Getting the Error “reference to non-static member function must be called ”

Question

I am solving a problem on LeetCode to make a pair sort and give the output basically link - https://leetcode.com/problems/the-k-weakest-rows-in-a-matrix/

this is my code

class Solution {
    public:
    
    bool sortbysec(const pair<int,int> &a,
              const pair<int,int> &b)
    {
        if(a.first == b.first)
            return (a.second < b.second);
        
        return (a.first > b.first);
    }
    
    vector<int> kWeakestRows(vector<vector<int>>& mat, int k) {
        
               
        vector<pair<int,int>>zeros;
        
        vector<int>res;
        
        for(int i = 0;i<mat.size();i++){
            zeros.push_back(make_pair(count(mat[i].begin(),mat[i].end(),0),i)) ;
        }
        sort(zeros.begin(),zeros.end(),sortbysec);
        int n = zeros.size();
        for(int i = 0;i<mat.size();++i){
            res.push_back(zeros[i].second);
        }
        return res;
    }
};

i get this error 1st time and don't know about it what to do to solve it

Answer 1

You cannot pass a non-static member function to std::sort in that way because it would need an instance of Solution to call it on.

Fortunately, the solution in this case is simple: since sortbysec doesn't reference any member variables, you can declare it static :

static bool sortbysec(const pair<int,int> &a,
          const pair<int,int> &b)

If you do need to call a non-static member function as your sort function, you can do it via a capturing lambda:

sort(zeros.begin(),zeros.end(),
     [this] (const pair<int,int> &a, const pair<int,int> &b) 
         { return sortbysec (a, b); });