How is user authentication handled by the mongocxx v3 driver?

The legacy driver interface included an auth() function which had input parameters username, password, database, and an output parameter errorMessage, and a boolean return type. In case of an authentication error the function would return false and errorMessage would contain the information. The mongocxx v3 driver doesn’t have a function, but rather a defined mongocxx::authentication_exception, so normally I’d suspect that in case of an authentication failure the driver would simply throw, and the message would be housed in e.what(). I tried to search the entire codebase for where this exception is being thrown and I couldn’t find it. What is the case here?

Thanks in advance!