How to use the istream and ostream?

Dear ladies and gentlemen,

    I created a header file followed by a book -- C++ Primer 3rd Edition.
The header file is as the following:

//////////////////////////////////////////////////////////////////

#ifndef STRING_CLASS_H
#define STRING_CLASS_H

#include <iostream>

istream& operator>>( istream&, String& );
ostream& operator<<( ostream&, const String& );

class String;

class String {
public:
  String();
  String( const char* );
  String( const String& );

  ~String();

  String& operator=( const String& );
  String& operator=( const char* );

  bool operator==( const String& );
  bool operator==( const char* );

  char& operator[]( int );

  int size() { return _size; }
  char* c_str() { return _string; }

private:
  int _size;
  char *_string;
};

#endif /* STRING_CLASS_H */

/////////////////////////////////////////////////////

    When I include this header file in my project and compile the project
with VC++ 7.0 which was in Visual Studio .NET 2003, the two lines would
cause error:

    istream& operator>>( istream&, String& );
    ostream& operator<<( ostream&, const String& );

    The compiler told me that lack ";" (before the "&"). What's the problem?
How can I handle this error? Help me, please. Thank you very much.

    Best regards.

    Xiaoming