Re: How to make this exception-safe

From:

Triple-DES <DenPlettfrie@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 20 Nov 2008 16:37:32 CST

Message-ID:

<d5115b99-b418-40d1-957e-74991d95b477@y18g2000yqn.googlegroups.com>

On 19 Nov, 22:27, Martin Bonner <martinfro...@yahoo.co.uk> wrote:

On Nov 18, 6:43 pm, Triple-DES <DenPlettf...@gmail.com> wrote:

Consider the following code:

#include <vector>

struct C {
   explicit C(int) {} // may throw
   private:
     C(const C&);
     C& operator=(const C&);

};

struct V {
   V() {
     // may leak if push_back or C::C(int) throws
      v_.push_back( new C(2) );
      v_.push_back( new C(1) );
      v_.push_back( new C(3) );
   }

   ~V() {} // delete all elems of v

private:
   std::vector<C*> v_;

};

How would you go about making an exception-safe version of V::V()?
This is what I could come up with:

// 1st attempt
#include <memory>

V::V() {
   v_.reserve(3);

   std::auto_ptr<C> c1( new C(2) );
   std::auto_ptr<C> c2( new C(1) );
   std::auto_ptr<C> c3( new C(3) );

   v_.push_back( c1.release() );
   v_.push_back( c2.release() );
   v_.push_back( c3.release() );

}

Which is tedious if you are inserting more objects. Does anyone have a
better solution?
PS! boost/tr1 shared_ptr or similar can not be used.

    V::cleanup()
    {
      while (v_.size() > 0) { delete v_.back(); v_.pop_back(); }
    }

    V() {
        try {
          v_.push_back( new C(2) );
          v_.push_back( new C(1) );
          v_.push_back( new C(3) );
        } catch (...) {
          cleanup();
          throw;
        }
    }

Looks straightforward enough, except: If a constructor call succeeds,
but push_back throws, you will leak. You would need to use reserve
first.

(you can then use cleanup from the destructor too).

If your management have a problem with boost, then a clone of
shared_ptr is probably the best solution though. (Why can't you use
it?)

To be more precise. No third-party libraries at all, only 100%
standard C++(03).

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