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[Page 819]

16.4. When to Use Exception Handling

Exception handling is designed to process synchronous errors, which occur when a statement executes. Common examples of these errors are out-of-range array subscripts, arithmetic overflow (i.e., a value outside the representable range of values), division by zero, invalid function parameters and unsuccessful memory allocation (due to lack of memory). Exception handling is not designed to process errors associated with asynchronous events (e.g., disk I/O completions, network message arrivals, mouse clicks and keystrokes), which occur in parallel with, and independent of, the program's flow of control.

Software Engineering Observation 16.3

Incorporate your exception-handling strategy into your system from the design process's inception. Including effective exception handling after a system has been implemented can be difficult.

Software Engineering Observation 16.4

Exception handling provides a single, uniform technique for processing problems. This helps programmers working on large projects understand each other's error-processing code.

Software Engineering Observation 16.5

Avoid using exception handling as an alternate form of flow of control. These "additional" exceptions can "get in the way" of genuine error-type exceptions.

Software Engineering Observation 16.6

Exception handling simplifies combining software components and enables them to work together effectively by enabling predefined components to communicate problems to application-specific components, which can then process the problems in an application-specific manner.

The exception-handling mechanism also is useful for processing problems that occur when a program interacts with software elements, such as member functions, constructors, destructors and classes. Rather than handling problems internally, such software elements often use exceptions to notify programs when problems occur. This enables programmers to implement customized error handling for each application.

Performance Tip 16.3

When no exceptions occur, exception-handling code incurs little or no performance penalties. Thus, programs that implement exception handling operate more efficiently than do programs that intermix error-handling code with program logic.

Software Engineering Observation 16.7

Functions with common error conditions should return 0 or NULL (or other appropriate values) rather than throw exceptions. A program calling such a function can check the return value to determine success or failure of the function call.

Complex applications normally consist of predefined software components and application-specific components that use the predefined components. When a predefined component encounters a problem, that component needs a mechanism to communicate the problem to the application-specific componentthe predefined component cannot know in advance how each application processes a problem that occurs.

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