C++ Lab 4

Copy Constructors, Assignment Operators,
Destructors and File IO

The following files are used in this lab:

date2.h - Declaration for the Date class
date2.cc - Definition/Implementation for Date class
testDateCopy.cc - A example program using the copy constructor and assignment with the Date class.
snode.h - Declaration file for the SimNode class
snode.cc - Definition/Implementation for the SimNode class
olist.h - Declaration for the OrderedList class
olist.cc - Definition for the OrderedList class
testListCopy.cc - Example program illustrating problems with default copy constructor and assignment on linked lists. Also shows when destructor is called.
FileIO.cc - Example program illustrating file IO

C++ provides default destructors, copy constructors, and assignment operators for objects. These work fine for classes that do not use pointers. However, classes with pointers and dynamically allocated memory should define versions of these to correctly allocate and deallocate memory. All three should be provided.

Copy Constructors

C++ provides a default copy constructor that is invoked with either of the following object declarations:

     ClassName objectName(objectToCopy);

     ClassName objectName = objectToCopy;

Either of these declarations sets up the object and copies each data member of the objectToCopy to the corresponding data member of the new object.

Warning For data members that are pointers the pointer is copied but NOT the actual data so the two pointers are now aliases for the same place in memory. This is called a shallow copy and causes huge problems!

Assignment Operator

C++ provides a default assignment operator for objects that copies each data member of the object. For example, the code

     object1 = object2;

will copy each data member of object2 into the corresponding data member of object1. As for the copy constructor, if the data member is a pointer only the pointer is copied.

Destructors

The destructor for a class is automatically called when an object goes out of scope. If there is no destructor provided for the class the default destructor is called; otherwise, the one provided in the class is called. The destructor should be responsible for freeing all memory used by an object.

Syntax for the declaration: ~ClassName()

If the class contains a linked list the default destructor frees only the pointer to the beginning of the list. The programmer should write code to delete all nodes from the list.

Exercises

Compile and run testDateCopy.cc and study the code. No copy constructor or assignment operator has been defined for the Date class so the program is using the defaults provided by C++.
Note that the code behaves as expected.
Now compile and run testListCopy.cc and study the code. The copy constructor and the overloaded assignment operator have been commented out of the OrderedList class. The destructor code only has a message that the destructor has been called. Study the output and answer the following (*** WRITE ANSWERS TO THESE QUESTIONS ***:
- newList1 is a copy of list. What happens when the first element of list is removed? Why? (Describe in terms of pointers and links - draw a picture!)
- The node is inserted back in the original list (but not at the beginning). What does newList1 now look like and why? Again draw a picture to show what is going on with the pointers.
- Note that the destructor is called at the end of the if block because an OrderedList was created inside the block.
- Similar pointer behavior occurs when the default assignment is used. For example, draw pictures to illustrate what is happening with newList1 and newList2 near the end of the program.
Remove the comments that comment out the copy constructor and the assignment operator (in both olist.h and olist.cc) and run the program. You should now observe correct behavior.
Finally, the current destructor does nothing but print a message. It should actually remove and delete each node on the list. Similarly the assignment operator should reclaim all nodes from the current list before reassigning the object. So, modify the class as follows:
- Add a function to actually delete the nodes in the list and return them to free store.
- Update the destructor to call the function to delete the nodes.
- Similarly add code in the assignment operator to delete the nodes in the current list before reassigning.

Using Files

The program FileIO.cc shows how to open files, use them (read from and write to), and close them. It reads a list of integers and finds the sum and average. It prints the number of integers read, the sum, and the average to another file. Note that the file stream must explicitly be closed.

Exercise:

Study the program to see what is going on.
Create an input file the meets the specifications - include at least 5 integers for the data. Run your program then examine the output file.
Run the program entering the name of a non-existent file for the input file. What happens?
Delete a data integer from your input file. Run the program to see what happens.
Change the parameter type to the function processInput from ifstream to istream. In the C++ library, an ifstream is derived from an istream; that is, an ifstream is an istream. Compile the program. What is the error and what does it mean?
Now, move the statement inFile.close() from inside the function processInput to main just before the call to the function writeOutput. Compile the program. Why are there no errors this time? Link and run the program to make sure it is correct.