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]

1
#
In this Clause, the type name FILE refers to the type FILE declared in <cstdio> ([cstdio.syn]).
2
#
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
#
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 begins execution.284
The objects are not destroyed during program execution.285
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.
4
#
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.
5
#
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
:
Users must still synchronize concurrent use of these objects and streams by multiple threads if they wish to avoid interleaved characters.
— end note
 ]
See also: ISO C 7.21.2
284)
If it is possible for them to do so, implementations should initialize the objects earlier than required.
⮥
285)
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]

🔗
istream cin;
1
#
The object cin controls input from a stream buffer associated with the object stdin, declared in <cstdio> ([cstdio.syn]).
2
#
After the object cin is initialized, cin.tie() returns &cout.
Its state is otherwise the same as required for basic_­ios<char>​::​init.
🔗
ostream cout;
3
#
The object cout controls output to a stream buffer associated with the object stdout, declared in <cstdio> ([cstdio.syn]).
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ostream cerr;
4
#
The object cerr controls output to a stream buffer associated with the object stderr, declared in <cstdio> ([cstdio.syn]).
5
#
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.
🔗
ostream clog;
6
#
The object clog controls output to a stream buffer associated with the object stderr, declared in <cstdio> ([cstdio.syn]).

29.4.4 Wide stream objects [wide.stream.objects]

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wistream wcin;
1
#
The object wcin controls input from a stream buffer associated with the object stdin, declared in <cstdio> ([cstdio.syn]).
2
#
After the object wcin is initialized, wcin.tie() returns &wcout.
Its state is otherwise the same as required for basic_­ios<wchar_­t>​::​init.
🔗
wostream wcout;
3
#
The object wcout controls output to a stream buffer associated with the object stdout, declared in <cstdio> ([cstdio.syn]).
🔗
wostream wcerr;
4
#
The object wcerr controls output to a stream buffer associated with the object stderr, declared in <cstdio> ([cstdio.syn]).
5
#
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.
🔗
wostream wclog;
6
#
The object wclog controls output to a stream buffer associated with the object stderr, declared in <cstdio> ([cstdio.syn]).