31 Input/output library [input.output]

31.3 Forward declarations [iostream.forward]

31.3.2 Overview [iostream.forward.overview]

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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, basic_filebuf, and basic_syncbuf.
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.
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The class template specialization basic_ostream<charT, traits> serves as a base class for class templates basic_ostringstream, basic_ofstream, and basic_osyncstream.
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.
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[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]
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Other typedef-names define instances of class templates specialized for char or wchar_t types.
8
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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]
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[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]