HP Integrity NonStop J-Series - Manual (Page 252)

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©Copyright 1996 Rogue Wave Software
External Errors
External errors are due to events beyond the scope of the program. As we mentioned in the
introduction, any non-trivial program should be prepared to recover from an external error. In
general, external errors are:
Not easily predicted in advance; ●
Encountered at more abstract levels; ●
Not costly to detect; ●
Detected in both the production and the debug versions of the library. ●
Examples of external errors would include: an attempt to set a bad date, such as 31 June 1992;
an attempt to invert a singular matrix; a stream write error; being out of memory. Tools.h++
would respond to these by throwing an exception inheriting from RWExternalErr, or providing
a test for object validity.
External errors may be run time errors. In an object-oriented environment, run time errors
frequently show up as an attempt to set an object into an invalid state, perhaps as a result of
invalid user input. The example given above, initializing a date object with the nonexistent date
31 June 1992, is an external run time error.
External errors often take the form of exceptions thrown by the operating system. Tools.h++
takes responsibility for detecting these exceptions and recovering the resources it has acquired;
it will close files and restore heap memory. As the user, however, you are responsible for all
resources acquired by code external to the Tools.h++ library during these kinds of exceptions.
Generally, Tools.h++ assumes that these exceptions will not be thrown during memory-related
C library calls such as memcpy. Tools.h++ make every effort to detect throws which occur
during operations on Tools.h++ objects or user-defined objects.
In theory, the Tools.h++ response to an external error is to throw an exception, or to provide a
test for object validity. It should never abort the program. In practice, however, some compilers
do not handle exceptions, so Tools.h++ provides an opportunity to recover in an error handler,
or to test for a status value. Here is an example of using the isValid function to validate user
input:
RWDate date;