template 
     
      class Iterator{public:    // Interface    virtual void First() = 0;    virtual void Next() = 0;    virtual bool IsDone() const = 0;    virtual T CurrentItem() const = 0;};template 
      
       class DequeIterator : public Iterator
       
        {    // data.    const deque
        
          &m_deque;    unsigned m_curPos;public:    DequeIterator(const deque
         
           &d) : m_deque(d), m_curPos(0) {}public:    // Interface    virtual void First() override    { m_curPos = 0; }    virtual void Next() override    { ++m_curPos; }    virtual bool IsDone() const override    { return m_curPos == m_deque.size(); }    virtual T CurrentItem() const override    { return m_deque[m_curPos]; }};template 
          
           class DequeReverseIterator : public Iterator
           
            { // data. const deque
            
              &m_deque; unsigned m_curPos;public: DequeReverseIterator(const deque
             
               &d) : m_deque(d), m_curPos(d.size()-1) {}public: // Interface virtual void First() override { m_curPos = m_deque.size()-1; } virtual void Next() override { --m_curPos; } virtual bool IsDone() const override { return m_curPos == -1; } // Magic and ugly. virtual T CurrentItem() const override { return m_deque[m_curPos]; }};int main(){ deque
              
                d; for (int i = 0; i < 10; ++i) { d.push_back(i); } // Forward iterator. for (DequeIterator
               
                 i(d); !i.IsDone(); i.Next()) { cout <
                
                 <
                 

i(d); !i.IsDone(); i.Next()) { cout <

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