The term "exception" is preferred to "error" because it does not imply that anything is wrong - a condition viewed as an error by one procedure or programmer may not be viewed that way by another. Taking a broad view, errors can be considered to be a proper subset of exceptions, and explicit error mechanisms such as errno can be considered (verbose) forms of exception handling. In languages without built-in exception handling such as C, routines would need to signal the error in some other way, such as the common return code and errno pattern. value is outside of the domain of a function), an unavailable resource (like a missing file, a hard disk error, or out-of-memory errors), or that the routine has detected a normal condition that requires special handling, e.g., attention, end of file.Įxception handling solves the semipredicate problem, in that the mechanism distinguishes normal return values from erroneous ones. Common exceptions include an invalid argument (e.g.
the programmer may deem division by zero to be undefined, hence an exception, or devise some behavior such as returning zero or a special "ZERO DIVIDE" value (circumventing the need for exceptions). The set of "normal" circumstances is defined entirely by the programmer, e.g. The precondition, and the definition of exception, is subjective. The exception handling mechanism then handles the exception. An exception handling mechanism allows the procedure to raise an exception if this precondition is violated, for example if the procedure has been called on an abnormal set of arguments. The definition of an exception is based on the observation that each procedure has a precondition, a set of circumstances for which it will terminate "normally". 5.9.2 Restarts separate mechanism from policy.5.4 Exception handling based on design by contract.5.2 Termination and resumption semantics.5 Exception support in programming languages.
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