Re: try...finally semantics

From:
Rolf Magnus <ramagnus@t-online.de>
Newsgroups:
comp.lang.c++
Date:
Sat, 25 Aug 2007 09:39:25 +0200
Message-ID:
<faombd$uqi$03$1@news.t-online.com>
Daniel Kraft wrote:

Hi all,

I just needed some kind of try...finally semantics in a method and was
wondering how to do this "the best way" -- there were two alternatives
which came to my mind:

1) RAII:

void A::method()
{
  // Stuff here

  // This is the "try...finally"
  {
   class Finally
   {
    private:
     A& me;
    public:
     inline Finally(A& m)
      : me(m)
     {}
     inline ~Finally()
     {
      me.foo=bar;
      me.doSomething();
     }
   } finally(*this);
   // Do the throwing stuff
  }

  // Stuff here
}

2) Catch and rethrow:

void A::method()
{
  // Stuff here

  #define DO_FINALLY \
   foo=bar; \
   doSomething();
  try
  {
   // throwing stuff
  } catch(...)
  {
   DO_FINALLY
   throw;
  }
  DO_FINALLY
  #undef DO_FINALLY

  // Stuff here
}

Of course the RAII thing is "really" like try...finally, as for instance
an early return is caught, too, but the same is not true for 2); and 1)
seems much "cleaner" to me. However, there is some amount of bloat in
1) which seems to make the code a bit unreadable for me.

I believe most C++ programmers must have had a need for try...finally
functionality at some point, and I'd like to know how you solved it /
how you would recommend to solve it. Is there maybe some way I did not
think about?


Usually, if your classes are properly designed with a destructor that cleans
up, you get RAII implicitly and there is no need for 'finally'. Dynamically
allocated objects can be handled with std::auto_ptr if you don't already
use some type of shared pointer.

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