CONCEPTS OF
PROGRAMMING LANGUAGES
TENTH EDITION
Robert W. Sebesta
Chapter 14-Exception Handling and Event Handling
Review question
1. Define exception, exception handler, raising an exception, disabling an exception, continuation, finalization, and built-in exception.
Answer: An exception is an unusual event that is detectable by either hardware or software and that may require special processing. The special processing that may be required when an exception is detected is called exception handling. The processing is done by a code unit or segment called an exception handler. An exception is raised when its associated event occurs. In some situations, it may be desirable to ignore certain hardware-detectable exceptions—for example, division by zero—for a time. This action would be done by disabling the exception. After an exception handler executes, either control can transfer to somewhere in the program outside of the handler code or program execution can simply terminate. We term this the question of control continuation after handler execution, or simply continuation. In some situations, it is necessary to complete some computation regardless of how subprogram execution terminates. The ability to specify such a computation is called finalization. Built-in exceptions have a built-in meaning, it is generally inadvisable to use these to signal program-specific error conditions. Instead we introduce a new exception using an exception declaration, and signal it using a raise expression when a run-time violation occurs. That way we can associate specific exceptions with specific pieces of code, easing the process of tracking down the source of the error.
2. When is an exception thrown or raised?
Answer: The exception thrown or raised when there is throw clause of a method lists that have checked exception which could throw and does not handle.
10. What are the four exceptions defined in the Standard package of Ada?
Answer: The four exception defined in the standard package of Ada are Constraint_Error, Program_Error, Storage_Error, Tasking_Error
11. What is the use of suppress pragma in Ada?
Answer: An Ada pragma is a directive to the compiler. Certain run-time checks that are parts of the built-in exceptions can be disabled in Ada programs by use of the Suppress pragma, the simple form of which is pragma Suppress(check_name) where check_name is the name of a particular exception check. The Suppress pragma can appear only in declaration sections. When it appears, the specified check may be suspended in the associated block or program unit of which the declaration section is a part. Explicit raises are not affected by Suppress. Although it is not required, most Ada compilers implement the Suppress pragma.
13. Describe three problems with Ada’s exception handling.
Answer: There are several problems with Ada’s exception handling. One problem is the propagation model, which allows exceptions to be propagated to an outer scope in which the exception is not visible. Also, it is not always possible to determine the origin of propagated exceptions. Another problem is the inadequacy of exception handling for tasks. For example, a task that raises an exception but does not handle it simply dies. Finally, when support for object-oriented programming was added in Ada 95, its exception handling was not extended to deal with the new constructs. For example, when several objects of a class are created and used in a block and one of them propagates an exception, it is impossible to determine which one raised the exception.
14. What is the name of all C++ exception handlers?
Answer: Each catch function is an exception handler. A catch function can have only a single formal parameter, which is similar to a formal parameter in a function definition in C++, including the possibility of it being an ellipsis (. . .). A handler with an ellipsis formal parameter is the catch-all handler; it is enacted for any raised exception if no appropriate handler was found. The formal parameter also can be a naked type specifier, such as float, as in a function prototype. In such a case, the only purpose of the formal parameter is to make the handler uniquely identifiable. When information about the exception is to be passed to the handler, the formal parameter includes a variable name that is used for that purpose. Because the class of the parameter can be any user-defined class, the parameter can include as many data members as are necessary.
15. Which standard libraries define and throw the exception out_of_range in C++?
Answer: The exception out_of_range in C++ thrown by library container classes
16. Which standard libraries define and throw the exception overflow_error in C++?
Answer: the exception overflow_error in C++ thrown by math library functions
19. State the similarity between the exception handling mechanism in C++ and Ada
Answer: In some ways, the C++ exception-handling mechanism is similar to that of Ada. For example, unhandled exceptions in functions are propagated to the function’s caller.
20. State the differences between the exception handling mechanism in C++ and Ada
Answer: There are no predefined hardware-detectable exceptions that can be handled by the user, and exceptions are not named. Exceptions are connected to handlers through a parameter type in which the formal parameter may be omitted. The type of the formal parameter of a handler determines the condition under which it is called but may have nothing whatsoever to do with the nature of the raised exception.
