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>;
}
1
#
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
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.

30.3.2 Overview [iostream.forward.overview]

1
#
The class template specialization basic_­ios<charT, traits> serves as a virtual base class for the class templates basic_­istream, basic_­ostream, and class templates derived from them.
basic_­iostream is a class template derived from both basic_­istream<charT, traits> and basic_­ostream<charT, traits>.
2
#
The class template specialization basic_­streambuf<charT, traits> serves as a base class for class templates basic_­stringbuf and basic_­filebuf.
3
#
The class template specialization basic_­istream<charT, traits> serves as a base class for class templates basic_­istringstream and basic_­ifstream.
4
#
The class template specialization basic_­ostream<charT, traits> serves as a base class for class templates basic_­ostringstream and basic_­ofstream.
5
#
The class template specialization basic_­iostream<charT, traits> serves as a base class for class templates basic_­stringstream and basic_­fstream.
6
#
Other typedef-names define instances of class templates specialized for char or wchar_­t types.
7
#
Specializations of the class template fpos are used for specifying file position information.
8
#
The types streampos and wstreampos are used for positioning streams specialized on char and wchar_­t respectively.
9
#
[ Note
:
This synopsis suggests a circularity between streampos and char_­traits<char>.
An implementation can avoid this circularity by substituting equivalent types.
One way to do this might be 
template<class stateT> class fpos { ... };      // depends on nothing
using _STATE = ... ;             // implementation private declaration of stateT

using streampos = fpos<_STATE>;

template<> struct char_traits<char> {
  using pos_type = streampos;
}
— end note
 ]