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.