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.
{handle (an exception occurrence)}
To
handle an exception occurrence is to respond
to the exceptional event.
{propagate}
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.]
Ramification: In other words, if the
execution of a given construct raises an exception, but does not handle
it, the exception is propagated to an enclosing execution (except in
the case of a
task_body).
Propagation involves re-raising the same exception
occurrence. For example, calling an entry of an uncallable task raises
Tasking_Error; this is not propagation.
Dynamic Semantics
{dynamically enclosing
(of one execution by another)} {execution
(dynamically enclosing)} 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.
{innermost
dynamically enclosing} The
innermost
dynamically enclosing execution of a given execution is the dynamically
enclosing execution that started most recently.
To be honest: {
included (one execution
by another)}
{
execution (included by another
execution)}
If the execution of
a dynamically
encloses that of
b, then we also say that the execution of
b
is
included in the execution of
a.
Ramification: Examples: The execution
of an
if_statement
dynamically encloses the evaluation of the
condition
after the
if (during that evaluation). (Recall that “execution”
includes both “elaboration” and “evaluation”,
as well as other executions.) The evaluation of a function call dynamically
encloses the execution of the
sequence_of_statements
of the function
body
(during that execution). Note that, due to recursion, several simultaneous
executions of the same construct can be occurring at once during the
execution of a particular task.
Dynamically enclosing is not defined across
task boundaries; a task's execution does not include the execution of
any other tasks.
Dynamically enclosing is only defined for executions
that are occurring at a given moment in time; if an
if_statement
is currently executing the
sequence_of_statements
after
then, then the evaluation of the
condition
is no longer dynamically enclosed by the execution of the
if_statement
(or anything else).
{raise
(an exception occurrence)} 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;
Ramification: When an exception is raised
by the execution of a
task_body,
there is no dynamically enclosing execution, so the exception does not
propagate any further. If the exception occurred during the activation
of the task, then the activator raises Tasking_Error, as explained in
9.2, “
Task Execution
- Task Activation”, but we don't define that as propagation;
it's a special rule. Otherwise (the exception occurred during the execution
of the
handled_sequence_of_statements
of the task), the task silently disappears. Thus, abnormal termination
of tasks is not always considered to be an error.
{propagate (an
exception occurrence by an execution, to a dynamically enclosing execution)}
Otherwise, the occurrence is
propagated to
the innermost dynamically enclosing execution, which means that the occurrence
is raised again in that context.
To be honest: {
propagate (an exception
by an execution)}
{
propagate (an exception
by a construct)}
As shorthands, we refer to the
propagation of an exception, and the
propagation by a construct,
if the execution of the construct propagates an exception occurrence.
{handle (an exception
occurrence)} {execution
(handler) [partial]} {elaboration
(choice_parameter_specification) [partial]} 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.
Ramification: {
AI95-00318-02}
This “replacement” semantics implies that the handler can
do pretty much anything the abandoned sequence could do; for example,
in a function, the handler can execute a return statement that applies
to the function.
Ramification: The rules for exceptions
raised in library units, main subprograms and partitions follow from
the normal rules, plus the semantics of the environment task described
in Section 10 (for example, the environment task of a partition elaborates
library units and calls the main subprogram). If an exception is propagated
by the main subprogram, it is propagated to the environment task, which
then terminates abnormally, causing the partition to terminate abnormally.
Although abnormal termination of tasks is not necessarily an error, abnormal
termination of a partition due to an exception is an error.
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