Re: How to make this exception-safe

From:

Maxim Yegorushkin <maxim.yegorushkin@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 19 Nov 2008 15:25:45 CST

Message-ID:

<0f9c239d-8f38-484a-bb3a-9e4139ff4b43@s1g2000prg.googlegroups.com>

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?

One solution is to use a vector of smart pointers.

Another is to use an intrusive list if your container does not have to
be an array. Allocate nodes, once a node has been allocated inserting
it in a list never throws:

    #include <memory>

    struct C {
        explicit C(int) {} // may throw

    private:
        C(const C&);
        C& operator=(const C&);
    };

    struct NodeC : C
    {
        explicit NodeC(int x)
            : C(x)
        {}

        std::auto_ptr<NodeC> next;
    };

    struct V
    {
        std::auto_ptr<NodeC> head, *tail;

        V()
            : tail(&head)
        {
            // if any of these throw, the destructor
            // of head takes care of cleaning it up

            *tail = new NodeC(1);
            tail = &(**tail).next;

            *tail = new NodeC(2);
            tail = &(**tail).next;

            *tail = new NodeC(3);
            tail = &(**tail).next;
        }
    };

--
Max

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