I'm new to ArduinoJSON - so perhaps it is a newbie's question.... but I wish to pass a StaticJsonDocument into a function as a parameter ( later on it should be implemented in a library ).

exmaple below shows test_1 what I wish to obtain, but by duplicating StaticJsonDocument, which I don't want to do.

How can test_1 should be written (as I tried in test_2)?

#include <ArduinoJson.h>

StaticJsonDocument<200> doc;

void createJSON() {
        doc["sensor"] = "gps";
        doc["time"] = 1351824120;

        JsonArray data = doc.createNestedArray("data");

        // Generate the minified JSON and send it to the Serial port.
        serializeJson(doc, Serial);
        Serial.println("JSON is created:");

        // Generate the prettified JSON and send it to the Serial port.
        serializeJsonPretty(doc, Serial);


bool test_1(StaticJsonDocument<100> _doc){
        serializeJson(_doc, Serial);
        return true;

bool test2(const JsonObject& _doc){
        serializeJson(_doc, Serial);

void setup() {



void loop() {
        // not used in this example
  • 1
    bool test_1(StaticJsonDocument<DOC_SIZE>& _doc) { use a define or const for the size. it must be the same
    – Juraj
    Apr 20 '19 at 14:40
  • @Juraj - why not posting it as an answer ? it works OK
    – guyd
    Apr 20 '19 at 16:51
  • it is not about Arduino, but OK. I even found a better solution. (I know it is working, I tested it)
    – Juraj
    Apr 20 '19 at 17:16

The StaticJsonDocument is a template class. The template value in <> is here only the size of the internal buffer, but every size used generates a different class. (memory usage!)

To have the parameter of the function take an StaticJsonDocument version instance, it must be the same version or a common base class. In this case the base class is JsonDocument.

If you don't use the reference symbol & the parameter is copied. In this case StaticJsonDocument<200> _doc or JsonDocument _doc would create a copy and that copy would be modified in the function. The object used as parameter value would be unchanged - empty.

So use

void test2(const JsonDocument& _doc) {
  serializeJson(_doc, Serial);
  • Hey @Juraj, thanks for this answer. I'm trying to do exactly the same thing as @Guy .D, except when implementing your solution I'm getting a compiler error: no known conversion for argument 2 from ArduinoJson6101_000::StaticJsonDocument<168u>' to 'const JsonObject& {aka const ArduinoJson6101_000::ObjectRef&} Any advice would be great and I can post a code snippet if required.
    – Peza
    May 5 '19 at 21:35
  • yes I'm using ArduinoJSON V6.
    – Peza
    May 7 '19 at 2:40
  • 1
    sorry, the code snippet was from Guy's code, not from my test. corrected
    – Juraj
    May 7 '19 at 3:40

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