30 Input/output library [input.output]

30.3 Forward declarations [iostream.forward]

30.3.1 Header <iosfwd> synopsis [iosfwd.syn]

namespace std {
  template<class charT> class char_traits;
  template<> class char_traits<char>;
  template<> class char_traits<char16_t>;
  template<> class char_traits<char32_t>;
  template<> class char_traits<wchar_t>;

  template<class T> class allocator;

  template <class charT, class traits = char_traits<charT>>
    class basic_ios;
  template <class charT, class traits = char_traits<charT>>
    class basic_streambuf;
  template <class charT, class traits = char_traits<charT>>
    class basic_istream;
  template <class charT, class traits = char_traits<charT>>
    class basic_ostream;
  template <class charT, class traits = char_traits<charT>>
    class basic_iostream;

  template <class charT, class traits = char_traits<charT>,
            class Allocator = allocator<charT>>
    class basic_stringbuf;
  template <class charT, class traits = char_traits<charT>,
            class Allocator = allocator<charT>>
    class basic_istringstream;
  template <class charT, class traits = char_traits<charT>,
            class Allocator = allocator<charT>>
    class basic_ostringstream;
  template <class charT, class traits = char_traits<charT>,
            class Allocator = allocator<charT>>
    class basic_stringstream;

  template <class charT, class traits = char_traits<charT>>
    class basic_filebuf;
  template <class charT, class traits = char_traits<charT>>
    class basic_ifstream;
  template <class charT, class traits = char_traits<charT>>
    class basic_ofstream;
  template <class charT, class traits = char_traits<charT>>
    class basic_fstream;

  template <class charT, class traits = char_traits<charT>>
    class istreambuf_iterator;
  template <class charT, class traits = char_traits<charT>>
    class ostreambuf_iterator;

  using ios  = basic_ios<char>;
  using wios = basic_ios<wchar_t>;

  using streambuf = basic_streambuf<char>;
  using istream   = basic_istream<char>;
  using ostream   = basic_ostream<char>;
  using iostream  = basic_iostream<char>;

  using stringbuf     = basic_stringbuf<char>;
  using istringstream = basic_istringstream<char>;
  using ostringstream = basic_ostringstream<char>;
  using stringstream  = basic_stringstream<char>;

  using filebuf  = basic_filebuf<char>;
  using ifstream = basic_ifstream<char>;
  using ofstream = basic_ofstream<char>;
  using fstream  = basic_fstream<char>;

  using wstreambuf = basic_streambuf<wchar_t>;
  using wistream   = basic_istream<wchar_t>;
  using wostream   = basic_ostream<wchar_t>;
  using wiostream  = basic_iostream<wchar_t>;

  using wstringbuf     = basic_stringbuf<wchar_t>;
  using wistringstream = basic_istringstream<wchar_t>;
  using wostringstream = basic_ostringstream<wchar_t>;
  using wstringstream  = basic_stringstream<wchar_t>;

  using wfilebuf  = basic_filebuf<wchar_t>;
  using wifstream = basic_ifstream<wchar_t>;
  using wofstream = basic_ofstream<wchar_t>;
  using wfstream  = basic_fstream<wchar_t>;

  template <class state> class fpos;
  using streampos  = fpos<char_traits<char>::state_type>;
  using wstreampos = fpos<char_traits<wchar_t>::state_type>;
}
Default template arguments are described as appearing both in <iosfwd> and in the synopsis of other headers but it is well-formed to include both <iosfwd> and one or more of the other headers.287
It is the implementation's responsibility to implement headers so that including <iosfwd> and other headers does not violate the rules about multiple occurrences of default arguments.