error C2146: syntax error : missing ';' before identifier 'value_type'

#pragma once
class iterator
{
public:
  typedef std::forward_iterator_tag iterator_category;
  typedef T                          value_type;
  typedef ptrdiff_t                  difference_type;
  typedef T*                         pointer;
  typedef T&                         reference;

  iterator();

  // Get the data element at this position
  reference operator*() const;
  pointer operator->() const;

  // Move position forward 1 place
  iterator& operator++();
  iterator operator++(int);

  // Comparison operators
  bool operator== (const iterator&) const;
  bool operator!= (const iterator&) const;
private:
  
};

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error C2146: syntax error : missing ';' before identifier 'value_type'