31 Input/output library [input.output]

31.4 Standard iostream objects [iostream.objects]

31.4.2 Overview [iostream.objects.overview]

In this Clause, the type name FILE refers to the type FILE declared in <cstdio>.
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.
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.263
Recommended practice: If it is possible for them to do so, implementations should initialize the objects earlier than required.
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]
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.
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
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.