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/
You will only need to modify the following files:
shape.{cpp,h}
circle.{cpp,h}
truck.{cpp,h}
flower.{cpp,h}
drawable.h
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 {
public:
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 {
public:
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 */
d->breed;
/* 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->name;
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
./lab_inheritance
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
- Investigate and fix the code so that the areas and the perimeters are the same.
- To fix this problem you should only need to modify
shape.cpp
and/orshape.h
.
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
- Investigate and fix the code so that the there is no more memory leak inside
of
test_destructor
. - To fix this problem you should only need to modify
drawable.h
andshape.h
.
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
- Investigate and fix the code so that the
Circle
is being constructed with the proper center and color. - To fix this problem you should only need to modify
circle.cpp
- The correct
test_constructor.png
should look like the following:
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
- Investigate and fix the code so that
test_pure_virtual
compiles, runs, and outputs aTruck
. - To fix this problem you should only need to modify
truck.h
andtruck.cpp
. - In order to have the
Truck
draw properly you will first need to have Exercise 3 completed. - The correct
test_pure_virtual.png
should look like the following:
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.
Exercise
- Investigate and fix the code so that
test_slicing
outputs aFlower
. - To fix this problem you should only need to modify
flower.h
andflower.cpp
. - You must use polymorphism!
- The correct
test_slicing.png
output should look like the following:
Testing Your Code
Run the Catch tests as follows (this requires your code to compile when run simply as make
):
make test
./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.