If you try to use the String class with ArduinoJson on a Particle device (Argon, Boron, Electron, Photon, Xenon…), you’ll probably get a compilation error like:

  • call of overloaded 'set(const String&)' is ambiguous
  • no type named 'Reader' in 'struct ArduinoJson::Internals::StringTraits<String, void>'
  • no matching function for call to 'serialize(const ArduinoJson6112_11000::BasicJsonDocument<ArduinoJson6112_11000::DefaultAllocator>&, String&)'

These errors occur because the support of the String class is disabled in ArduinoJson.

ArduinoJson enables the support of the String class when the ARDUINO symbol is defined. Particle doesn’t define ARDUINO unless you include Arduino.h in your project.

To solve this problem, simply include Arduino.h:

#include <Arduino.h>
#include <ArduinoJson.h>

Alternatively, you can force the support of the String class by defining ARDUINOJSON_ENABLE_ARDUINO_STRING:

#include <ArduinoJson.h>