11. Templates and friend

• 
  #include <iostream>
  
  using namespace std;
  
  // Forward declare class template
  template < typename ValueT > class Container;
  
  // Forward declare function template
  template < typename ValueT > ostream& operator<<( ostream& os, Container< ValueT > const& v );
  
  template < typename ValueT >
  class Container {
  public:
      // This friend declaration is a template of ValueT
      // function because of the extra <> after the name:
      friend ostream& operator<< <>( ostream& os, Container< ValueT > const& v );
  private:
      ValueT m_data;
  };
  
  int main()
  {
      Container< int > vect_i;
      cout << vect_i;
      return 0;
  }
  
  template < typename ValueT > ostream& operator<<( ostream& os, Container< ValueT > const& v )
  {
      os << v.m_data;
      return os;
  }
  
  

• If we left out the < > part, we would get a

      Warning: friend declaration declares a non-template function
      if this is not what you intended, make sure the function
      template has already been declared and add 
        <>
      after the function name here.
      Linker error: unresolved external symbol...