Your code has two error:

1) You cannot define friend function body in the class declaration

2) You must declare friend function as template

3) In push function you must increment _index

The following code compile and run right

Qt Code: 
  1. #include <iosfwd>
  2.  
  3. template <class T = int, size_t N = 100>
  4. class Stack {
  5.         T _data[N];
  6.         size_t _index;
  7.  
  8.     public:
  9.         Stack() : _index(0) { }
  10.         void push(const T& value) {
  11.             if ( _index != N) 
  12.             {
  13.             	_data[_index] = value;
  14.             	++_index;
  15.             }
  16.             else 
  17.             	printf("stack full\n");
  18.         }
  19.  
  20.         template <typename TYPE, size_t SIZE>
  21.         friend std::ostream& operator<<(std::ostream& os, const Stack<TYPE, SIZE>& stack);
  22. };
  23.  
  24. template <typename T, size_t N>
  25. std::ostream& operator<<(std::ostream& os, const Stack<T, N>& stack)
  26. {
  27.     for (size_t i=0; i < stack._index; ++i)
  28.         os << stack._data[i] << "  ";
  29.     return os;
  30. }
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Qt Code: 
  1. int main()
  2. {
  3. 	Stack<int, 10> st;
  4. 	st.push(2);
  5. 	st.push(5);
  6. 	st.push(3);
  7.  
  8. 	std::cout << st << std::endl;
  9.  
  10. 	return 0;
  11. }
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