How do i fix these errors : reference to non-static member function must be called and invalid use of member 'mat' in static member function?

Question

I was writing a program where I had to use custom sort, so I made a custom comparator "comp", but using it inside C++ class, gives "reference to non-static member function must be called", I fixed this using static bool comp(). But after making my comparator functions static, I am getting the error "invalid use of member 'mat' in static member function". How do I get through these?

I have called sort(str.begin(), str.end(), comp()), and from "comp", I haved called "comp_()". I had to sort the string in a different manner.

class Solution {
public:
     int mat[26][26];
       static bool comp_(char a, char b, int i) {
        if (i == 26) {
            return a<b;
        }
        if (mat[a-'A'][i] > mat[b-'A'][i]) {
            return true;
        }
        else if (mat[a-'A'][i] < mat[b-'A'][i]) {
            return false;
        }
        return comp_(a,b,i+1);
    }

      static bool comp(char a, char b) {
        return comp_(a,b,0);
    }

Answer 1

The trick is to define a call-operator on your Solution class so that it can be used as a Comparator object that is passed to the std::sort() call, like this:

#include <iostream>
#include <vector>

using namespace std;

class Solution {
public:
     int mat[26][26];

     bool comp(char a, char b, int i) const {
        if (i == 26) {
            return a<b;
        }
        if (mat[a-'A'][i] > mat[b-'A'][i]) {
            return true;
        }
        else if (mat[a-'A'][i] < mat[b-'A'][i]) {
            return false;
        }
        return comp(a,b,i+1);
    }

    bool operator()(const char & a, const char & b) const
    {
       return comp(a, b, 0);
    }
};

int main(int argc, char ** argv)
{
   std::vector<char> str;
   str.push_back('d');
   str.push_back('c');
   str.push_back('b');
   str.push_back('a');

   Solution sol;
   std::sort(str.begin(), str.end(), sol);

   for (size_t i=0; i<str.size(); i++) std::cout << str[i] << std::endl;
   return 0;
}

That way the methods in your Solution object do not have to be static, and therefore they can access the mat member variable.