Problem with STL sort.

Hi all,
I am using the STL sort to a vector whose every element is an instance
type "struct data" as defined below:

However, when I am trying to compile the code, I am getting the
following error:

/usr/include/c++/3.4.3/bits/stl_algo.h:90: error: passing 'const
SerialNumbers::data' as 'this' argument of 'bool
SerialNumbers::data::operator<(const SerialNumbers::data&)' discards
qualifiers

Can anybody please explain to me what that error means?

I am using the g++ to compile my code.

The code follows:

#include <iostream>
#include <string>
#include <vector>
#include <stack>
#include <algorithm>
#include <sstream>

using namespace std;

class Test {
    public:

    struct data
    {
         string str;

        bool operator < (const data& d2)
        {

            return false;

       }

    };

    vector <string> sortSerials(vector <string> inp)
    {
        vector<data> table;
        for(int i=0; i < inp.size(); ++i)
        {
                        data d;
            d.str = inp.at(i);
            table.push_back(d);
        }

        sort(table.begin(),table.end());

        vector<string> ret;
        for(int i=0; i < table.size(); ++i)
        {
          ret.push_back(table.at(i).str);
        }

        return ret;

    } // the method ends here.
};