EECS 280 Project 4: Text Editor

Due 8:00pm Tuesday April 2, 2024. You may work alone or with a partner (partnership guidelines).

Change Log

2023-03-15 We’ve released an updated version of the starter files:

Implementation details of the project have not changed.

2023-03-22 We’ve updated the REQUIRES clause and implementation of the -- operators included in the starter code to explicitly prohibit decrementing an iterator pointing at the first element (since there is nothing before the first element). This better matches the interface for std::list and its iterators. Double check your TextBuffer.cpp implementations to ensure your code does not decrement begin iterators.

Here are the updated -- operator definitions, which you can copy/paste over the previously provided versions. The REQUIRES clauses and corresponding assertions in the prefix operator implementation are updated, while the rest of the code is unchanged.

// This operator will be used to test your code. Do not modify it.
// REQUIRES: Iterator is decrementable. All iterators associated with a
//           list are decrementable, including end iterators, except for
//           begin iterators (those equal to begin() on the list)
// EFFECTS:  moves this Iterator to point to the previous element
//           and returns a reference to this Iterator
Iterator& operator--() { // prefix -- (e.g. --it)
  assert(*this != list_ptr->begin());
  if (node_ptr) {
    node_ptr = node_ptr->prev;
  } else { // decrementing an end Iterator moves it to the last element
    node_ptr = list_ptr->last;
  return *this;

// This operator will be used to test your code. Do not modify it.
// REQUIRES: Iterator is decrementable. All iterators associated with a
//           list are decrementable, including end iterators, except for
//           begin iterators (those equal to begin() on the list)
// EFFECTS:  moves this Iterator to point to the previous element
//           and returns a copy of the original Iterator
Iterator operator--(int /*dummy*/) { // postfix -- (e.g. it--)
  Iterator copy = *this;
  return copy;


The learning goals of this project include Container ADTs, Dynamic Memory, The Big Three, Linked Lists, and Iterators. You will gain experience with new and delete, constructors and destructors, and the List data structure that is similar to std::list from the standard library.

When you’re done, you’ll have implemented the basic features of a text editor that is usable through a terminal-based visual editor, similar to other terminal editors such as Pico, nano, Emacs, and vi.


Set up your visual debugger and version control, then submit to the autograder.

Visual debugger

During setup, name your project p4-editor. Use this starter files link: https://eecs280staff.github.io/p4-editor/starter-files.tar.gz

VS Code Visual Studio Xcode

You should end up with a folder with starter files that look like this. You may have already renamed files like List.hpp.starter to List.hpp.

$ ls
List.hpp.starter               e0.cpp
List_compile_check.cpp         femto.cpp
List_public_tests.cpp          line.cpp
List_tests.cpp.starter         line_test1.in
Makefile                       line_test1.out.correct
TextBuffer.hpp                 line_test2.in
TextBuffer_public_tests.cpp    line_test2.out.correct
TextBuffer_tests.cpp.starter   unit_test_framework.hpp

Here’s a short description of each starter file.

File(s) Description
List.hpp.starter Starter code for the List class template.
List_tests.cpp.starter Starter code for your List unit tests.
List_compile_check.cpp Compile check test for List.
List_public_tests.cpp A very small set of test cases for List.
Makefile Helper commands for building.
TextBuffer.hpp Interface specification for the TextBuffer class.
TextBuffer_public_tests.cpp Public test cases for the TextBuffer class.
TextBuffer_tests.cpp.starter Starter code for your TextBuffer unit tests.
e0.cpp A basic visual editor for testing the TextBuffer class.
femto.cpp A more functional visual editor that uses the TextBuffer class.
line.cpp A simple command line tool to test the TextBuffer class.
Input and correct output for system tests with line.cpp.

Version control

Set up version control using the Version control tutorial.

After you’re done, you should have a local repository with a “clean” status and your local repository should be connected to a remote GitHub repository.

$ git status
On branch main
Your branch is up-to-date with 'origin/main'.

nothing to commit, working tree clean
$ git remote -v
origin	https://github.com/awdeorio/p4-editor.git (fetch)
origin	https://githubcom/awdeorio/p4-editor.git (push)

You should have a .gitignore file (instructions).

