access a field with operator[]

[sarikh]

assuming I have a field called "aaa" that does not always exist in my json, is there a different way to access it other than the following:
int a; if (req_json.contains("aaa")) { a= req_json["aaa"]; } else { a= 0; }

[nlohmann]

For this use case, you can use the value function (https://json.nlohmann.me/api/basic_json/value/):

int a = req_json.value("aaa", 0);