Migrating code written for Arduino JSON v3 to v4

Arduino JSON v4 was a major rewrite of the library, and the API changed significantly.

Includes

Arduino JSON v3 had two include files:

#include <JsonParser.h>
#include <JsonGenerator.h>

Arduino JSON v4 only has one:

#include <ArduinoJson.h>

Arduino JSON v3 had two namespaces:

using namespace ArduinoJson::Parser;
using namespace ArduinoJson::Generator;

Arduino JSON v4 doesn't require the using namespace statement. It has a namespace but the using namespace is done in the header file.

Arduino JSON v3 had different memory allocation models for the 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 model:

StaticJsonBuffer<128> buffer; // 128 being the capacity in bytes

Arduino JSON v3 returned value types:

JsonArray array = parser.parseArray(json);
JsonObject object = parser.parseObject(json);

Arduino JSON v4 returns references types:

JsonArray& array = buffer.parseArray(json);
JsonObject& object = buffer.parseObject(json);

Everything else is compatible

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.

Arduino JSON v3 used to implement the Printable interface, which allowed statements like:

Serial.print(array);

But Arduino JSON v4 doesn't, instead you need to write this:

array.printTo(Serial);

Note: there was a good reason for removing that feature, and it's reducing the size of JsonArray and JsonObject.