It is used to input-Output forward declarations,this header provides forward declarations for the types of the standard input/output library.
Following is the defines for iosfwd function.
#define _GLIBCXX_IOSFWD
In below example explains about iosfwd function.
amespace std { template<class charT> struct char_traits; template<> class char_traits<char>; 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 Alloc=allocator<charT> > class basic_stringbuf; template <class charT, class traits=char_traits<charT>, class Alloc=allocator<charT> > class basic_istringstream; template <class charT, class traits=char_traits<charT>, class Alloc=allocator<charT> > class basic_ostringstream; template <class charT, class traits=char_traits<charT>, class Alloc=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; template<class stateT> class fpos; typedef basic_ios<char> ios; 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_ios<wchar_t> wios; 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; typedef fpos<mbstate_t> streampos; typedef fpos<mbstate_t> wstreampos; }
In below example explains about iosfwd function.
namespace std { template<class charT> struct 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 Alloc=allocator<charT> > class basic_stringbuf; template <class charT, class traits=char_traits<charT>, class Alloc=allocator<charT> > class basic_istringstream; template <class charT, class traits=char_traits<charT>, class Alloc=allocator<charT> > class basic_ostringstream; template <class charT, class traits=char_traits<charT>, class Alloc=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; template<class stateT> class fpos; typedef basic_ios<char> ios; 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_ios<wchar_t> wios; 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; typedef fpos<mbstate_t> streampos; typedef fpos<mbstate_t> wstreampos; }