3.5.3 Exceptional situations

A variety of exceptional situations are distinguished in this report, among them violations of syntax, violations of a procedure’s specification, violations of implementation restrictions, and exceptional situations in the environment. When an exceptional situation is detected by the implementation, an exception is raised, which means that a special procedure called the current exception handler is called. A program can also raise an exception, and override the current exception handler; stdlib exceptions.

When an exception is raised, an object is provided that describes the nature of the exceptional situation. The report uses the condition system described in library section “Conditions” to describe exceptional situations, classifying them by condition types.

Some exceptional situations allow continuing the program if the exception handler takes appropriate action. The corresponding exceptions are called continuable. For most of the exceptional situations described in this report, portable programs cannot rely upon the exception being continuable at the place where the situation was detected. For those exceptions, the exception handler that is invoked by the exception should not return. In some cases, however, continuing is permissible, and the handler may return. stdlib exceptions

Implementations must raise an exception when they are unable to continue correct execution of a correct program due to some implementation restriction. For example, an implementation that does not support infinities must raise an exception with condition type &implementation-restriction when it evaluates an expression whose result would be an infinity.

Some possible implementation restrictions such as the lack of representations for NaNs and infinities are anticipated by this report, and implementations typically must raise an exception of the appropriate condition type if they encounter such a situation.

This report uses the phrase “an exception is raised” synonymously with “an exception must be raised”. This report uses the phrase “an exception with condition type t” to indicate that the object provided with the exception is a condition object of the specified type. The phrase “a continuable exception is raised” indicates an exceptional situation that permits the exception handler to return.