30 Input/output library [input.output]

30.3 Forward declarations [iostream.forward]

30.3.2 Overview [iostream.forward.overview]

1
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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>.
2
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The class template specialization basic_­streambuf<charT, traits> serves as a base class for class templates basic_­stringbuf and basic_­filebuf.
3
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The class template specialization basic_­istream<charT, traits> serves as a base class for class templates basic_­istringstream and basic_­ifstream.
4
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The class template specialization basic_­ostream<charT, traits> serves as a base class for class templates basic_­ostringstream and basic_­ofstream.
5
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The class template specialization basic_­iostream<charT, traits> serves as a base class for class templates basic_­stringstream and basic_­fstream.
6
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Other typedef-names define instances of class templates specialized for char or wchar_­t types.
7
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Specializations of the class template fpos are used for specifying file position information.
8
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The types streampos and wstreampos are used for positioning streams specialized on char and wchar_­t respectively.
9
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[ Note
:
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
using _STATE = ... ;             // implementation private declaration of stateT

using streampos = fpos<_STATE>;

template<> struct char_traits<char> {
  using pos_type = streampos;
}
— end note
 ]