c++ - Infinite loop, removing duplicates from a stack? -

i infinite loop when printing new stack that's returned removeduplicates function, don't know what's causing it? other functions working perfectly. functions should keeps first occurrence if element duplicated. code:

class stack {        node *head;         public: stack() {            head = null;        };         ~stack() {           while (head) {               node * temp = head;               head = head->next;               delete temp;           }       }        void push(int data);        int pop();        bool isempty();       void print();       stack removeduplicates();  };    void stack::push(int data) {     node *temp = new node(data);     temp->next = head;     head = temp;  }   int stack::pop() {      if (head != null ) {         int x = head->data;         node *temp = head;         head = head->next;         delete temp;         return x;     } else {         cout << "the stack empty!";         return -1;     }  }   bool stack::isempty(){     return head == null;    }    void stack::print() {      node * temp = head;      while(temp != null ) {          cout << temp->data << " ";          temp = temp->next;      }  }   stack stack::removeduplicates(){     stack st;     node *temp = null;     bool flag;      while (head != null) {         if (st.head == null) {             st.push(head->data);             temp = st.head;         } else {             flag = true;             while (temp != null) {                 if (head->data == temp->data)                     flag = false;                 temp = temp->next;             }             if (flag == true)                st.push(head->data);          }          node *del = head;          head = head->next;          delete del;      }      return st;  } 

what head declared outside function?

does code outside function modify head?

if head null going function function won't anything, , depending on print code didn't present, might loop forever reason alone.

in first if statement have if (st.head == null) ... , set temp st.head, e.g. temp = null, when temp ever going non-null use in else branch of if statement?

why don't initialize variables explicitly inside function instead of merely declaring them , associating them types, since, if they're stack variables undefined. helps understand , maintain code if format easier parse visually. you'd surprised how many errors can catch discipline.

  stack stack::removeduplicates() {     stack st;     node *temp = null;     bool flag;      while (head != null) {         if (st.head == null) {             st.push(head->data);             temp = st.head;         } else {             flag = true;             while (temp != null) {                 if (head->data == temp->data)                     flag = false;                 temp = temp->next;             }             if (flag)                st.push(head->data);         }         node *del = head;         head = head->next;         delete del;      }      return st;   }