Global basic_json< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer >::basic_json (std::istream &i, const parser_callback_t cb=nullptr): This constructor is deprecated and will be removed in version 3.0.0 to unify the interface of the library. Deserialization will be done by stream operators or by calling one of the parse functions, e.g. parse(std::istream&, const parser_callback_t). That is, calls like json j(i); for an input stream i need to be replaced by json j = json::parse(i);. See the example below.