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11.4 Exception Handling

When an exception occurrence is raised, normal program execution is abandoned and control is transferred to an applicable exception_handler, if any. To handle an exception occurrence is to respond to the exceptional event. To propagate an exception occurrence is to raise it again in another context; that is, to fail to respond to the exceptional event in the present context.

Dynamic Semantics

Within a given task, if the execution of construct a is defined by this International Standard to consist (in part) of the execution of construct b, then while b is executing, the execution of a is said to dynamically enclose the execution of b. The innermost dynamically enclosing execution of a given execution is the dynamically enclosing execution that started most recently.

When an exception occurrence is raised by the execution of a given construct, the rest of the execution of that construct is abandoned; that is, any portions of the execution that have not yet taken place are not performed. The construct is first completed, and then left, as explained in 7.6.1. Then:

• If the construct is a task_body, the exception does not propagate further;

• If the construct is the sequence_of_statements of a handled_sequence_of_statements that has a handler with a choice covering the exception, the occurrence is handled by that handler;

• Otherwise, the occurrence is propagated to the innermost dynamically enclosing execution, which means that the occurrence is raised again in that context.

When an occurrence is handled by a given handler, the choice_parameter_specification, if any, is first elaborated, which creates the choice parameter and initializes it to the occurrence. Then, the sequence_of_statements of the handler is executed; this execution replaces the abandoned portion of the execution of the sequence_of_statements.

NOTES

(1) Note that exceptions raised in a declarative_part of a body are not handled by the handlers of the handled_sequence_of_statements of that body.

Subclauses

1. The Package Exceptions
2. Example of Exception Handling

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