CS 2005, B Term 1999
Techniques of Programming
Lab 2 (Nov. 10)

Instructions

The objective of this lab session is to provide additional practice with classes that use dynamic memory. In particular, you will see what can go wrong when a copy constructor and overloaded assignment operator are not provided for a dynamic class, and you will implement these crucial member functions for a dynamic class that shares some features of the DPoly class used for HW2. The insight gained through this process should also be helpful for Test1.

  1. Sign in with the TA. You should both print your name and sign the sheet.
  2. Listen to the TA's mini-lecture.
  3. Log onto your CCC Unix account and change the directory to /cs/cs2005/samples/lab2/. That directory contains the following files for a dynamic class named DynStuff:
  4. Do the problems listed below. Feel free to ask the TA questions about the problems. Actively working on the lab assignment during the lab session is required. Don't worry if you can't finish the full assignment before the lab session is over. You can finish afterwards.

Problems

  1. Copy the files in /cs/cs2005/samples/lab2/ to one of your own personal directories. Study the test program source file dyntest.cxx. This program constructs two DynStuff objects named dyn1 and dyn2, prints the contents of dyn1 and dyn2, assigns dyn1=dyn2, modifies the contents of dyn2, and prints the final contents of both dyn1 and dyn2. Type make at the Unix prompt in the directory to which you copied the files provided in /cs/cs2005/samples/lab2/. This compiles the DynStuff class definition, compiles the test program, links the DynStuff class implementation to it, and produces an executable file named dyntest.

  2. Run the executable file dyntest. Are the results consistent with the expected behavior of the program based on the source code in dyntest.cxx? (Answer: no). Problems occur here because the DynStuff class definition uses dynamic memory. The assignment dyn1=dyn2 invokes automatic assignment mechanisms which merely copy the member variables of dyn2 to the corresponding member variables of dyn1. Since the member variable data is a pointer, this makes dyn1.data point to the same storage area as dyn2.data. When dyn2 is subsequently modified, the array dyn1.data is also affected, since it's actually the same array as dyn2.data.

  3. Fix the above problems by adding three new member functions for the DynStuff class: a copy constructor, an overloaded assignment operator, and a destructor. Debug your design as needed, and test it by linking it with the test program in dyntest.cxx. Refer to section 4.3 of Main and Savitch for examples of similar functions in the context of a dynamic Bag class.

  4. Use any remaining time to work on HW2.