29 Input/output library [input.output]

29.4 Standard iostream objects [iostream.objects]

29.4.1 Header <iostream> synopsis [iostream.syn]

#include <ios> // see [ios.syn] #include <streambuf> // see [streambuf.syn] #include <istream> // see [istream.syn] #include <ostream> // see [ostream.syn] namespace std { extern istream cin; extern ostream cout; extern ostream cerr; extern ostream clog; extern wistream wcin; extern wostream wcout; extern wostream wcerr; extern wostream wclog; }

29.4.2 Overview [iostream.objects.overview]

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In this Clause, the type name FILE refers to the type FILE declared in <cstdio>.
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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.
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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.288
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Recommended practice: If it is possible for them to do so, implementations should initialize the objects earlier than required.
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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.
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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.
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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 1:
Unsynchronized concurrent use of these objects and streams by multiple threads can result in interleaved characters.
— end note]
See also: ISO C 7.21.2
288)
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.
 ⮥

29.4.3 Narrow stream objects [narrow.stream.objects]

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istream cin;
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The object cin controls input from a stream buffer associated with the object stdin, declared in <cstdio>.
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After the object cin is initialized, cin.tie() returns &cout.
Its state is otherwise the same as required for basic_­ios<char>​::​init.
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ostream cout;
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The object cout controls output to a stream buffer associated with the object stdout, declared in <cstdio>.
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ostream cerr;
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The object cerr controls output to a stream buffer associated with the object stderr, declared in <cstdio>.
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After the object cerr is initialized, cerr.flags() & unitbuf is nonzero and cerr.tie() returns &cout.
Its state is otherwise the same as required for basic_­ios<char>​::​init.
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ostream clog;
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The object clog controls output to a stream buffer associated with the object stderr, declared in <cstdio>.

29.4.4 Wide stream objects [wide.stream.objects]

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wistream wcin;
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The object wcin controls input from a stream buffer associated with the object stdin, declared in <cstdio>.
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After the object wcin is initialized, wcin.tie() returns &wcout.
Its state is otherwise the same as required for basic_­ios<wchar_­t>​::​init.
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wostream wcout;
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The object wcout controls output to a stream buffer associated with the object stdout, declared in <cstdio>.
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wostream wcerr;
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The object wcerr controls output to a stream buffer associated with the object stderr, declared in <cstdio>.
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After the object wcerr is initialized, wcerr.flags() & unitbuf is nonzero and wcerr.tie() returns &wcout.
Its state is otherwise the same as required for basic_­ios<wchar_­t>​::​init.
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wostream wclog;
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The object wclog controls output to a stream buffer associated with the object stderr, declared in <cstdio>.