I was trying to sort a Map by value , using the std::sort and a custom compare function. But I got a compile error .

I don't know how to use Lambda functions. I just know they can be used. But for now I need to learn what went wrong with the custom compare function.

I have already tried using iterator variables as arguments in the dcompare function.

class Solution {
public:
    int foo(int n) {
        unordered_map<int,int> M ;
        // Inputs = {2,4,5,2,4,2,1}
        M[4] = 2;
        M[5] = 1;
        M[2] = 3;
        M[1] = 1;

        for(auto it : M){
            cout<<it.first<<" : "<<it.second<<endl;
        }

        sort(M.begin(), M.end(), dcompare);

        for(auto it : M){
            cout<<it.first<<" : "<<it.second<<endl;
        }

        return 0;
    }

private:
    static bool dcompare(const std::pair<int,int> itL, const std::pair<int,int> itR) {
        return (itL.second < itR.second);
    }
};

/*
I get the same error when I changed the variables to iterators

static bool dcompare(const map<int,int>::iterator itL, const map<int,int>::iterator itR) 
{ 
return (itL->second < itR->second); 
}
*/

Compile error:

/usr/local/include/c++/8.2.0/bits/stl_algo.h: In instantiation of 'void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = std::_Rb_tree_iterator >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter, std::pair)>]': /usr/local/include/c++/8.2.0/bits/stl_algo.h:4866:18: required from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = std::_Rb_tree_iterator >; _Compare = bool (*)(std::pair, std::pair)]' Line 15: Char 42: required from here /usr/local/include/c++/8.2.0/bits/stl_algo.h:1969:22: error: no match for 'operator-' (operand types are 'std::_Rb_tree_iterator >' and 'std::_Rb_tree_iterator >') std::__lg(__last - __first) * 2,

PS:
I can't share the actual question , So I am sharing just the sorting part of it with some debug prints.