CSC 2210, SPA 5: Copy-on-write Strings

Description

This assignment will give you experience with the "big 3" for a new string class. If a program does a lot of text manipulation, it can be helpful if it uses a string class that does basic, automated memory management. For example, if two string variables are assigned to each other, it would be useful to have them point to the same underlying buffer (to avoid duplication), but to reassign the variable if the string is modified. You are going to create a string library that implements this.

In particular, you will implement a class cow_string. In this case "cow" stands for "copy on write", most definitely note an implied reference to livestock frequently found in Wisconsin. The class will store arrays of characters and be implemented using the C-style string operations like strcpy. To avoid duplication, cow_string objects point to shareable_string objects which contain both a string and a count of the number of cow_string objects that share that string.

More specifically, download the following code and get it to run with no errors:

• generic_string.h
• generic_string.cpp
• cow_string.h
• cow_string.cpp
• main.cpp

Note the main.cpp contains code to implement a simple editor. This editor responds to the following commands:

• a: Append a line to the end of the buffer a given number of times
• e: Edit line n at index i to character c
• r: Replace line n with new text
• d: Delete line n
• p: Print the buffer starting at line n The code in main is designed using the stream-of-consciousness model (that is, not well structured). This is meant to illustrate why writing code using this model is not recommended.

Constraints

• Change only cow_string.cpp. Talk to your instructor if you believe you need to change other files.

• Do not add additional includes to cow_string.cpp. In particular, do not use std::string. Use the C-style string (strcpy, strlen, etc.) to process strings.

• The original code does not compile because code needs to be added to specify the argument to the generic_string constructor. Adding this code is part of the exercise.

Submitting

When you are finished, submit just cow_string.cpp to esubmit as 5cow. Your solution will only be graded for correctness, not style issues. You may discuss your solution with others, but each student is to submit their own.

Frequent Problems

• If you are getting linker errors when building in CLion, edit CMakeLists.txt and make sure both generic_string.cpp and cow_string.cpp (but not the .h files) are listed between the parens for add_executable. For example, your file should look similar to CMakeLists.txt, though you may have a different assignment name or other differences.

• If you get a message about there being bytes that did not get released, double-check your logic for updating the underlying_string->count and deleting the underlying_string object. The deletion should happen whenever the count reaches zero.

• If you are getting an error message about double deletes, check that your code maintains the underlying string counter correctly. That counter should be a count of the number of cow_string objects that refer to a given underlying string object, so the count should be two after executing the code

        cow_string a = "hello";
        cow_string b = a;