implement string class with custom behavior

Question

In one of our class sir said that template allows one to customize behavior of class, and then he gave example of string class, that with few lines of code we can customize string class from STL, as in, we can make it to treat 'a' and 'z' same, 'b' and 'y' same, 'c' and 'x' same and so on. Similary 'A' and 'Z' same etc.

"abc" == "zyx" is true;
"Abc" == "zyx" is false;
"Abc == "Zyx" is true;

etc

I was thinking of implementing such string class, but I am not able to do so. How can we implement such string class using templates?

Answer 1

It's very tricky. All you need to write your own traits class, specifically you need to derive it from char_traits<> class template, and redefine eq() and compare() function (Note: only redefining eq() would not work; even though there is no change in the redefinition of compare(), you've to write it in your derived class as such!). Lets say this traits class sequence_traits and call your custom string sequence. After all, string is a sequence of characters!

Note : What I understand from your post that you want alphabets[i] == alphabets[25-i] to be treated as same, that means, first letter and last letter same, second letter and second last letter same, and so on!

struct sequence_traits : char_traits<char>
{
    //'a' and 'z' are equal
    //'b' and 'y' are equal
    //'c' and 'x' are equal, and so on.
    //that implies, 'a' + 'z' == 'b' + 'y' == 'c' + 'x' == 'd'  + 'w == so on
    //same for upper cases!
    static bool eq(const char& left, const char& right)
    {   
        return ( left == right) || (left + right == 'a' + 'z') || ( left + right == 'A' + 'Z') ;
    }
    static int compare(const char *first1, const char *first2, size_t count)
    {   
        for (; 0 < count; --count, ++first1, ++first2)
            if (!eq(*first1, *first2))
                return (lt(*first1, *first2) ? -1 : +1);
        return (0);
    }
};

And then you can do this typedef for easy use:

typedef basic_string<char, sequence_traits> sequence;

You're done. You can use sequence now. :-)

Working example : http://www.ideone.com/ByBRV

---

Read this article to know how it works in detail : http://www.gotw.ca/gotw/029.htm

Answer 2

You'll want to look at the char_traits type. This is an example of one type of traits class that you can use in with basic_string in order to get a working string type. If you define your own traits class, you can build a custom string like this:

class CustomTraits { ... };
typedef basic_string<char, CustomTraits> CustomString;

And now the traits defined by CustomTraits will be used to determine how the string works.

As an example along the lines of what you were saying, you could do something like this:

class CustomTraits: public char_traits<char> {
public:
    /* Redefine equality to compare 'a' and 'z' equal. */
    static bool eq(char one, char two) {
         return one == two || (one == 'a' && two == 'z' || one == 'z' && two == 'a');
    }
};
typedef basic_string<char, CustomTraits> StringWithAAndZEqual;

Now, you can use the new type and the class will treat 'a' and 'z' identically.

Answer 3

You want to make your custom char_traits and instantiate std::basic_string with that.