Nesting of Exceptions

Nesting of exceptions, that is, throwing an exception while another remains unhandled, is allowed only in restricted circumstances. From the point when an exception is thrown to the point when the matching catch clause is entered, the exception is unhandled. Functions that are called along the way, such as destructors of automatic objects being destroyed, may throw new exceptions, as long as the exception does not escape the function. If a function exits via an exception while another exception remains unhandled, the terminate() function is called immediately.

Once an exception handler has been entered, the exception is considered handled, and exceptions may be thrown again.

You can determine whether any exception has been thrown and is currently unhandled. See "Calling the uncaught_exception() Function".