Re: [boost] [scope_exit] D-style scope(failure) and scope(success) in C++

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

Hello,
> 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: 
https://github.com/panaseleus/stack_unwinding/blob/master/examples/uncaught_exception_count_floating_object.cpp 
(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,
Evgeny