Re: Ideas for strong guarantee?

From:

David Abrahams <dave@boost-consulting.com>

Newsgroups:

comp.lang.c++.moderated

Date:

17 Jul 2006 22:40:38 -0400

Message-ID:

<uhd1g6jgu.fsf@boost-consulting.com>

thiago.adams@gmail.com writes:

Hi All,

I'm using exceptions to report errors in my code and I think that it
is a good idea. But in some circumstances is hard to implement a strong
guarantee for functions.

Then don't. You should only implement strongest guarantee that is
natural for your operations. The strong guarantee is neither always
necessary, nor always sufficient, for writing exception-safe code.

http://www.boost.org/more/generic_exception_safety.html

I would like suggestions for the code below:

class X {

  std::vector<Type*> m_vec1;
  std::vector<Type*> m_vec2;

  void func()
  {
     Type * p1 = new Type();
     Type * p2 = new Type();
     m_vec1.push_back(p1);
     m_vec2.push_back(p2);
  }

};

// partial solution...
void func()
{
   std::auto_ptr<Type> sp1(new Type());
   std::auto_ptr<Type> sp2(new Type());

   m_vec1.push_back(sp1.get()); //
   sp1.release();

   m_vec2.push_back(sp2.get()); //
   sp2.release();
}

This solution is almost correct, but if some exceptions occurs at
second push_back I need to keep m_vec1 unchanged.

How handle this situation?

If you really need the strong guarantee:

   void func()
   {
      std::auto_ptr<Type> sp1(new Type());
      std::auto_ptr<Type> sp2(new Type());

      m_vec1.push_back(sp1.get()); //

      try
      {
         m_vec2.push_back(sp2.get()); //
      }
      catch(...)
      {
         m_vec1.pop_back();
         throw;
      }
      sp1.release();
      sp2.release();
   }

HTH,

--
Dave Abrahams
Boost Consulting
www.boost-consulting.com

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