how to know the parse is error

[alongL]

string str = ""; //give a empty string
auto j3 = json::parse(str); //exceptin occours

auto j4 = json::parse(str, nullptr, false); //no exception
j4["action"];  //exception  occours

How to know the parse is right?

[nlohmann]

When you switch off exceptions as you did when parsing to j4, the result type will be discarded in case of a parse error (see documentation of parse). You can check this with j4.is_discarded() which should be true.

[alongL]

ok. Problem Solved