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Postings from June 2016
* John Siracusa <siracusa@gmail.com> [2016-06-23 18:24]:
> eval {
> my $foo = Foo->new;
> };
>
> ok(!$@, 'exception handled in destructor (no extra final statement in eval)');
This one fails because there isn’t any exception during the eval. So the
Foo instance is returned as the value of the `eval` block. If you put
a `warn` in front of the `eval` you can see that. Then at the end of the
statement the Foo instance goes out of scope and only at that point does
its DESTROY fire.
So the $@ declobbering change is in fact irrelevant to the behaviour of
this particular test.
> eval {
> my $foo = Foo->new;
> die ($foo = 'Died in eval');
> };
>
> like($@, qr/^Died in eval/, 'exception thrown in eval (object destroyed in last statement)');
This can be provoked into failing in the same way by various alternate
phrasings such as
$foo = 1, die 'Died in eval';
The key is to put $foo on the stack, then overwrite it. The result as
best as I can surmise from my not-actually-a-guts-hacker understanding
is that when $foo goes out of scope, it is no longer pointing to the Foo
instance, which therefore survives until the end of the statement… which
once again is outside the `eval` block, and therefore clears the hurdle.
Basically, the specifics of the timing of how far things survive when it
comes to the last statement of a block are fiddly and you must carefully
control what the last statement is and what it does to avoid surprises.
Regards,
--
Aristotle Pagaltzis // <http://plasmasturm.org/>
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