29 Input/output library [input.output]

29.3 Forward declarations [iostream.forward]

29.3.1 Header <iosfwd> synopsis [iosfwd.syn]

namespace std { template<class charT> struct char_traits; template<> struct char_traits<char>; template<> struct char_traits<char8_t>; template<> struct char_traits<char16_t>; template<> struct char_traits<char32_t>; template<> struct 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_spanbuf; template<class charT, class traits = char_traits<charT>> class basic_ispanstream; template<class charT, class traits = char_traits<charT>> class basic_ospanstream; template<class charT, class traits = char_traits<charT>> class basic_spanstream; 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 Allocator = allocator<charT>> class basic_syncbuf; template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>> class basic_osyncstream; 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 spanbuf = basic_spanbuf<char>; using ispanstream = basic_ispanstream<char>; using ospanstream = basic_ospanstream<char>; using spanstream = basic_spanstream<char>; using filebuf = basic_filebuf<char>; using ifstream = basic_ifstream<char>; using ofstream = basic_ofstream<char>; using fstream = basic_fstream<char>; using syncbuf = basic_syncbuf<char>; using osyncstream = basic_osyncstream<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 wspanbuf = basic_spanbuf<wchar_t>; using wispanstream = basic_ispanstream<wchar_t>; using wospanstream = basic_ospanstream<wchar_t>; using wspanstream = basic_spanstream<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>; using wsyncbuf = basic_syncbuf<wchar_t>; using wosyncstream = basic_osyncstream<wchar_t>; template<class state> class fpos; using streampos = fpos<char_traits<char>::state_type>; using wstreampos = fpos<char_traits<wchar_t>::state_type>; using u8streampos = fpos<char_traits<char8_t>::state_type>; using u16streampos = fpos<char_traits<char16_t>::state_type>; using u32streampos = fpos<char_traits<char32_t>::state_type>; }
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.274
274)274)
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.

29.3.2 Overview [iostream.forward.overview]

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, basic_­filebuf, and basic_­syncbuf.
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, basic_­ofstream, and basic_­osyncstream.
The class template specialization basic_­iostream<charT, traits> serves as a base class for class templates basic_­stringstream and basic_­fstream.
[Note 1:
For each of the class templates above, the program is ill-formed if traits​::​char_­type is not the same type as charT ([char.traits]).
— end note]
Other typedef-names 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.
[Example 1:
The types streampos and wstreampos are used for positioning streams specialized on char and wchar_­t respectively.
— end example]
[Note 2:
This synopsis suggests a circularity between streampos and char_­traits<char>.
An implementation can avoid this circularity by substituting equivalent types.
— end note]