Inserting a value into a linked list using iterator position

[Forgetful]

Im trying to insert a new element into a Linked list but instead of inserting the value my program replaces the value with the new value in that location for example my code output would be

H,e,l,l,o
H,!,l,l,o

but I want it to be

H,e,l,l,o
H,e,!,l,l,o

how can I fix this?

#include <initializer_list>
#include <iostream>

template <typename T>
class List {
  struct Node;  
  class It;
 public:
  List(std::initializer_list<T> ele) {
    this->operator=(ele);  
  }

  void InsertB(T ele) {
    Node* node = new Node{ele};
    if (this->size == 0) {
      this->start = this->fin = node;
    } else {
      this->fin->next = node;
      node->prev = this->fin;
      this->fin = node;
    }
    ++this->size;
  }

  void InsertF(T ele) {
    Node* node = new Node{ele};
    if (this->size == 0) {
      this->start = this->fin = node;
    } else {
      node->next = this->start;
      this->start->prev = node;
      this->start = node;
    }
    ++this->size;
  }

  List& operator=(std::initializer_list<T> ele) {
    this->size = 0;
    for (auto&& val : ele) this->InsertB(val);
    return *this;
  }

  friend std::ostream& operator<<(std::ostream& out, const List& list) {
    for (auto val = list.start; val; val = val->next) {
      out << val->data;
      if (val->next) out << ",";
    }
    return out;
  }

  template <class It>
  List(It begin, It end) {
    for (; begin != end; ++begin) this->InsertB(*begin);
  }

  
  It begin() {
    return It(start);
  }
  
  It end() {
    return It(NULL);
  }

  It InsertIt(It pos, const T& ele) {
   if(pos == begin()) {
     InsertF(ele);
     return begin();
   } else if(pos == end()) {
     InsertB(ele);
     return fin;
   } else {
     Node* temp = new Node{ele, pos.cur->next, pos.cur};

     pos.cur->prev->next = temp;
     pos.cur->prev = temp;

     ++size;
     return temp;
   }
  }

private:
  struct Node {
    T data;
    Node* next = nullptr;
    Node* prev = nullptr;

  };

  class It {
    friend class List;
    public:
    It& operator=(const It& that) {
      List<char> first(that.begin(), that.end());
    }

    It& operator++() {
      this->cur = cur->next;
      return *this;
    }

    It operator++(int) {
      It temp(*this);
      this->operator++();
      return temp;
    }

    T const *operator->() const { return &(**this); }

    T *operator->() { return &(**this); }

    T& operator*() const {return this->cur->data;}
    
    bool operator!=(It val) const { 
       return !(this->operator==(val));
    }

    bool operator==(It val) const { 
       return this->cur == val.cur;
    }

     private:

     Node* cur;
     It (Node *ptr) : cur(ptr) {}
  };

  Node* start = nullptr;
  Node* fin = nullptr;
  std::size_t size = 0;
};

int main() {
List<char> test = {'H', 'e', 'l', 'l', 'o'};
std::cout << test << std::endl;
List<char> test2 (test.begin(), test.end());
auto iter = test2.begin();
iter++;
test2.InsertIt(iter, '!');
std::cout << test2 << std::endl;
}

[JLBorges]

> I want it to be H,e,!,l,l,o

Since the iterator is 'pointing' to the second element 'e', it should be H,!,e,l,l,o
(Insert the new element at the position specified by the iterator)

} else {
    // Node* temp = new Node{ele, pos.cur->next, pos.cur}; // *** line 73  
    Node* temp = new Node{ ele /*data*/ , pos.cur/*next*/, pos.cur->prev/*prev*/ };

    pos.cur->prev->next = temp;
    pos.cur->prev = temp;

    ++size;
    return temp;
}

[JLBorges]

Insert appears to work correctly when I try it:

int main() {
    List<char> test = {'H', 'e', 'l', 'l', 'o'};
    std::cout << test << std::endl;
    List<char> test2 (test.begin(), test.end());
    auto iter = test2.begin(); // H,e,l,l,o
    iter++;
    test2.InsertIt(iter, '!');
    std::cout << test2 << std::endl; // H,!,e,l,l,o

    // **** added now **** //////////////////

    auto inserted = test2.InsertIt(++iter, '*');
    std::cout << test2 << std::endl; // H,!,e,*,l,l,o
    std::cout << *inserted << std::endl; // *

    test2.InsertIt( test2.begin(), '+');
    std::cout << test2 << std::endl; // +,H,!,e,*,l,l,o

    int n = 0 ;
    for( auto it = test2.begin() ; it != test2.end() ; ++it )
        it = test2.InsertIt( ++it, char( n++ + '0' ) );
    std::cout << test2 << std::endl; // +,0,H,1,!,2,e,3,*,4,l,5,l,6,o,7

    ////////////////////////////////////////////////////////////
}

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