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; typedef basic_ios<char> ios; typedef basic_ios<wchar_t> wios; typedef basic_streambuf<char> streambuf; typedef basic_istream<char> istream; typedef basic_ostream<char> ostream; typedef basic_iostream<char> iostream; typedef basic_stringbuf<char> stringbuf; typedef basic_istringstream<char> istringstream; typedef basic_ostringstream<char> ostringstream; typedef basic_stringstream<char> stringstream; typedef basic_filebuf<char> filebuf; typedef basic_ifstream<char> ifstream; typedef basic_ofstream<char> ofstream; typedef basic_fstream<char> fstream; typedef basic_streambuf<wchar_t> wstreambuf; typedef basic_istream<wchar_t> wistream; typedef basic_ostream<wchar_t> wostream; typedef basic_iostream<wchar_t> wiostream; typedef basic_stringbuf<wchar_t> wstringbuf; typedef basic_istringstream<wchar_t> wistringstream; typedef basic_ostringstream<wchar_t> wostringstream; typedef basic_stringstream<wchar_t> wstringstream; typedef basic_filebuf<wchar_t> wfilebuf; typedef basic_ifstream<wchar_t> wifstream; typedef basic_ofstream<wchar_t> wofstream; typedef basic_fstream<wchar_t> wfstream; template <class state> class fpos; typedef fpos<char_traits<char>::state_type> streampos; typedef fpos<char_traits<wchar_t>::state_type> wstreampos; }
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.294
[ Note: 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>.
The class template specialization basic_streambuf<charT,traits> serves as a base class for class templates basic_stringbuf and basic_filebuf.
The class template specialization basic_istream<charT,traits> serves as a base class for class templates basic_istringstream and basic_ifstream.
The class template specialization basic_ostream<charT,traits> serves as a base class for class templates basic_ostringstream and basic_ofstream.
The class template specialization basic_iostream<charT,traits> serves as a base class for class templates basic_stringstream and basic_fstream.
Other typedefs define instances of class templates specialized for char or wchar_t types.
Specializations of the class template fpos are used for specifying file position information.
The types streampos and wstreampos are used for positioning streams specialized on char and wchar_t respectively.
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 typedef ... _STATE; // implementation private declaration of stateT typedef fpos<_STATE> streampos; template<> struct char_traits<char> { typedef streampos pos_type; }
— end note ]
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.