CS222 Lab 5

Lab 5 - Copy Constructors and Assignment Operators

The purpose of this lab is to see why copy constructors and assignment operators are vital for class which have dynamic array member variables.

Copy the class list.cpp to your current directory using

cp /usr/users/mdanfor/public_html/cs222-w07/list.cpp .

Compile this program exactly as given and run the executable. Notice that the pointer addresses are copied over when assignment or cloning occurs. For example, after clone = DoubleList(c), both c and clone are pointing to the same chunk of memory. This means if one is deleted, the other would be pointing to "free" memory space which can be given to someone else to use. If the system is under load, you may notice random values in the last line of output corresponding to the values stored in a after a = b + c;. This is because the + operator creates a temporary DoubleList to store the new concatenated array. The temporary variable is deleted after a = b + c;, but a continues to point to the allocated space. The system thinks that memory is free, so it MAY allocate it to another program in the short space between a = b + c; and cout << a. If that occurs, the other program that has allocated the memory can write whatever it wants into that memory space, which can lead to random values being displayed when you call cout << a.

To prevent this from occuring, you need to define a copy constructor and an assignment operator. I have given the prototypes for those functions in the code, but commented them out so you could run the above experiment. Both functions will do the same basic tasks:

Set the size of the current object ('this') to the size of the 'right' object.
Try to allocate 'size' elements to the current object.
If the allocation is successful, copy all the elements from 'right' into the current object
If the allocation fails, set the size of the current object to 0.

See the body of the constructor that takes a double array for hints on how to accomplish these steps.

Compile the new program with the copy constructor and assignment operators defined. If you have defined them correctly, you should now see that the array pointer address changes for clone and a when it did not change before.

Email the source code for this final program to me.