From cppreference.com
explicitoperatorbool()constnoexcept;(since C++11)Checks if the error code value is valid, i.e. non-zero.
Return value
false if value()==0, true otherwise.
Notes
Although this operator is often used as a convenient shorthand to check if any error was returned, as in if(ec){/* handle error */}, such use is not robust: some error codes, for example, HTTP status code 200, may indicate success as well.
Example