Exception

Continuing with the previous example, in Layer3 we have function z which again reports failures via exceptions. It will call function h from Layer2_old which returns outcome<int> (which may store an int or a std::error_code or a std::exception_ptr). The goal is to unpack it to either the successful return value int or to throw an appropriate exception: if we are storing an std::exception_ptr, just rethrow it. If we are storing a std::error_code throw it as std::system_error, which is designed to store std::error_code’s: »