two-dimension vector custom predicate greater function

I have a two-dimension vector like below.

vector<vector<int>> vec = {{2,5}, {3, 4}, {0,1}, {1,2}}

I want to sort this array based on the second element of each vec[i]. So the end result should be

{{2,5},{3,4},{1,2}, {0,1}}

I want to use something like

sort(vec.begin(), vec.end(), secondGreater);

bool secondGreater(vector<int> a, vector<int> b){
  return a[1]>b[1];
}

My thinking is: each element of vec is a one-dimension vector, so i provide one function to compare two one-dimension vector. But somehow it doesn't work. Any hint?

Thanks.

//-------------------------------------------------

The code is like below, and the error message is

Line 6: Char 46: error: reference to non-static member function must be called sort(boxTypes.begin(), boxTypes.end(), secondGreater); ^~~~~~~~~~~~~ 1 error generated.

class Solution {
public:
    int maximumUnits(vector<vector<int>>& boxTypes, int truckSize) {
      int cnt=0;
      
      sort(boxTypes.begin(), boxTypes.end(), secondGreater);
      
      for(auto i=0;i<boxTypes.size();i++){
        if(truckSize > boxTypes[i][0]){
          cnt+= boxTypes[i][0] * boxTypes[i][1];
          truckSize -= boxTypes[i][0];
        }
        else if(truckSize>0 && truckSize < boxTypes[i][0]){
          cnt+= boxTypes[i][1] * truckSize;
          break;
        }
        else break;
      }
      return cnt;
      
    }
    
    bool secondGreater(vector<int> boxA, vector<int> boxB){
      return boxA[1] > boxB[1];
    }
};

Solution

secondGreater is a member function. But you are trying to call it as if it were a free function with no associated object.

Since it accesses no member variables, it doesn't need to be a member function, move it outside the class:

bool secondGreater(vector<int> boxA, vector<int> boxB){
   return boxA[1] > boxB[1];
}

class Solution

If you for some reason needed it to be in the class, you could make it static:

static bool secondGreater( ...

If you need it for some reason to be a non-static member function, aka you need state from the class in the future, then you could bind this to it:

sort(boxTypes.begin(), boxTypes.end(), std::bind(secondGreater, this, _1, _2)); 
// This code is only psuedo code.

Or since it is so simple, just replace the whole thing with a lambda:

sort(boxTypes.begin(), boxTypes.end(), [](vector<int> boxA, vector<int> boxB){
   return boxA[1] > boxB[1];
});