member function of an object of class map

Question

I somehow not sure what is wrong with call to insert for the object "history" where history is of type map. I have checked that there should be an insert function for a map object where the insert function would take in a pair as an argument. However I keep getting the error of

#include <map>
using namespace std;
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        std::map<ListNode, int> history; \\ so here history is created to be an object of type map
        
        ListNode *current = head;
        
        if(head == NULL) {
            return nullptr;
        } else {
            while(current->next) {
                history.insert(make_pair(current, current->val)); \\ this line has the error of no matching member function for call to "insert"

                if(history.count(current->next)) {
                    return current->next;
                }
            }
        }
        return nullptr;
    }
};

could someone points out what is wrong here? or maybe my understanding in the c++ map class?

sorry I am pretty new to c++ and my uses of c++ terminology might not even be right.

thank you

Sam Varshavchik · Accepted Answer · 2022-07-25 00:11:29Z

std::map<ListNode, int> history;

This map's key is a ListNode.

ListNode *current = head;

current is a ListNode *, a pointer to a ListNode.

history.insert(make_pair(current, current->val));

This attempts to insert a ListNode * key into a map whose key is a ListNode. This is ill-formed. If a map's key is an int, an inserted key must be an int. If the map's key is a ListNode the inserted key must be a ListNode. If the map's key is a ListNode * the inserted key must be a ListNode *.

Either your map's key is wrong, or your insert() call is wrong.

P.S. You appear to be using an outdated C++ textbook to learn C++, or an old C++ compiler that does not implement the current C++ standard. Modern C++ uses a cleaner "uniform initialization" syntax with insert() instead of std::make_pair. You should see if you can find a more recent textbook and/or compiler, that support modern C++.

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