24. What is the difference between checked and unchecked exceptions in Java?
Answer: Exceptions of class Error and RuntimeException and their descendants are called unchecked exceptions. All other exceptions are called checked exceptions. Unchecked exceptions are never a concern of the compiler. However, the compiler ensures that all checked exceptions a method can throw are either listed in its throws clause or handled in the method. Note that checking this at compile time differs from C++, in which it is done at run time. The reason why exceptions of the classes Error and RuntimeException and their descendants are unchecked is that any method could throw them. A program can catch unchecked exceptions, but it is not required.
26. How can an exception handler be written in Java so that it handles any exception?
Answer: The exception handlers of Java have the same form as those of C++, except that every catch must have a parameter and the class of the parameter must be a descendant of the predefined class Throw-able. The syntax of the try construct in Java is exactly as that of C++, except for the finally clause.
28. What is the purpose of Java finally clause?
Answer: A finally clause is placed at the end of the list of handlers just after a complete try construct. The semantics of this construct is as follows: If the try clause throws no exceptions, the finally clause is executed before execution continues after the try construct. If the try clause throws an exception and it is caught by a following handler, the finally clause is executed after the handler completes its execution. If the try clause throws an exception but it is not caught by a handler following the try construct, the finally clause is executed before the exception is propagated
Problem Set
1 . What mechanism did early programming languages provide to detect or attempt to deal with errors?
Answer:
Early programming languages were designed and implemented in such a way that the user program could neither detect nor attempt to deal with such errors. In these languages, the occurrence of such an error simply causes the program to be terminated and control to be transferred to the operating system. The typical operating system reaction to a run-time error is to display a diagnostic message, which may be meaningful and therefore useful, or highly cryptic. After displaying the message, the program is terminated.
2. Describe the approach for the detection of subscript range errors used in C and Java.
Answer:
Java compilers usually generate code to check the correctness of every subscript expression (they do not generate such code when it can be determined at compile time that a subscript expression cannot have an out-of-range value, for example, if the subscript is a literal). In C, subscript ranges are not checked because the cost of such checking was (and still is) not believed to be worth the benefit of detecting such errors. In some compilers for some languages, subscript range checking can be selected (if not turned on by default) or turned off (if it is on by default) as desired in the program or in the command that executes the compiler.
5. From a textbook on FORTRAN, determine how exception handling is done in FORTRAN programs.
Answer:
For example, a Fortran “Read” statement can intercept input errors and end-of-file conditions, both of which are detected by the input device hardware. In both cases, the Read statement can specify the label of some statement in the user program that deals with the condition. In the case of the end-of-file, it is clear that the condition is not always considered an error. In most cases, it is nothing more than a signal that one kind of processing is completed and another kind must begin. In spite of the obvious difference between end-of-file and events that are always errors, such as a failed input process, Fortran handles both situations with the same mechanism.
6. In languages without exception-handling facilities, it is common to have most subprograms include an “error” parameter, which can be set to some value representing “OK” or some other value representing “error in procedure”. What advantage does a linguistic exception-handling facility like that of Ada have over this method?
Answer:
There are several advantages of a linguistic mechanism for handling exceptions, such as that found in Ada, over simply using a flag error parameter in all subprograms. One advantage is that the code to test the flag after every call is eliminated. Such testing makes programs longer and harder to read. Another advantage is that exceptions can be propagated farther than one level of control in a uniform and implicit way. Finally, there is the advantage that all programs use a uniform method for dealing with unusual circumstances, leading to enhanced readability.
7. In a language without exception handling facilities, we could send an error-handling procedure as a parameter to each procedure that can detect errors that must be handled. What disadvantages are there to this method?
Answer:
There are several disadvantages of sending error handling subprograms to other subprograms. One is that it may be necessary to send several error handlers to some subprograms, greatly complicating both the writing and execution of calls. Another is that there is no method of propagating exceptions, meaning that they must all be handled locally. This complicates exception handling, because it requires more attention to handling in more places.
Special thanks to Mr. Tri Djoko Wahjono, Ir., M.Sc.
