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Insidious Inheritance

Assignment Description

In this lab you will get experience with some of the implementation issues and conceptual details of inheritance. Inheritance is a mechanism for increasing the reusability and reliability of C++ code. It is worth mentioning that inheritance is a characteristic of all object oriented programming languages. Our goal is to give you a glimpse of the functionality of inheritance, so that you can make informed design decisions in the future. Please read through the entire lab before you begin.

Lab Insight

This lab teaches some of the characteristics observed in object oriented programming. These characteristics are useful in software design when building code bases for large projects, APIs, and production code bases. Some classes that further extend on these concepts include CS 427, CS 242, and CS 422. This lab will help you build clean code bases where the OOP (Object-Oriented Programming) characteristics help minimize having to rewrite redundant code as well as make use of virtual inheritance to treat custom sub-classes the same way as their base classes without losing the custom functionality of these subclasses.

Getting Set Up

From your CS 225 git directory, run the following on EWS:

git fetch release
git merge release/lab_inheritance -m "Merging initial lab_inheritance files"

If you’re on your own machine, you may need to run:

git fetch release
git merge --allow-unrelated-histories release/lab_inheritance -m "Merging initial lab_inheritance files"

Upon a successful merge, your lab_inheritance files are now in your lab_inheritance directory.

The code for this activity resides in the lab_inheritance/ directory. Get there by typing this in your working directory:

cd lab_inheritance/

Class Hierarchy

To help us understand class hierarchies better here is an example of a simple class hierarchy showing that a Dog is a Animal.

The code would look something like the following:

class Animal {
      string name;

      virtual void speak() = 0;
      /* The = 0 at the end of the method means that the method is a pure virtual method
       * meaning that it does not have an implementation and it delegates the task
       * of implementing the method to the classes that is derived from it */

class Dog : public Animal {
      string breed;

      /* Dog inherits speak from Animal */
      void speak();


void Dog::speak() {
    cout << "Woof Woof" << endl;

In this example Animals have a name and can speak but since speak is a pure virtual method we CANNOT construct an Animal by itself. That is Animal is an abstract class and it can only be constructed by one of its derived classes. For example, a Dog is a derived class of Animal. This means that a Dog is a Animal, and, therefore, it inherits a name and a speak method from Animal. However, since the Animal’s speak does not have an implementation, Dog must implement the speak method.

Here is an example of how we could use a Dog object:

Dog* d = new Dog();

/* Like usual we can access all the public methods and member variables of a
 * Dog */

/* But now since a Dog is an Animal we can also do this too */
d->name;     // inherited from Animal
d->speak();  // inherited from Animal and since it is a Dog speak() will print
             // "Woof Woof"

/* Additionally we can treat our Dog only like an Animal like this */
Animal* a = d;

/* But now we can only do the following */
a->speak();  // Still prints "Woof Woof" because speak is a virtual method.

a->breed     // ERROR! This will NOT work since we perceive it as an Animal now

/* Additionally, if we try to have our Animal pointer point back to a Dog
 * pointer this will cause a problem because an Animal Is NOT A Dog. */
Dog* d2 = a;  // ERROR!  Animal Is NOT A Dog

/* Furthermore, since Animal is abstract and has a pure virtual method
 * we CANNOT construct one! */
Animal a2;  // ERROR! Animal is an abstract class

Now that we can understand a simple class hierarchy, let’s look at a more complex one. Here is a diagram depicting the class hierarchy that is used in this lab. (Note: This diagram is missing some information, e.g. methods, member variables, etc.., for demonstration purposes)

This means everything is a Drawable and will have a draw method. Code like the following is perfectly acceptable:

Drawable* triangle  = new Triangle(....);
Drawable* circle    = new Circle(...);
Drawable* rectangle = new Rectangle(....);
Drawable* truck     = new Truck(...);
Drawable* flower    = new Flower(....);

/* Now the only thing we can use on triangle, circle, rectangle, truck, and
 * flower is draw but what gets drawn will change depending on what type the
 * pointer is actually pointing to. This is called polymorphism, the behavior
 * changes depending on the actual type of the object being pointed to. */

PNG canvas;
triangle->draw(&canvas);   // draws a Triangle even though triangle is a Drawable*
circle->draw(&canvas);     // draws a Circle even though circle is a Drawable*
rectangle->draw(&canvas);  // draws a Rectangle even though rectangle is a Drawable*
truck->draw(&canvas);      // draws a Truck even though truck is a Drawable*
flower->draw(&canvas);     // draws a Flower even though flower is a Drawable*

Look at main.cpp for a working example executable in action. main.cpp gets compiled and linked into an executable named lab_inheritance. Follow the instructions below to build, run, and view the output:

The Makefile provided for this MP will create an executable when you run make. It will generate lab_inheritance.

For example when you run


You could also run Valgrind on the normal executable:

valgrind --leak-check=full ./lab_inheritance

This lab will use all of these objects in interesting ways but as you will quickly see they are not working the way the should. Your objective for this lab is to go through the 5 test executables and fix the code to work correctly by modifying how the classes in the hierarchy declare and implement their methods.

Once you have fixed all the Valgrind errors, you can test your program output following the directions below.

Exercise 1: Fix the Virtual Methods

Please build and run test_virtual:

make test_virtual                   # compile to produce test_virtual executeble
valgrind ./test_virtual             # run test_virtual with valgrind

As you will see when you run test_virtual, the output will say:

The Perimeters are NOT the same.
The Areas are NOT the same.

However, if you look closely at the code they should be the same because both of the pointers in test_virtual.cpp point to the same object!

Exercise 2: Fix the Destructor

Please build and run test_destructor:

make test_destructor                         # compile to produce test_destructor executeble
valgrind --leak-check=full ./test_destructor # run test_destructor in valgrind

When you run test_destructor in Valgrind you will see that test_destructor is leaking memory. However, if you look closely, Triangle does have a valid destructor and it is being called in test_destructor!

Exercise 3: Fix the Constructor

Please build and run test_constructor:

make test_constructor # make test_constructor
./test_constructor    # run test_constructor

When you run test_constructor you will see the following output:

Circle's color is NOT correct!
Circle's center is NOT correct!

If you look closely, we are constructing a Circle with a valid center and color. However, when it is being drawn and when we ask for the Circle’s center and color they are not the same!

Exercise 4: Fix the Pure Virtual Method

Please build and run test_pure_virtual.

make test_pure_virtual # make test_pure_virtual
./test_pure_virtual    # run test_pure_virtual

When you try to make test_pure_virtual you will see that it does not compile.

However, if you look at the truck.{h,cpp} it is a fully featured class! Why is it not compiling?

Exercise 5: Fix the Slicing

Please build and run test_slicing with:

make test_slicing # make test_slicing
./test_slicing    # run test_slicing

After you run test_slicing open up its output test_slicing.png. You will see that a Flower has NOT been drawn. For some reason just a bunch of X’s has been drawn and a red circle.

If you look at flower.h and flower.cpp, we have all of the proper member variables set up. However, when we try to draw them they are drawn incorrectly.

Testing Your Code

Run the Catch tests as follows (this requires your code to compile when run simply as make):

make test

Cleaning up files

To clean up your working repository and remove the test images produced by your program, you can type the following command:

make clean

Submitting Your Work

The following files are used to grade this assignment:

  • shape.cpp
  • shape.h
  • circle.cpp
  • circle.h
  • truck.cpp
  • truck.h
  • flower.cpp
  • flower.h
  • drawable.h

All other files including any testing files you have added will not be used for grading.