scalar deleting destructor issue

Question

I can\'t figure out why I get error for the code below.

The instances of object A will be pushed into a vector (vectorA.push_back(A a)) continu

Answer 1

Your copy constructor does a shallow copy. So, now you have two objects that both have the same recording pointer.

You should either do a deep copy, or ensure the ownership is properly transferred (by using something like std::unique_ptr if C++11 is available.

See This question on the difference between deep and shallow copies.

Let's look at some examples:

class ABadCopyingClass
{
public:
   ABadCopyingClass()
   {
       a_ = new int(5);
   }

   ~ABadCopyingClass()
   {
       delete a_;
   }

private:
    int* a_;   
};

The above class is bad because the default copy constructor and assignment operator will perform a shallow copy, and lead to two objects both thinking that they own the underlying a_ object. When one of them goes out of scope, the a_ will be deleted, and the other one will be left with a dangling pointer that will eventually lead to a crash.

class ABetterCopyingClass
{
public:
    ABetterCopyingClass()
       a_(new int(5))
    {
    }

    ABetterCopyingClass(const ABetterCopyingClass& r)
    {
        a_ = new int(*r.a_);
    }

    ABetterCopyingClass& operator=(const ABetterCopyingClass& r)
    {
        // in the case of reassignment...
        delete a_;

        a_ = new int(*r.a_);
        return *this;
    }

    ~ABetterCopyingClass()
    {
        delete a_;
    }

private:
    int* a_;    

};

This class improved our situation a little (note, that the normal error checking is left out in this simple example). Now the copy constructor and assignment operator properly perform the necessary deep copying. The drawback here is the amount of boilerplate code we had to add -- it's easy to get that wrong.

class ACannonicalCopyingClass
{
public:
   ACannonicalCopyingClass()
      : a_(new int(5))
   {
   }

   ACannonicalCopyingClass(ACannonicalCopyingClass&& moved_from)
   {
       a_ = std::move(moved_from.a_);
   }
private:
   std::unique_ptr a_;
};

This example (C++11 only) is even better. We've removed a significant amount of boilerplate code, however the semantics here are a bit different. Instead of deep copying, we get in this case transfer of ownership of the underlying a_ object.

The easiest version (C++11 only) to implement is the version that provides shared ownership of the underlying a_ object. This is the version that is most similar to your provided example, with the added bonus that it does not cause a crash.

class ASharedCopyingClass
{
public:
   ASharedCopyingClass()
      : a_(std::make_shared(5))
   {
   }

private:
    std::shared_ptr a_;
};

This version can be copied at will, and the underlying a_ object will happily be reference counted. The last copy to go out of scope will set the reference count to 0, which will trigger the memory deallocation.