c++ - Implementation of map - how to differ two ends of the map in iterator -

iterator implementation:

 class iterator {         private:             node<pair>* _ptr = nullptr;         public:              iterator(node<pair>* ptr = nullptr) : _ptr(ptr) { }              iterator(const iterator& itr) = default;              ~iterator() = default;              iterator& operator=(const iterator&) = default;              iterator operator++(int) {                 node<pair> *cur = this->_ptr;                 if (this->_ptr) {                     this->_ptr = _ptr->next();                 }                 return cur;             }              iterator& operator++() {                 if (this->_ptr) {                     this->_ptr = _ptr->next();                 }                 return *this;             }              pair& operator*() {                 if (!this->_ptr) {                     throw mapelementnotfoundexception();                 }                 return this->_ptr->data();             }              bool operator==(const iterator& itr) const {                 if ((this->_ptr == nullptr && itr._ptr == nullptr) ||                     (this->_ptr == itr._ptr)) {                     return true;                 }                 return false;             }              friend bool operator!=(const iterator& itr1, const iterator& itr2) {///// non member                 return !(itr1 == itr2);             }         }; 

private of map class:

    node<pair> *_head;     valuetype _default; /////////////////////////////////////////////////     int _size; 

the methods begin , end:

iterator begin() const{             return iterator(this->_head);         }          iterator end() const{             return iterator(nullptr); //////////////////////////////////////////////         } 

how can implement iterator , end method work:

assert_equals(false, map9.end() == map7.end()); 

my goal implement iterator in such way, in implement correctly operator == , != iterators, 2 iterators identical if point same object in same map, , different in other way.

edit: (solution)

private:             node<pair>* _ptr/* = nullptr */;             mtmmap<valuetype, keytype, comparefunction>* _map_ptr;         public:              iterator(node<pair>* ptr/* = nullptr*/,const mtmmap<valuetype, keytype, comparefunction> * mtm_ptr)                 : _ptr(ptr), _map_ptr(mtm_ptr){ } 

why compiler says there here const-correctnes problem. can't cast const mtmmap * mtm_ptr mtmmap * mtm_ptr. copy of ptr.