$ head .gitignore
# This is a sample .gitignore file that's useful for C++ projects.

Group registration

Register your partnership (or working alone) on the Autograder. Then, submit the code you have.

Linked list

Implement your doubly-linked list in List.hpp. List.hpp.starter provides prototypes for each function. Because List is a templated container, function implementations go in List.hpp. There is no List.cpp.

While the List from lecture was singly linked, this List is doubly linked. This List also contains an iterator interface. The iterator keeps track of both the current node as well as the List to which it belongs – this allows us to go backwards from an end iterator by looking up the last node in the List.

Do not modify the public interface of the List class. Implement a doubly-linked list. No arrays or vectors, etc. Manage memory allocation so that there are no memory leaks (Leak checking tutorial).

Compile and run the provided compile check and List tests.

$ make List_compile_check.exe
$ make List_public_tests.exe
$ ./List_public_tests.exe

Write tests for List in List_tests.cpp using the Unit Test Framework. You’ll submit these tests to the autograder. See the Unit Test Grading section.

$ make List_tests.exe
$ ./List_tests.exe

Pro-tip: Getting an error about typename? Take a look at our reference on Typename.


Rename these files (VS Code (macOS), VS Code (Windows), Visual Studio, Xcode, CLI):

Edit List.hpp, adding a function stub for each member function declared in List.hpp, either inside or outside the class definition: For example:

// Inside the class definition:
template <typename T>
class List {
  bool empty() const {


// Or, outside the class definition:
template<typename T>
bool List<T>::empty() const {

The List tests should compile and run. The public tests will fail until you implement the functions. The file for your test cases (List_tests.cpp) will pass because it initially only contains ASSERT_TRUE(true).

$ make List_public_tests.exe
$ ./List_public_tests.exe
$ make List_tests.exe
$ ./List_tests.exe

At this point, we haven’t written the List Iterator, so List_compile_check.exe won’t compile. You’ll need to take a look at the lecture about iterators and write your own tests. After you do, use the provided compile check like this:

$ make List_compile_check.exe

Configure your IDE to debug either the public tests or your own tests.

Public tests Your own tests
VS Code (macOS)

Set program name to:

Set program name to:

VS Code (Windows)

Set program name to:

Set program name to:


Include compile sources:
List_public_tests.cpp, List.hpp

Include compile sources:
List_tests.cpp, List.hpp

Visual Studio

Exclude files from the build:

  • Include List_public_tests.cpp
  • Exclude List_compile_check.cpp, List_tests.cpp, TextBuffer_public_tests.cpp, TextBuffer_tests.cpp, e0.cpp, femto.cpp, line.cpp, main.cpp (if present)

Exclude files from the build:

  • Include List_tests.cpp
  • Exclude List_compile_check.cpp, List_public_tests.cpp, TextBuffer_public_tests.cpp, TextBuffer_tests.cpp, e0.cpp, femto.cpp, line.cpp, main.cpp (if present)


A text buffer holds an editable sequence of characters and at the same time has the ability to report the current row and column (You can display this information in Pico/nano with Ctrl-C, or in Emacs by typing M-x line-number-mode and M-x column-number-mode.). The text buffer keeps track of the current cursor position where all edits are done and allows scrolling using the arrow keys to move the cursor through the text.

In this section, you will implement the TextBuffer class according the interface defined in the TextBuffer.hpp file. The data representation of a TextBuffer is a doubly linked list of characters (either your implementation from part 1 or std::list from the C++ standard library). The TextBuffer also stores an iterator that indicates the current position of the cursor, the current row and column position, and the index of the cursor with respect to the entire buffer (i.e. how many characters from the start to the cursor).

