ArduinoJson/examples/ProgmemExample/ProgmemExample.ino

// ArduinoJson - arduinojson.org
// Copyright Benoit Blanchon 2014-2018
// MIT License
//
// This example shows the different ways you can use Flash strings with
// ArduinoJson.
//
// Use Flash strings sparingly, because ArduinoJson duplicates them in the
// JsonDocument. Prefer plain old char*, as they are more efficient in term of
// code size, speed, and memory usage.

#include <ArduinoJson.h>

void setup() {
#ifdef PROGMEM  // <- check that Flash strings are supported

  DynamicJsonDocument doc(1024);

  // You can use a Flash String as your JSON input.
  // WARNING: the string in the input  will be duplicated in the JsonDocument.
  deserializeJson(doc, F("{\"sensor\":\"gps\",\"time\":1351824120,"
                         "\"data\":[48.756080,2.302038]}"));
  JsonObject obj = doc.as<JsonObject>();

  // You can use a Flash String to get an element of a JsonObject
  // No duplication is done.
  long time = obj[F("time")];

  // You can use a Flash String to set an element of a JsonObject
  // WARNING: the content of the Flash String will be duplicated in the
  // JsonDocument.
  obj[F("time")] = time;

  // You can set a Flash String to a JsonObject or JsonArray:
  // WARNING: the content of the Flash String will be duplicated in the
  // JsonDocument.
  obj["sensor"] = F("gps");

  // It works with serialized() too:
  obj["sensor"] = serialized(F("\"gps\""));
  obj["sensor"] = serialized(F("\xA3gps"), 3);

  // You can compare the content of a JsonVariant to a Flash String
  if (obj["sensor"] == F("gps")) {
    // ...
  }

#else

#warning PROGMEM is not supported on this platform

#endif
}

void loop() {
  // not used in this example
}

// Visit https://arduinojson.org/v6/example/progmem/ for more.