Array Class -- no copy constuctor error

Using Array objects -- What happens if you don't have a copy constructor?
Assume the Array class is identical to the example given, except that it doesn't have a copy constructor. It has everything else, destructor, operator=, etc. And let's say you have a main that looks something like


   Array x(20);
   // set the elements of x's int array to valid int values
   Array y(x);                        // construct y to look like x

A memory picture of x and y look as follows. The Array object x is allocated memory for an int array and its size is set to 20 by the default constructor.

Because we're assuming no copy constructor was written, the default copy constructor, which does a memberwise copy, is used. The address in arrayPtr is copied and the int in size is copied. So that means that x and y don't have their own array. Both objects contain pointers that point to the same int array.


Object x:
  +-------------+
  |         __  |          _________________________
  |arrayPtr| -|---------->|  some valid int values  |
  |         --  |          -------------------------
  |      __     |         ^
  |size |20|    |        /
  |      --     |       /
  +-------------+      /
                      /
Object y:            /
  +-------------+   /
  |         __  |  /
  |arrayPtr| -|----
  |         --  |  
  |      __     |
  |size |20|    |
  |      --     |
  +-------------+

Every time main changes a value in x's array, y's array is also getting changed. And vice versa, when y is changed. That is very bad!

Moreover, after the program appears to end, it crashes. Do you know why?

Here's why. At the end of main, when objects x and y go out scope, the destructor is called for them. First the destructor is called for y and memory the int array's memory is deallocated, and then the data member's memory are deallocated. The memory picture for x and y now looks like:


Object x:
  +-------------+
  |         __  |          
  |arrayPtr| -|----------> memory has been deallocated
  |         --  |          
  |      __     |          
  |size |20|    |
  |      --     |
  +-------------+
                
Object y:   memory has been deallocated

When the destructor is called for x, it crashes when trying to execute the line of code: delete [] arrayPtr; because the int array has already been deallocated.

And, no, the fix is not to comment out the delete in the destructor! The fix is to correctly write the copy constructor.