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Subject: Re: [boost] [scope_exit] D-style scope(failure) and scope(success) in C++
From: Evgeny Panasyuk (evgeny.panasyuk_at_[hidden])
Date: 2012-10-03 06:52:36

> This library seems very interresting. But you say that
> uncaught_exception_count() has flaws in your code.
> Can you give any exemple of the cases where your library can be fooled?

As far as I know, uncaught_exception_count itself does not have flaws.

There is some flaw in unwinding_indicator based on uncaught_exception_count.
It works as follows:
1. store uncaught_exception_count in constructor
2. member function unwinding() returns true if state stored in
constructor does not equals to current uncaught_exception_count. I.e.
returns true on transition of state.

It can be fooled in following case: when object is created during stack
unwinding (i.e. in some destructor), and that object outlived unwinding
process, for instance it was stored in global variable.
I have just added example to repo showing such case:
(it has output at bottom) .
As you can see there is transition in uncaught_exception_count state,
butdestructor ( ~ExpCountPrinter() ) is not called due to unwinding.

However, despite of this flaw, scope(failure) and scope(success)
features can be reliably implemented on top of such unwinding_indicator,
because it's objects live only in code blocks (compound-statements), and
can't leave them.
Especially in case of proposed BOOST_SCOPE_FAILURE and
BOOST_SCOPE_SUCCESS - user even don't see auxiliary variables.

Thank you!

Best Regards,

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