C++11 scope exit guard, a good idea?

Question

I\'ve written a small utility class for C++11 which I use as a scope guard for easier handling of exception safety and similar things.

Seems somewhat like a hack. Bu

Answer 1

Scope guards are definitely a good idea. I think the scope guard concept is potent tool for exception safety. If you can make a safer, cleaner version that Boost's ScopeExit using C++0x syntax, I think it would be well worth your time.

Similar to Alexandrescu's ScopeGuard and Boost's ScopeExit , the D programming language has direct syntax for this sort of thing. The D programming team thought the scope guard was a good enough idea that they added it directly to the language (ie it's not implemented in a library).

Example.

void foo( bool fail )
{
   scope(exit)
   {
      writeln("I'm always printed");
   }

   scope(success) writeln("The function exited normally");

   scope(error)
      writeln("The function exited with an exception.");

   if( fail )
      throw new Exception("Die Die Die!");
}

The scope based guards aren't anything new. It's functionality can easily be replicated with a class destructor (RAII and all that). It's also possible to replace with try/finally in C# or Java. Heck, even pthreads provides a rudimentary scope guard, called pthread_cleanup_push.

What makes scope guards so powerful is when you have multiple scope(*) statements in the function. It scales incredibly well, as opposed to try/finally which require super human powers to manage anything more than two.

Answer 2

Using Boost:

#include <boost/preprocessor/cat.hpp>

template<class Fn>
class ScopeGuardDetails {
    const Fn m_fn;
public:
    constexpr ScopeGuardDetails(Fn &&fn) : m_fn(fn) {}
    ~ScopeGuardDetails() { m_fn(); }
};
#define ScopeGuardName BOOST_PP_CAT(BOOST_PP_CAT(__scope_guard, _), BOOST_PP_CAT(BOOST_PP_CAT(__LINE__, _), __COUNTER__))
#define defer(stmt) const auto ScopeGuardName = [](const auto _fn) { \
    return ScopeGuardDetails<decltype(_fn)> { std::move(_fn) }; \
}([&] { stmt });

Usage:

if (gdiplus::GdiplusStartup(&token, &startupInput, nullptr) == Gdiplus::Ok) {
    defer({
        gdiplus::GdiplusShutdown(token);
    });
    ...
}

Answer 3

my $0.02

struct at_scope_end
{
    std::function < void () > Action;

    at_scope_end (std::function < void () > Action) :
        Action (Action)
    {
    }

    ~at_scope_end ()
    {
        Action ();
    }
};

#define AT_SCOPE_END_CAT(x,y)    x##y
#define AT_SCOPE_END_ID(index)   AT_SCOPE_END_CAT(__sg, index)
#define AT_SCOPE_END(expr)      at_scope_end AT_SCOPE_END_ID(__LINE__) ( [&] () { expr; } );

Answer 4

For the record, there is scope_exit in the TS 3

Answer 5

For the record, there is Boost ScopeExit.

Answer 6

We could omit the ugly [&] stuff by putting it in the define:

#define UTILITY_SCOPE_EXIT(f) const auto& _UTILITY_EXIT_SCOPE_LINENAME(EXIT, __LINE__) = ::detail::create_scope_exit([&]f)

Then:

UTILITY_SCOPE_EXIT({myfile.close();});

Tested with MSVC++ 11.0 (VS2012). Regards.