stl - C++ iterator over certain elements in vector -

please forgive me if trivial question, i'm learning c++ , try wrap head around concepts. when comes iterators, i'm lost.

say have custom class representing data structure, , 1 of members vector of integers. want write bidirectional iterator class, outputs numbers in vector. there easy , instructive way? i'd prefer not using libraries other stl.

 #include <vector> #include <iostream>  class x {  public:    class eveniterator {   public:     eveniterator(std::vector<int>::iterator it, std::vector<int>::iterator end) : it(it), end(end) {       while (true) {         if (iseven(*it)) {           break;         } else if (it == end) {           break;         }         it++;       }     }      bool operator != (const eveniterator& evenit) {       return evenit.it != this->it;     }      int operator * () {       return *it;     }      eveniterator operator ++ () {       while (true) {         it++;         if (iseven(*it)) {           return eveniterator(it, end);         } else if (it == end) {           return eveniterator(it, end);         }       }     }   private:     std::vector<int>::iterator it;         std::vector<int>::iterator end;       };    static bool iseven(int number) {     return number % 2 == 0;   }    void add(int number) {     v.push_back(number);   }    eveniterator evenbegin() {     return eveniterator(v.begin(), v.end());   }    eveniterator evenend() {     return eveniterator(v.end(), v.end());   }  private:   std::vector<int> v;  };  int main() {   x x;   x.add(1);   x.add(2);   x.add(3);   x.add(2);   x.add(2);   x.add(31);   x.add(56);   x.add(101);    (x::eveniterator = x.evenbegin(); != x.evenend(); ++it){     std::cout << *it << std::endl; // prints numbers   } }