31 Input/output library [input.output]

31.4 Standard iostream objects [iostream.objects]

31.4.2 Overview [iostream.objects.overview]

1
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In this Clause, the type name FILE refers to the type FILE declared in <cstdio>.
2
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The header <iostream> declares objects that associate objects with the standard C streams provided for by the functions declared in <cstdio>, and includes all the headers necessary to use these objects.
3
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The objects are constructed and the associations are established at some time prior to or during the first time an object of class ios_base​::​Init is constructed, and in any case before the body of main ([basic.start.main]) begins execution.
The objects are not destroyed during program execution.264
4
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Recommended practice: If it is possible for them to do so, implementations should initialize the objects earlier than required.
5
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The results of including <iostream> in a translation unit shall be as if <iostream> defined an instance of ios_base​::​Init with static storage duration.
Each C++ library module ([std.modules]) in a hosted implementation shall behave as if it contains an interface unit that defines an unexported ios_base​::​Init variable with ordered initialization ([basic.start.dynamic]).
[Note 1: 
As a result, the definition of that variable is appearance-ordered before any declaration following the point of importation of a C++ library module.
Whether such a definition exists is unobservable by a program that does not reference any of the standard iostream objects.
— end note]
6
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Mixing operations on corresponding wide- and narrow-character streams follows the same semantics as mixing such operations on FILEs, as specified in the C standard library.
7
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Concurrent access to a synchronized ([ios.members.static]) standard iostream object's formatted and unformatted input ([istream]) and output ([ostream]) functions or a standard C stream by multiple threads does not result in a data race ([intro.multithread]).
[Note 2: 
Unsynchronized concurrent use of these objects and streams by multiple threads can result in interleaved characters.
— end note]
See also: ISO/IEC 9899:2018, 7.21.2
264)264)
Constructors and destructors for objects with static storage duration can access these objects to read input from stdin or write output to stdout or stderr.