The current_exception function must not be called outside of a catch block.
- An exception_ptr that refers to the currently handled exception or a copy of the currently handled exception.
- If the function needs to allocate memory and the attempt fails, it returns an exception_ptr that refers to an instance of std::bad_alloc.
- It is unspecified whether the return values of two successive calls to current_exception refer to the same exception object.
- Correct implementation of current_exception may require compiler support, unless enable_current_exception was used at the time the currently handled exception object was passed to throw. Whenever current_exception fails to properly copy the current exception object, it returns an exception_ptr to an object of type that is as close as possible to the original exception type, using unknown_exception as a final fallback. All such types derive from boost::exception, and: