Lab 2, A simple C++ class

Goals for this lab

By the time you have completed this lab, you should be able to:

  • Fundamentally define a C++ class, and know how it differs from a struct
  • Use objects of a class in application progroms
  • Implement a class’s methods outside the class definition
  • Better understand the distinction between an abstraction, its application and its implementation - and how it can be realized with C++

Pairing Reminder

You must work as a pair on the labs. If you were paired up last week, you need to work with the same pair. If you did not get paired up last week, or your pair is no longer in the class, please see the TA.

Lab Preparation

Before you begin, let’s prepare your cs32 directory in your home directory if you don’t already have one. Note that these instructions assume you are using either a CSIL machine or one of the lab machines:

Create the directory:

mkdir cs32

Change the permissions such that only your account can access the files inside this directory. This step is incredibly important. If omitted, other students can peek at your work and you may be held partially responsible.

chmod 700 cs32

Change into the cs32 directory, and create a lab2 directory:

cd cs32
mkdir lab2

Note that you do not need to change the permissions of the lab2 directory or any files inside of cs32 because permissions cascade. If you wanted to lock down your entire account you could run chmod 700 ~; however, no one would be able to access your website if that were the case.

Change into the lab2 directory and copy files you will need for this lab:

cd lab2
cp ~cs32/public_html/code/lab2/* .

Distinguish procedural from an object-oriented design

Look over grading1.cpp, the less object-oriented version of today’s program. It does use a struct to manage a student’s collection of grades, but that’s as object-oriented as it gets. Essentially, this is a C program in C++ clothing - the overall program design is procedural. Still you should make sure you know what it does and how it works before improving it.

This program solves the textbook’s Programming Project 1 on page 594. Briefly, it is a grading program for a class with these policies:

  • two quizzes worth 10 points each
  • one midterm and one final exam worth 100 points each
  • overall grade based on 50 percent for the final exam, 25 percent for midterm and 25 percent for quizzes. Further, 90+ earns A, 80-90 earns B, 70-80 earns C, 60-70 earns D, and otherwise F.

The program reads 4 numbers from stdin, and calculates and prints the associated letter grade. It does no error-checking. Compile via:

make grading1

Execute grading1 via:


You should see the following output (note you’ll have to type 9 9 40 90 followed by return):

Enter scores in this order: 2 quizzes, midterm, final: 9 9 40 90
Grade is B

Notice an important feature of the code in grading1.cpp: the data values in the global structure definition are directly accessable to all parts of the program. Unless otherwise specified, all members of a C++ struct are public. These data values are manipulated in three of the external functions: getScores, overallGrade, and main. It would be difficult to reuse parts of this code in other programs, and also difficult for multiple programmers to work together on separate parts of the code.

Be sure you understand why this design is not optimal, before continuing to the next step.

Convert a struct to a class

Now it’s time to do (a variation of) the textbook’s Programming Project 2 on page 595, which is to “Redo Programming Project 1 … but this time make the student record type a class type instead of a structure type.”

Open grading2.cpp with an editor (e.g., emacs). No need to read the code, as it is unchanged from grading1.cpp - only the comments are new. We’ll guide you through this first step of the conversion, to properly define class Record. Make just the following changes to the file:

  1. Redefine a Record object to be a class instead of a struct, by changing line 7 as instructed.

  2. The first section of a class definition traditionally is its public interface. In C++ that means adding the keyword public and a colon, :, on a line by itself - otherwise, all parts of a class by default are private. Add that line now.

  3. In a moment you will make the data private, but that means you must provide a way for clients of the class to set data values, but only because this problem requires that clients have that capability. To that end, declare a method named setGrades in the public section. This method should take four double arguments, and its return type should be void. Just declare the method here - it will be implemented later. The declaration does not require names for the parameters, only types, but if you want to name them here anyway, we suggest q1, q2, m and f, in that order.

  4. Before the data values are declared, add the keyword private and a colon to identify the rest of the class as private parts.

  5. It’s time for some cut and paste, and then some editing. Move the function prototype of letterEquiv into the private section of the class definition - it will become a class member function (“method”), but only to be used by other class members. Also add the keyword const to the end of its signature, so it can support constant objects.

  6. Now move the function prototype of overallGrade into the class’s public section, so it becomes part of a Record’s interface. This method will calculate a Record object’s own letter grade, so it won’t need any parameter! Therefore, remove the parameter from the signature, and add the keyword const to the end.

By the way, be sure to leave the function prototype of getScores in-between the class definition and main - it will remain as an external function.

The program will not successfully compile now, but the part inside the class definition should compile without errors. Try compiling it now to verify that the first error is not until the second statement in main:

make grading2

Verify that the first error reported is located somewhere in main. In case you need more guidance with the instructions in this section (or the next one), have a look at example.cpp. The example program defines, applies and implements a simple date class, in a manor that isvery similar to the grading2.cpp program.

Finish implementing the object-oriented design

The (uppercase) comments in the rest of grading2.cpp include specific instructions as well as hints about what needs to be done to complete the conversion. Here are some general guidelines to help you understand why these changes must be made.

  • In main, the function overallGrade is not being used properly anymore, because overallGrade is now a member function of the Record class, and it no longer accepts an argument. Actually, now the object named rec is able to calculate and return its own grade. Use the object’s name and the dot, ., operator to invoke the method (without any arguments).

  • In function getScores, currently the data values of the Record object (result) are being set directly. That can’t work anymore. The only way to set the values of the object’s data is to use the public method setGrades.

  • The function definition signatures for letterEquiv and overallGrade both must be changed in two ways: (a) they must identify themselves as part of class Record - using the scope resolution operator, ::, correctly; and (b) they must be declared const to match their new signatures in the class definition.

  • Also remember that overallGrade has no parameter anymore. So that means no variable named r to use. That’s okay though, because now this function is a class member, so it can access the calling object’s private data by name alone.

Compile and test the program to be sure it works correctly after all your changes. Fix any problems before submitting.

Submission Instructions

Please make only one submission per pair; this isn’t essential, it just makes life easier for the TA.

~cs32/submit lab2@cs32 grading2.cpp

You may submit up to 20 times. Please check the feedback email to ensure you submitted correctly, and are satisfied with your final score. If you are not, feel free to revise and submit again. Please also review the automated feedback instructions as needed.

Special Thanks

Special thanks to Michael Costanzo for allowing me use of his CS32 material. This lab was copied almost verbatim from his Spring 2012 CS32 class.


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