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Showing content from https://arduinojson.org/v6/api/jsonobject/isnull/ below:

JsonObject::isNull() | ArduinoJson 6

Description

JsonObject::isNull() tests whether the JsonObject points to an object or not.

You can use this function to:

check if the object was successfully parsed, or
check if the object was successfully allocated.

As an alternative, you can use the conversion to bool; for example, if(object) instead of if(!object.isNull())

Signature Return value

JsonObject::isNull() returns a bool that tells if the JsonObject points to something:

true if the JsonObject is null,
false if the JsonObject is valid and points to an object.

Examples

Example 1: parsing success:

StaticJsonDocument<200> doc;
deserializeJson(doc, "{\"hello\":\"world\"}");
JsonObject object = doc.as<JsonObject>();
Serial.println(object.isNull()); // false

Example 2: parsing failure:

StaticJsonDocument<200> doc;
deserializeJson(doc, "[\"hello\",\"world\"]");
JsonObject object = doc.as<JsonObject>();
Serial.println(object.isNull()); // true

Example 3: allocation success:

StaticJsonDocument<200> doc;
JsonObject object = doc.to<JsonObject>();
Serial.println(object.isNull()); // false

Example 4: allocation failure:

StaticJsonDocument<1> doc;
JsonObject object = doc.to<JsonObject>();
Serial.println(object.isNull()); // true