Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

Boost Exception

unknown_exception

#include <boost/exception_ptr.hpp>

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

This type is used by the exception_ptr support in Boost Exception. Please see current_exception.