Parameter return when exceptions occur

Parameter return when exceptions occur

[Steve]

The "variable lenght strings thread" has touched on the case of using
Ada.Text_Io.Get_Line with a string on the last line of a file that does not
contain a file terminator.  In this case an End_Error exception is raised.

Question: when the End_Error exception occurs, is anything known about the
value of the parameters to Get_Line?  Does the procedure still return
meaningful values?

I suspect it is compiler dependent, since I didn't manage to find any
explanation in the LRM.

Steve
(The Duck)

Re: Parameter return when exceptions occur

[Martin Dowie]

Steve wrote:
> The "variable lenght strings thread" has touched on the case of using
> Ada.Text_Io.Get_Line with a string on the last line of a file that
> does not contain a file terminator.  In this case an End_Error
> exception is raised.
>
> Question: when the End_Error exception occurs, is anything known
> about the value of the parameters to Get_Line?  Does the procedure
> still return meaningful values?
>
> I suspect it is compiler dependent, since I didn't manage to find any
> explanation in the LRM.

If a subprogram raises an exception you can assume /nothing/ about
the contents of the returning values.

Cheers

-- Martin

Re: Parameter return when exceptions occur

[Christoph Karl Walter Grein]

> > Question: when the End_Error exception occurs, is anything known
> > about the value of the parameters to Get_Line?  Does the procedure
> > still return meaningful values?
> >
> > I suspect it is compiler dependent, since I didn't manage to find any
> > explanation in the LRM.
> 
> If a subprogram raises an exception you can assume /nothing/ about
> the contents of the returning values.

In fact, there even isn't a returned value since you "return" to a different place:
the exception handler (if any).
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Re: Parameter return when exceptions occur

[Martin Dowie]

Christoph Karl Walter Grein wrote:
>>> Question: when the End_Error exception occurs, is anything known
>>> about the value of the parameters to Get_Line?  Does the procedure
>>> still return meaningful values?
>>>
>>> I suspect it is compiler dependent, since I didn't manage to find
>>> any explanation in the LRM.
>>
>> If a subprogram raises an exception you can assume /nothing/ about
>> the contents of the returning values.
>
> In fact, there even isn't a returned value since you "return" to a
> different place: the exception handler (if any).

The parameters used in the call might still be in scope though - but you
can't assume anything about their contents.

Cheers

-- Martin

Re: Parameter return when exceptions occur

[Matthew Heaney]

"Martin Dowie" <martin.dowie@baesystems.com> writes:

> If a subprogram raises an exception you can assume /nothing/ about the
> contents of the returning values.

This is only true if this is a pass-by-copy type (scalar, etc).  If the
type is pass-by-reference (tagged, etc), then yes, you can depend on the
values "returned" by the subprogram, even when there's an exception.
(In fact, the AI-302 containers depend on this behavior.)

Re: Parameter return when exceptions occur

[Steve]

It would be nice if Text_Io.Get_Line would work this way and return a count
and string when an end error exception occurs.  I think it actually works
with some compilers.

Steve
(The Duck)


"Matthew Heaney" <matthewjheaney@earthlink.net> wrote in message
news:u654rj65p.fsf@earthlink.net...
> "Martin Dowie" <martin.dowie@baesystems.com> writes:
>
> > If a subprogram raises an exception you can assume /nothing/ about the
> > contents of the returning values.
>
> This is only true if this is a pass-by-copy type (scalar, etc).  If the
> type is pass-by-reference (tagged, etc), then yes, you can depend on the
> values "returned" by the subprogram, even when there's an exception.
> (In fact, the AI-302 containers depend on this behavior.)

Re: Parameter return when exceptions occur

[Jeffrey Carter]

Steve wrote:
> It would be nice if Text_Io.Get_Line would work this way and return a
> count and string when an end error exception occurs.  I think it
> actually works with some compilers.

Neither of these are pass-by-reference parameters. Last is a 
pass-by-copy parameter, and Item is a pass-however-the-compiler-chooses 
parameter.

-- 
Jeff Carter
"We use a large, vibrating egg."
Annie Hall
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