Re: Ideas for strong guarantee?

From:

"Maxim Yegorushkin" <maxim.yegorushkin@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

21 Jul 2006 10:39:33 -0400

Message-ID:

<1153469240.973157.79390@m73g2000cwd.googlegroups.com>

thiago.adams@gmail.com wrote:

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.
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.

It can be solved by using scope guards. I find custom local guards most
flexible (compared to Alexandrescu's scope guard).

#include <vector>

int main()
{
     std::vector<int*> a, b;

     struct guard
     {
         std::vector<int*> *a, *b;
         int *p, *q;
         void release() { a = 0; b = 0; p = 0; q = 0; }
         ~guard()
         {
             if(a)
                 a->pop_back();
             if(b)
                 b->pop_back();
             delete p;
             delete q;
         }
     } guard = {};

     a.push_back(guard.p = new int(1));
     guard.a = &a;
     b.push_back(guard.q = new int(2));
     guard.b = &b;

     // more code that may throw

     guard.release();
}

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