Re: How to make this exception-safe

From:

Yechezkel Mett <ymett.on.usenet@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 19 Nov 2008 15:28:40 CST

Message-ID:

<abd8c05e-df50-4b91-b494-a899b9b7158a@v22g2000pro.googlegroups.com>

On Nov 18, 8:43 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.

Why not? (It's important to know what constraints we are working
under.)

I would suggest using a container which holds pointers and manages
lifetimes -- boost has a few under the name Pointer Container.

Otherwise:

namespace
{
  std::auto_ptr<C> new_C(int n) { return std::auto_ptr<C>(n); }
}

V::V() {
   std::auto_ptr<C> a[] = {
      new_C(2),
      new_C(1),
      new_C(3)
   };

   const size_t count = sizeof a / sizeof a[0]

   v_.reserve(count);
   std::transform(a, a + count, std::back_inserter(v_),
      boost::bind(&std::auto_ptr<C>::release, _1));
}

Or, of course:

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

   try {
      v_.push_back( new C(2) );
      v_.push_back( new C(1) );
      v_.push_back( new C(3) );
   }
   catch (...) {
      <call cleanup function>
      throw;
   }
}

Yechezkel Mett

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