[boost] [scope_exit] Boost.ScopeExit capturing 'this'

Subject: [boost] [scope_exit] Boost.ScopeExit capturing 'this'
From: adam.butcher_at_[hidden]
Date: 2009-02-27 06:40:46

I wasn't sure whether I had the correct address for Boost.ScopeExit's 
author Alexander Nasonov, so I've submitted to this list just in case and 
to get feedback from other users.

I have been playing with BOOST_SCOPE_EXIT -- it it very convenient.
One thing I would have liked would be to capture 'this' inside member
functions.  I have created a patch which adds BOOST_SCOPE_EXIT_THIS to
do just that.  It provides an operator->() in the generated struct
which can be used to access the enclosing class's members.

The syntax is:
 
    BOOST_SCOPE_EXIT_THIS( (&a) (&b) )
    {
        a; b;               // local captures
        (*this)->member;    // member access
    }
    BOOST_SCOPE_EXIT_END;

and can be

    BOOST_SCOPE_EXIT_THIS()
    {
        (*this)->member;
    }
    BOOST_SCOPE_EXIT_END;

providing the preprocessor allows for empty parameter lists (which gcc
does and vc does albeit after disabling a warning to remove noise).

The type pointed to by the result of operator->() is correctly cv 
qualified.

It has been tested on gcc 4.3, gcc 4.2, msvc 7.1 and msvc 9.0.  I don't 
have access to other compilers.

Boost.Typeof worked fine on gcc for typeof(this) -- mostly because gcc
has a native typeof I guess.

On visual studio (vc71 and vc90) it was not so good -- a compiler bug
  (
http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=331418
,
   
http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=365549
)
that has been marked as 'Won't fix' means that 'this' cannot be used
in expressions within template argument lists -- even if those
expressions are innocuous typeid()s or sizeof()s for type-inference.

The common workaround (capturing a compile-time integer with an
enumerator and using the enumerator in the template parameter list)
doesn't work with the vc71-specific implementation of Boost.Typeof
since it is based on typeid() -- this cannot be captured in a
compile-time expression (or I know of no way to that doesn't involve
the expression existing in a template argument list).

All is not lost though -- the general-purpose sizeof() based approach
for the other VC versions works fine for VC71 for determining the
typeof 'this'.  As such my patch includes a VC>=1300 specific
workaround for typeof 'this' determination.  This is unfortunate but I
couldn't think of any other non-intrusive way of doing it.

Please find the attached patch and let me know what you think -- I'm
sure this feature would be useful to users -- it saves having to do:

    TypeOfMyClassWhichCouldBeTedious* self = this
    BOOST_SCOPE_EXIT( (&self) )
    {
        self->member;
    }
    BOOST_SCOPE_EXIT_END;

which removes the convenience of BOOST_SCOPE_EXIT in these contexts
since something like:

   struct at_end { TypeOfMyClassWhichCouldBeTedious* self; ~at_end()
   {
       self->member;
   }
   } at_end = {this};

can be done without any dependencies to achieve the same result.

Regards,
Adam.

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