ArduinoJSON - How to check if array is empty

Question

Parameters file defines the creation of entities and stored in flash. Its structure is given, and all fields ( keys & values ) are given. In current phase - those parameters are hard-coded in a const char* cont_params, as shown below.

My goal, is when array is empty, for example RF_2entity - means that current MCU does not uses its RF abilities, and will not init any RF entities. The way RF_2entity is defined below, as empty array, [] , ArduinoJSON's isNull() function return 0 in that case.

How can I keep parameters file's structure and logic, rather that fill its values with 255 for example.

  Serial.begin(115200);
  StaticJsonDocument<1200> doc;
  const char *cont_params = "{\"entityType\": [1,1,1,1],\
                    \"SWname\": [\"SW_0\",\"SW_1\",\"SW_2\",\"SW_3\"],\
                    \"SW_buttonTypes\": [2,1,1,1],\
                    \"SW_timeout\": [10,11,12,13],\
                    \"SWvirtCMD\":[0,0,0,0],\
                    \"Winname\": [],\
                    \"WextInputs\": [],\
                    \"WinvirtCMD\":[],\
                    \"RF_2entity\": [],\
                    \"v_file\": 0.5}";
  DeserializationError err = deserializeJson(doc, cont_params);
  JsonArray array = doc["Winname"].as<JsonArray>();
  serializeJsonPretty(array,Serial);
  Serial.println(array.isNull());

output:

[]0

Answer 1

The method isNull() is not what you want. It tells you whether the instance holds an array at all or not. And of course, your specific instance holds an array, though an empty one. Therefore, the method correctly returns false, aka 0.

As the API documentation shows, there are more methods to use.

What you want is size(), which returns the number of elements in the array. It will even return 0 on an instance without an array.