5.1.1 Types of Exceptions

There are two types of exceptions: address exceptions and status exceptions. An exception is initialized as an address exception, but it can be modified (before it is used) by defining a status value for it. Following are the primary differences between address and status exceptions:

• Different exception objects can be declared with the same status value, and those exceptions are considered identical by the exception package. For example, if one exception is raised, it can be caught by specifying another exception object having the same status. Two different address exceptions can never match each other.

• If the platform supports a universal definition of error status, then status exceptions can often be used to import and export system status values. When a facility called by DECthreads raises a system exception, DECthreads and its clients can catch the exception using a DECthreads status exception. Similarly, when a function raises a DECthreads exception, a caller might be able to handle it using facilities provided by the language or platform.

Status values used in exceptions can be interpreted, handled, and reported in a universal manner, regardless of which facility defined the status value. Use address exceptions if your code does not have a range of status codes assigned to it. Address exceptions are always unique so you do not risk colliding with another facility's status codes and inadvertently handling the wrong exception. Also, address exceptions are more portable because status codes are likely to be different on each platform.