The definition of the TextBuffer class is shown below:

class TextBuffer {
  using CharList = std::list<char>;
  using Iterator = std::list<char>::iterator;
  CharList data;           // linked list that contains the characters
  Iterator cursor;         // current position within the list
  int row;                 // current row
  int column;              // current column
  int index;               // current index
  // ... public interface not shown

We use CharList as a type alias for either List<char>, or std::list<char> (your choice – the autograder will use std::list<char> to avoid any issues with your List implementation), representing a doubly-linked list of characters. The following list contains the text spaces:

To use a linked list as a text buffer, we need to keep track of the cursor, which represents the position where edits can be made in the buffer. In many visual text editors, the cursor appears as a colored rectangle, so that if the cursor was pointing to the node containing the character 'c', we’d see it displayed as

Pressing the left arrow key in a text editor moves the cursor one character backward (to the left).

We can now draw the linked list corresponding to this text buffer along with the cursor:

Deletions in a text buffer take place at the cursor. If we delete a character in the previous picture, it will remove the character at the cursor:

Insertions happen to the left of the cursor. If we next typed the i key, that character would be entered in to the left of the cursor.

One consequence of this design is that, in order for additions to be made to the end of the buffer, the cursor needs to be able to go to the right of all the text. In other words, it must be possible for the cursor member variable to be equal to the end iterator for the underlying list. Starting from the buffer above, we can see what that looks like from the editor’s point of view:

As a doubly-linked list, this final buffer looks like this:

A new, empty text buffer containing no text starts with the cursor at the end iterator.

Rows and Columns

One thing we care a great deal about in a text editor is which characters are newlines, because that is what lets us know our position in the document: the row and column. In Emacs (and the remaining tasks of this assignment), the first row is row 1, and the first column is column 0. In these Emacs buffers, you can see the (row, column) displayed in the lower-right corner:

We can calculate the column of the cursor by working backwards until we find a newline, and we can calculate the row of the cursor by working backwards to the beginning of the buffer and counting the newlines. Note that in the middle example, the cursor is atop a cell containing a newline \n, but the cursor is at the end of row 1, not the beginning of row 2.

By tracking the row and column in the data structure, we can report this information to the user without ever having to recalculate the row. It’s good to avoid this, because calculating the row every time an edit is made can be expensive to run. You should also keep track of the column field but sometimes you might need to recalculate this field depending on the type of edit. Any single row is usually relatively short (80 columns maximum, if you’re using good style), so this should be fast. Specifically, we only need to recalculate the column when we move left from the beginning of one line to the end of the previous line.

Pro-tip: The TextBuffer class has a private compute_column member function that is intended to be a helper function for backward. Implement compute_column before you start implementing backward.

Efficiently implement the interface functions for manipulating editors given in the definition of the TextBuffer class in TextBuffer.hpp.


Rename these files (VS Code (macOS), VS Code (Windows), Visual Studio, Xcode, CLI):

Create a new file TextBuffer.cpp. Add function stubs for all the functions in TextBuffer.hpp. Remember to #include "TextBuffer.hpp".

Testing the TextBuffer

The TextBuffer tests should compile and run once you have created TextBuffer.cpp and added function stubs. The public tests will fail until you implement the functions. The file for your test cases (TextBuffer_tests.cpp) will pass because it initially only contains ASSERT_TRUE(true).

$ make TextBuffer_public_tests.exe
$ ./TextBuffer_public_tests.exe
$ make TextBuffer_tests.exe
$ ./TextBuffer_tests.exe

Interactive Testing

You can also test your TextBuffer implementation interactively by compiling and running the provided line.cpp, which visualizes the contents of a text buffer.

$ make line.exe
$ ./line.exe
LINE Is Not an Editor -- it is a linear visualization of a TextBuffer.
The '<' character mimics a call to backward()
The '>' character mimics a call to forward()
The '#' character mimics a call to remove()
The '^' character mimics a call to up()
The '!' character mimics a call to down()
The '[' character mimics a call to move_to_row_start()
The ']' character mimics a call to move_to_row_end()
The '@' character mimics a call to insert() with a newline
All other characters just mimic insert() with that character

Give initial input (empty line quits):

Try entering steady^<<<<^>>^>>^@<<@^^ as the initial input. Here are the first few lines of the result:

Give initial input (empty line quits):
start : |       :(1,0 )
add   : s|      :(1,1 )
add   : st|     :(1,2 )
add   : ste|    :(1,3 )
add   : stea|   :(1,4 )
add   : stead|  :(1,5 )
add   : steady| :(1,6 )
up    : steady| :(1,6 )
left  : stead|y :(1,5 )
left  : stea|dy :(1,4 )

The special commands directly invoke the corresponding member functions in the TextBuffer class. If an operation cannot be performed (e.g., invoking the up() member function to move the cursor up a line when it is already at the first row), the function should leave the text buffer unchanged instead of raising an error or assertion violation.

