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7.4 Exceptions

You should exercise care when using predefined exceptions because aspects of their treatment may vary between implementations. Implementation-specific exceptions must, of course, be avoided. See Guidelines 4.3 an 5.8 for further information on exceptions. See Guideline 7.1.6 for further information on vendor-supplied features.

Predefined and User-Defined Exceptions​

guideline​

  • Do not depend on the exact locations at which predefined exceptions are raised.
  • Do not rely on the behavior of Ada.Exceptions beyond the minimum defined in the language.

rationale​

The Ada Reference Manual (1995, §11) states that, among implementations, a predefined exception for the same cause may be raised from different locations. You will not be able to discriminate between the exceptions. Further, each of the predefined exceptions is associated with a variety of conditions. Any exception handler written for a predefined exception must be prepared to deal with any of these conditions.

Guideline 5.6.9 discusses the use of blocks to define local exception handlers that can catch exceptions close to their point of origin.

Implementation-Specific Exceptions​

guideline​

  • Do not raise implementation-specific exceptions.
  • Convert implementation-specific exceptions within interface packages to visible user-defined exceptions.

rationale​

No exception defined specifically by an implementation can be guaranteed to be portable to other implementations whether or not they are from the same vendor. Not only may the names be different, but the range of conditions triggering the exceptions may be different also.

If you create interface packages for the implementation-specific portions of your program, those packages can catch or recognize implementation-specific exceptions and convert them into user-defined exceptions that have been declared in the specification. Do not allow yourself to be forced to find and change the name of every handler you have written for these exceptions when the program is ported.