$12.1 BASIC CONCEPTS OF EXCEPTION HANDLING 413 Definition:exception cases An exception may occur during the execution of a routine r as a result of any of the following situations: EI.Attempting a qualified feature call a./and finding that a is void. E2 Attempting to attach a void value to an expanded target. E3.Executing an operation that produces an abnormal condition detected by the hardware or the operating system. E4 .Calling a routine that fails. E5.Finding that the precondition ofr does not hold on entry. E6 .Finding that the postcondition ofr does not hold on exit. E7 .Finding that the class invariant does not hold on entry or exit. E8 .Finding that the invariant of a loop does not hold after the from clause or after an iteration of the loop body. E9 Finding that an iteration of a loop's body does not decrease the variant. E10.Executing a check instruction and finding that its assertion does not hold. El 1.Executing an instruction meant explicitly to trigger an exception. Case E3 follows from signals that the operating system sends to an application when it detects an abnormal event,such as a fault in an arithmetic operation (underflow, overflow)or an attempt to allocate memory when none is available. Case E4 arises when a routine fails,as a result of an exception that happened during its own execution and from which it was not able to recover.This will be seen in more detail below,but be sure to note the rule that results from case E4: Failures and exceptions A failure of a routine causes an exception in its caller. See“Monitoring Cases E5 to E10 can only occur ifrun-time assertion monitoring has been enabled at assertions at run the proper level:at least assertion(require)for E5,assertion (loop)for E8 and E9 etc. time".page 393. Case E11 assumes that the software may include calls to a procedure raise whose sole goal is to raise an exception.Such a procedure will be introduced later. Causes of failure Along with the list of possible exception cases,it is useful for the record to define when a failure(itself the source of an exception in the caller,as per case E4)can occur:§12.1 BASIC CONCEPTS OF EXCEPTION HANDLING 413 Case E3 follows from signals that the operating system sends to an application when it detects an abnormal event, such as a fault in an arithmetic operation (underflow, overflow) or an attempt to allocate memory when none is available. Case E4 arises when a routine fails, as a result of an exception that happened during its own execution and from which it was not able to recover. This will be seen in more detail below, but be sure to note the rule that results from case E4: Cases E5 to E10 can only occur if run-time assertion monitoring has been enabled at the proper level: at least assertion (require) for E5, assertion (loop) for E8 and E9 etc. Case E11 assumes that the software may include calls to a procedure raise whose sole goal is to raise an exception. Such a procedure will be introduced later. Causes of failure Along with the list of possible exception cases, it is useful for the record to define when a failure (itself the source of an exception in the caller, as per case E4) can occur: Definition: exception cases An exception may occur during the execution of a routine r as a result of any of the following situations: E1 • Attempting a qualified feature call a ● f and finding that a is void. E2 • Attempting to attach a void value to an expanded target. E3 • Executing an operation that produces an abnormal condition detected by the hardware or the operating system. E4 • Calling a routine that fails. E5 • Finding that the precondition of r does not hold on entry. E6 • Finding that the postcondition of r does not hold on exit. E7 • Finding that the class invariant does not hold on entry or exit. E8 • Finding that the invariant of a loop does not hold after the from clause or after an iteration of the loop body. E9 • Finding that an iteration of a loop’s body does not decrease the variant. E10 • Executing a check instruction and finding that its assertion does not hold. E11 • Executing an instruction meant explicitly to trigger an exception. Failures and exceptions A failure of a routine causes an exception in its caller. See “Monitoring assertions at run time”, page 393