Boost Exception

unknown_exception

namespace
boost
    {
    class
    unknown_exception:
        public std::exception
        public boost::exception
        {
        ---unspecified---
        };
    }

This type is used by the cloning support in Boost Exception.

To allow an exception to be cloned, enable_exception_cloning() must be used at the time the exception object is passed to throw. Otherwise, calling clone_exception() returns an exception_ptr which refers to an instance of unknown_exception.