StaticJsonBuffer
+
+Arduino JSON v3 had different memory allocation models for parser:
+
+	JsonParser<16> parser; // 16 being the capacity in "tokens"
+
+and for the generator:
+
+	JsonArray<4> array; // 4 being the number of element
+	JsonObject<4> object;
+
+Arduino JSON v4 only has one memory allocation mode:
+
+	StaticJsonBuffer<128> buffer; // 128 being the capacity in bytes
+
+## Return values for the parser
+
+Arduino JSON v3 returned `JsonArray` and `JsonObject`:
+
+	JsonArray array = parser.parseArray(json);
+	JsonObject object = parser.parseObject(json);
+
+Arduino JSON v4 returns references:
+
+	JsonArray& array = buffer.parseArray(json);
+	JsonObject& object = buffer.parseObject(json);
+
+Everything else is compatible
+
+## Creating arrays and objects
+
+Arduino JSON v3 allowed to create `JsonArray` and `JsonObject` directly:
+
+	JsonArray<4> array;
+	JsonObject<4> object;
+
+Arduino JSON v4 requires that you use a `StaticJsonBuffer` for that:
+
+	JsonArray& array = buffer.createArray();
+	JsonObject& object = buffer.createObject();
+
+Note: you don't have to specify the capacity anymore.
+
+## Printable interface
+
+Arduino JSON v3 used to implement the Printable interface, that allowed that kind of statement:
+
+    Serial.print(array);
+
+Arduino JSON v4 doesn't, so you need to write this:
+
+	array.printTo(Serial);
+
+Note: there is a good reason for that: reducing the size of `JsonArray` and `JsonObject`.