6.5.1 Nonreturning Procedures
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Specifying aspect No_Return to have the value True indicates that a procedure
cannot return normally[; it may propagate an exception or loop forever].
Discussion: Aspect No_Deposit
will have to wait for Ada 2020. :-)
Static Semantics
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For a procedure or generic procedure, the following language-defined
representation aspect may be specified:
No_Return
The type of aspect No_Return is Boolean. When aspect No_Return is True
for an entity, the entity is said to be
nonreturning.
If directly specified, the
aspect_definition
shall be a static expression. [This aspect is never inherited;] if not
directly specified, the aspect is False.
Aspect Description for No_Return:
A procedure will not return normally.
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If a generic procedure is nonreturning, then so are its instances. If
a procedure declared within a generic unit is nonreturning, then so are
the corresponding copies of that procedure in instances.
Legality Rules
Reason: A null procedure cannot have
the appropriate nonreturning semantics, as it does not raise an exception
or loop forever.
Ramification: {
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The procedure can be abstract. If a nonreturning procedure is renamed
(anywhere) calls through the new name still have the nonreturning semantics.
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A procedure shall be nonreturning if it overrides a dispatching nonreturning
procedure.
In addition to the places where Legality
Rules normally apply (see
12.3), this rule
applies also in the private part of an instance of a generic unit.
Reason: This ensures that dispatching
calls to nonreturning procedures will, in fact, not return.
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If a renaming-as-body completes a nonreturning procedure declaration,
then the renamed procedure shall be nonreturning.
Reason: This ensures that no extra code
is needed to implement the renames (that is, no wrapper is needed) as
the body has the same property.
Paragraph 8 was deleted.
Dynamic Semantics
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If the body of a nonreturning procedure completes normally, Program_Error
is raised at the point of the call.
Discussion: Note that there is no name
for suppressing this check, since the check represents a bug, imposes
no time overhead, and minimal space overhead (since it can usually be
statically eliminated as dead code).
Implementation Note: If a nonreturning
procedure tries to return, we raise Program_Error. This is stated as
happening at the call site, because we do not wish to allow the procedure
to handle the exception (and then, perhaps, try to return again!). However,
the expected run-time model is that the compiler will generate raise
Program_Error at the end of the procedure body (but not handleable by
the procedure itself), as opposed to doing it at the call site. (This
is just like the typical run-time model for functions that fall off the
end without returning a value). The reason is indirect calls: in P.all(...);,
the compiler cannot know whether P designates a nonreturning procedure
or a normal one. Putting the raise Program_Error in the procedure's
generated code solves this problem neatly.
Similarly, if one passes a nonreturning procedure
to a generic formal parameter, the compiler cannot know this at call
sites (in shared code implementations); the raise-in-body solution deals
with this neatly.
Examples
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procedure Fail(Msg : String) --
raises Fatal_Error exception
with No_Return;
--
Inform compiler and reader that procedure never returns normallyExtensions to Ada 95
Extensions to Ada 2005
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
