Exceptions

An exception is an abnormal condition that interrupts a program’s normal flow of execution. In many programming languages, exceptions are objects that describe an error or unusual condition and can be raised when something goes wrong, such as a failed file operation or an invalid input. When an exception is raised, the runtime transfers control to an exception handler. If no handler is found, the exception propagates up the call stack and may terminate the program.

Exception handling provides mechanisms to recover from errors and to ensure resources are released. Common constructs include try, catch, and finally blocks, or equivalents. Implementations typically unwind the call stack until a matching handler is found. During unwinding, finally blocks or equivalent clean up resources, such as closing files or releasing locks.

Languages distinguish between different kinds of exceptions. Some use checked exceptions, which must be declared or caught by the programmer, while others treat all exceptions as unchecked at runtime. Examples of typical exception categories include input/output errors, arithmetic errors, and null or invalid references. Descriptive messages and stack traces aid debugging but should not be relied on for program logic.

Outside computing, the term exception also denotes an instance that does not conform to a general rule, such as an irregular form in linguistics or a provision that exempts a case from a rule in law or policy.