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: ecyrbe (ecyrbe_at_[hidden])
Date: 2012-10-03 06:09:54

Hi,

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?

thanks,

ecyrbe

2012/10/2 Evgeny Panasyuk <evgeny.panasyuk_at_[hidden]>

> Hello all,
>
> D language has scope(exit), scope(success) and scope(failure) features -
> http://dlang.org/statement.**html#ScopeGuardStatement
>:

> "...
> scope(exit) executes NonEmptyOrScopeBlockStatement when the scope exits
> normally or when it exits due to exception unwinding. scope(failure)
> executes NonEmptyOrScopeBlockStatement when the scope exits due to
> exception unwinding. scope(success) executes NonEmptyOrScopeBlockStatement
> when the scope exits normally.
> ..."
> Andrei Alexandrescu describes some of uses cases for scope(exit),
> scope(success), scope(failure) at his recent talk :
> http://channel9.msdn.com/**Events/Lang-NEXT/Lang-NEXT-**
> 2012/Three-Unlikely-**Successful-Features-of-D<http://channel9.msdn.com/Events/Lang-NEXT/Lang-NEXT-2012/Three-Unlikely-Successful-Features-of-D>
>
> There is Boost.ScopeExit library which emulates D's scope(exit) via
> BOOST_SCOPE_EXIT macro, but there are no analogues for D's scope(failure)
> and scope(success).
> Current alternative for these features is ScopeGuard idiom (
> http://en.wikibooks.org/wiki/**More_C%2B%2B_Idioms/Scope_**Guard
>,

> http://www.drdobbs.com/cpp/**generic-change-the-way-you-**
> write-excepti/184403758<http://www.drdobbs.com/cpp/generic-change-the-way-you-write-excepti/184403758>).
>
> Boost.ScopeExit library itself suggests to use ScopeGuard -
> http://www.boost.org/doc/libs/**1_51_0/libs/scope_exit/doc/**
> html/scope_exit/alternatives.**html#scope_exit.alternatives.**
> the_d_programming_language<http://www.boost.org/doc/libs/1_51_0/libs/scope_exit/doc/html/scope_exit/alternatives.html#scope_exit.alternatives.the_d_programming_language>:
> "
> Boost.ScopeExit is similar to scope(exit) feature built into the D
> programming language.
> A curious reader may notice that the library does not implement
> scope(success) and scope(failure) of the D language. Unfortunately, these
> are not possible in C++ because failure or success conditions cannot be
> determined by calling std::uncaught_exception (see Guru of the Week #47 for
> details about std::uncaught_exception and if it has any good use at all).
> However, this is not a big problem because these two D's constructs can be
> expressed in terms of scope(exit) and a bool commit variable (similarly to
> some examples presented in the Tutorial section).
> "
>
> I have made small library ( https://github.com/panaseleus/**
> stack_unwinding#d-style-scope-**guardsactions<https://github.com/panaseleus/stack_unwinding#d-style-scope-guardsactions>) which supplies primitives (class unwinding_indicator,
> UNWINDING_AWARE_DESTRUCTOR macro) to determining when object destructor is
> called due to stack-unwinding or due to normal scope leaving.
> Currently it is implemented on top of platform-specific implementation of
> uncaught_exception_count function and tested on:
> {MSVC2005, MSVC2008, MSVC2010, MSVC2012, GCC4.1.2, GCC4.4.6,
> Clang3.2}x{x32, x64}x{default settings}.
> uncaught_exception_count is a function similar to std::uncaught_exception
> from standard library, but instead of boolean result it returns unsigned
> int showing current count of uncaught exceptions.
>
> Such primitives allow us to implement scope(success) and scope(failure)
> features in C++. I have made several proof-of-concepts:
> * https://github.com/panaseleus/**stack_unwinding/blob/master/**
> examples/scope_actions.cpp<https://github.com/panaseleus/stack_unwinding/blob/master/examples/scope_actions.cpp>
> * https://github.com/panaseleus/**stack_unwinding/blob/master/**
> examples/example_from_dlang_**dot_org.cpp<https://github.com/panaseleus/stack_unwinding/blob/master/examples/example_from_dlang_dot_org.cpp>
> * https://github.com/panaseleus/**stack_unwinding/blob/master/**
> examples/scope_guard.cpp<https://github.com/panaseleus/stack_unwinding/blob/master/examples/scope_guard.cpp>
> * https://github.com/panaseleus/**stack_unwinding/blob/master/**
> examples/boost_scopes.cpp<https://github.com/panaseleus/stack_unwinding/blob/master/examples/boost_scopes.cpp>
>
> boost_scopes.cpp example shows usage of BOOST_SCOPE_FAILURE and
> BOOST_SCOPE_SUCCESS:
> try
> {
>     cout << "Case #1: stack unwinding" << endl;
>     BOOST_SCOPE_EXIT(void) { cout << "exit" << endl; } BOOST_SCOPE_EXIT_END
>     BOOST_SCOPE_FAILURE(void) { cout << "failure" << endl; }
> BOOST_SCOPE_FAILURE_END
>     BOOST_SCOPE_SUCCESS(void) { cout << "success" << endl; }
> BOOST_SCOPE_SUCCESS_END
>     throw 1;
> } catch(int){}
> {
>     cout << "Case #2: normal exit" << endl;
>     BOOST_SCOPE_EXIT(void) { cout << "exit" << endl; } BOOST_SCOPE_EXIT_END
>     BOOST_SCOPE_FAILURE(void) { cout << "failure" << endl; }
> BOOST_SCOPE_FAILURE_END
>     BOOST_SCOPE_SUCCESS(void) { cout << "success" << endl; }
> BOOST_SCOPE_SUCCESS_END
> }
> Output is:
> "
> Case #1: stack unwinding
> failure
> exit
> Case #2: normal exit
> success
> exit
> "
>
> Currently BOOST_SCOPE_FAILURE and BOOST_SCOPE_SUCCESS are implemented
> based on boost_1_51_0/boost/scope_exit.**hpp by copy-paste and replace
> (just for proof-or-concept, of course it is possible solution with much
> less duplication).
> Semantically meaningful changes can be viewed as diff at
> https://github.com/panaseleus/**stack_unwinding/commit/**
> d5513404323490ea99d9b6b77ff1b8**8f454a4a42<https://github.com/panaseleus/stack_unwinding/commit/d5513404323490ea99d9b6b77ff1b88f454a4a42>
>
> Before posting here, I made posts on several C++ forums, but unfortunately
> get only a few feedbacks regarding ScopeFailure and ScopeSuccess.
> What do you think?
>
> Thank you.
>
> P.S. Related discussion: http://lists.boost.org/**
> Archives/boost/2010/09/171051.**php<http://lists.boost.org/Archives/boost/2010/09/171051.php>
>
> Best Regards,
> Evgeny Panasyuk
>
>
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