A thrown exception is considered caught, when its corresponding handler has been entered (or, in case such a handler cannot be found, when unexpected() function has been invoked). Sometimes you have to check whether an uncaught exception is currently being processed. Consider a destructor that may throw an exception. If the destructor has been invoked due to stack unwinding caused by another exception, throwing an additional exception from this destructor will yield an undefined behavior. To check whether a thrown exception is currently being processed, you can use the standard function uncaught_exception():

 class FileException{};File::~File() throw (FileException);{   if ( close(file_handle) != success) // failed to close current file?  {    if (uncaught_exception()  == true ) // is there any uncaught exception being processed currently?       return;  // if so, do not throw an exception    throw FileException(); // otherwise, it is safe to throw an exception to signal an error  }return; // success}