<iosfwd>
声明向前对多个模板类使用在 (mfc) 中。 所有此类模板类在其他标准头中定义。 您将此头显式，只在需要其声明之一时，但是，不是其定义。
#include <iosfwd>
Typedef
typedef T1 streamoff;
typedef T2 streamsize;
typedef fpos streampos;
// wchar_t TYPE DEFINITIONS
typedef basic_ios<char, char_traits<char> > ios;
typedef basic_streambuf<char, char_traits<char> > streambuf;
typedef basic_istream<char, char_traits<char> > istream;
typedef basic_ostream<char, char_traits<char> > ostream;
typedef basic_iostream<char, char_traits<char> > iostream;
typedef basic_stringbuf<char, char_traits<char> > stringbuf;
typedef basic_istringstream<char, char_traits<char> > istringstream;
typedef basic_ostringstream<char, char_traits<char> > ostringstream;
typedef basic_stringstream<char, char_traits<char> > stringstream;
typedef basic_filebuf<char, char_traits<char> > filebuf;
typedef basic_ifstream<char, char_traits<char> > ifstream;
typedef basic_ofstream<char, char_traits<char> > ofstream;
typedef basic_fstream<char, char_traits<char> > fstream;
// wchar_t TYPE DEFINITIONS
typedef basic_ios<wchar_t, char_traits<wchar_t> > wios;
typedef basic_streambuf<wchar_t, char_traits<wchar_t> > wstreambuf;
typedef basic_istream<wchar_t, char_traits<wchar_t> > wistream;
typedef basic_ostream<wchar_t, char_traits<wchar_t> > wostream;
typedef basic_iostream<wchar_t, char_traits<wchar_t> > wiostream;
typedef basic_stringbuf<wchar_t, char_traits<wchar_t> > wstringbuf;
typedef basic_istringstream<wchar_t, char_traits<wchar_t> > wistringstream;
typedef basic_ostringstream<wchar_t, char_traits<wchar_t> > wostringstream;
typedef basic_stringstream<wchar_t, char_traits<wchar_t> > wstringstream;
typedef basic_filebuf<wchar_t, char_traits<wchar_t> > wfilebuf;
typedef basic_ifstream<wchar_t, char_traits<wchar_t> > wifstream;
typedef basic_ofstream<wchar_t, char_traits<wchar_t> > wofstream;
typedef basic_fstream<wchar_t, char_traits<wchar_t> > wfstream;
};
前向声明/模板类
template<class _Statetype>
class fpos;
template<class Elem>;
class char_traits;
class char_traits<char>;
class char_traits<wchar_t>;
template<class _Ty>
class allocator;
class ios_base;
template<class Elem, class Tr = char_traits<Elem> >
class basic_ios;
template<class Elem, class Tr = char_traits<Elem> >
class istreambuf_iterator;
template<class Elem, class Tr = char_traits<Elem> >
class ostreambuf_iterator;
template<class Elem, class Tr = char_traits<Elem> >
class basic_streambuf;
template<class Elem, class Tr = char_traits<Elem> >
class basic_istream;
template<class Elem, class Tr = char_traits<Elem> >
class basic_ostream;
template<class Elem, class Tr = char_traits<Elem> >
class basic_iostream;
template<class Elem, class Tr = char_traits<Elem> >
class basic_stringbuf;
template<class Elem, class Tr = char_traits<Elem> >
class basic_istringstream;
template<class Elem, class Tr = char_traits<Elem> >
class basic_ostringstream;
template<class Elem, class Tr = char_traits<Elem> >
class basic_stringstream;
template<class Elem, class Tr = char_traits<Elem> >
class basic_filebuf;
template<class Elem, class Tr = char_traits<Elem> >
class basic_ifstream;
template<class Elem, class Tr = char_traits<Elem> >
class basic_ofstream;
template<class Elem, class Tr = char_traits<Elem> >
class basic_fstream;