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.

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