We have provided two test cases that use line.exe, which you can run as follows:

$ ./line.exe < line_test1.in > line_test1.out
$ diff -qB line_test1.out line_test1.out.correct
$ ./line.exe < line_test2.in > line_test2.out
$ diff -qB line_test2.out line_test2.out.correct

Alternatively, use

$ make test

to run all List and TextBuffer tests.

Visual Text Editors

The starter files include two visual frontends that use your TextBuffer class, a basic one that is useful for testing, and a more functional one that is similar to Pico.

You may need to install the ncurses library before you can use the visual editors. On WSL, run:

$ sudo apt install libncurses5-dev

On macOS, run:

$ brew install ncurses

Close your terminal and reopen your terminal.

The first visual editor is called E0, and you can compile and run it as follows:

$ make e0.exe
$ ./e0.exe

This will launch a visual editor that will allow you to type text, use delete or backspace to remove characters, and use the arrow and home/end keys to navigate the text. The key combination Ctrl-X exits the editor.

The second visual editor is called FEMTO, and it provides much of the functionality of terminal editors such as Pico. Use the following to compile and run it:

$ make femto.exe
$ ./femto.exe

You can also specify a filename at the command line. For instance, we can examine the visual editor’s own source code with

$ ./femto.exe femto.cpp

You should see something like the following:

You can scroll between pages with the page-up and page-down keys (or just the up and down keys at the top and bottom rows), and you can use the special commands listed at the bottom – for example, ^X means the combination Ctrl-X, which you can use to exit.

Submission and grading

Submit these files to the autograder.

This project will be autograded for correctness, comprehensiveness of your test cases, and programming style. See the style checking tutorial for the criteria and how to check your style automatically on CAEN.


Check for memory leaks using the Leak checking tutorial.

Run all the unit tests and system tests. This includes the public tests we provided and the unit tests that you wrote.

$ make test

Pro-tip: Run commands in parallel with make -j.

$ make -j4 test

Unit Test Grading

We will autograde your List unit tests. We will not grade your TextBuffer tests.

Your unit tests must use the unit test framework.

A test suite must complete less than 5 seconds and contain 50 or fewer TEST() items. One test suite is one _tests.cpp file.

To grade your unit tests, we use a set of intentionally buggy instructor solutions. You get points for catching the bugs.

  1. We compile and run your unit tests with a correct solution.
    • Tests that pass are valid.
    • Tests that fail are invalid, they falsely report a bug.
  2. We compile and run all of your valid tests against each buggy solution.
    • If any of your tests fail, you caught the bug.
    • You earn points for each bug that you catch.

Requirements and restrictions

It is our goal for you to gain practice with good C++ code, classes, and dynamic memory.

Modify .cpp files and List.hpp Modify the signature of any public functions in List.hpp or TextBuffer.hpp
For List, make helper member functions private Modify the public interface of List or TextBuffer
Use any part of the STL except for containers in your List implementation Use STL containers in your implementation of List
#include a library to use its functions Assume that the compiler will find the library for you (some do, some don’t)
Use C++ strings Use C-strings
Pass large structs or classes by reference Pass large structs or classes by value
Pass by const reference when appropriate “I don’t think I’ll modify it …”
Use the Address Sanitizer to check for memory errors “It’s probably fine…”


This project has been adapted from the course 15-122 (Principles of Imperative Computation) offered at Carnegie Mellon University, which is taught in a safe subset of C called C0. This project was ported to C++ by Saquib Razak.

This document is licensed under a Creative Commons Attribution-NonCommercial 4.0 License. You’re free to copy and share this document, but not to sell it. You may not share source code provided with this document.