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Date: Fri, 10 May 2013 00:50:21 +0200
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To: Rasmus Schultz <me@rasmus-schultz.com>
Cc: PHP internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] exceptions during __toString()
From: colder@php.net (Etienne Kneuss)

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On Fri, May 10, 2013 at 12:00 AM, Rasmus Schultz <me@rasmus-schultz.com>wrote:

> I just ran into this issue again:
>
>
> http://stackoverflow.com/questions/2429642/why-its-impossible-to-throw-exception-from-tostring
>
> Instead of throwing some nonsense "you're not allowed to throw from here"
> error-message, how about actually unwinding the stack and passing the
> exception to the global exception-handler?
>
> I understand there's a technical limitation making it difficult/impossible
> to handle exceptions during __toString() - and it looks like you can still
> try {...} catch (Exception $e) in __toString() just fine, but then what?
> You have the exception, but you have no way to pass it to the standard
> global exception-handler.
>
> If the script has to terminate either way, why not terminate the same way
> an unhandled exception would normally cause the script to terminate?
>
> E.g. output the actual error-message, or pass it to whatever was registered
> with register_exception_handler() so it can be output or handled in the
> usual way?
>


The reason is that toString can be triggered from a lot of internal places.
If an exception is thrown and caught within toString, it's not a problem.
The problem is if the exception escapes toString, because it thus means
that the code invoking toString must be interrupted.

We in fact have no guarantee that all those internal places will work
consistently/correctly in the presence of an exception, and if the engine
will remain in a state that allows executing code afterward.
Not executing code means we need to bypass error handlers as well.

Handling an exception from internal code is a "manual" procedure, and code
written that relies on conversions to strings may not have been written
with support for exceptions in mind.

I guess the alternative approach is to allow exceptions, review the code
(i.e. a lot of work), and eventually fix reports as they come in.

Best,

-- 
Etienne Kneuss

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