Question

I’m working on one of the programming challenges in the book Starting Out With C++ Early Objects 7th Edition and one of the assignments asks to create a class which is derived from the STL string class. I’m posting the question for the purpose of understanding what I am allowed to do and how I am supposed to implement the solution so that no one offers more advanced suggestions.

— Question as it is written in the text —

Palindrome Testing

A Palindrome is a string that reads the same backward as forward. For example, the words mom, dad, madam, and radar are palindromes. Write a class Pstring that is derived from the STL string class. The Pstring class adds a member function

bool isPalindrome()

that determines whether the string is a palindrome. Include a constructor that takes an STL string object as a parameter and passes it to the string base class constructor. Test your class by having a main program that asks the user to enter a string. The program uses the string to initialize a Pstring object and then calls isPalindrome() to determine whether the string entered is a palindrome.

You may find it useful to use the subscript operator [] of the string class: if str is a string object and k is an integer, then str[k] returns the caracter at the position k in the string.

— End —

My main question is how do I access the member variable which holds my string object if the class I am deriving Pstring from is a class I did not write and I do not know how it implements its members?

For example,

#include <string>
using namespace std;

class Pstring : public string
{
public:
  Pstring(std::string text)
   : string(text) { }

  bool isPalindrome()
  {
    // How do I access the string if I am passing it to the base class?

    // What I think I should do is...
    bool is_palindrome = true;
    auto iBegin = begin();
    auto iEnd   = end() - 1;

    while (iBegin < iEnd && is_palindrome)
    {
      if (*iBegin++ != *iEnd--)
        is_palindrome = false;
    }

    return is_palindrome;

    // But I think this is wrong because...
    // #1 The book did not discuss the keyword auto yet
    // #2 The book discussed when a class is derived from another class,
    //    how the members from super class will be accessible to the sub class.
    //    However, with this assignment, I don't see how to access the members.
  }
}

The reason I feel like I am doing this incorrectly is because the assignment mentions using subscript notation, however, I don’t understand how to use the subscript notation if I don’t know the name of the variable where the string is stored.

Any help would be greatly appreciated because the author does not provide the solutions unless I am an instructor which is pretty lame in my opinion. It probably has to do with the fact that this is an academic text.

Answer 1

You shouldn’t inherit from std::string, as it wasn’t designed for that, nor do you need to in order to find a palindrome.

See this: Inheriting and overriding functions of a std::string?

Palindrome solution (from this question: Check if a string is palindrome Linked from this: C++ Palindrome finder optimization)

#include <algorithm>

bool isPal(const string& testing) {
    return std::equal(testing.begin(), testing.begin() + testing.size() / 2, testing.rbegin());
}

That book’s quality seems questionable. Free functions (depending on who you ask) are almost always preferred over member functions, and especially preferred over inheritance.

---

If you must use inheritance:

class Pstring : public string
{
    //...

    bool isPalindrome()
    {
      return std::equal(begin(), begin() + size() / 2, rbegin());

      // as a side-note, 'iterator' will refer to the inherited return type of begin()
      // Also, 'operator[](x)' will call the subscript operator
    }
};