[iostream.objects]

# 31 Input/output library [[input.output]](./#input.output)

## 31.4 Standard iostream objects [iostream.objects]

### [31.4.1](#iostream.syn) Header <iostream> synopsis [[iostream.syn]](iostream.syn)

[🔗](#header:%3ciostream%3e)

#include <ios> // see [[ios.syn]](ios.syn "31.5.1 Header <ios> synopsis")#include <streambuf> // see [[streambuf.syn]](streambuf.syn "31.6.1 Header <streambuf> synopsis")#include <istream> // see [[istream.syn]](istream.syn "31.7.1 Header <istream> synopsis")#include <ostream> // see [[ostream.syn]](ostream.syn "31.7.2 Header <ostream> synopsis")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;}

### [31.4.2](#overview) Overview [[iostream.objects.overview]](iostream.objects.overview)

[1](#overview-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L468)

In this Clause, the type name FILE refers to
the type FILE declared in [<cstdio>](cstdio.syn#header:%3ccstdio%3e "31.13.1 Header <cstdio> synopsis [cstdio.syn]")[.](#overview-1.sentence-1)

[2](#overview-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L472)

The header [<iostream>](#header:%3ciostream%3e "31.4.1 Header <iostream> synopsis [iostream.syn]") declares objects that associate objects with the
standard C streams provided for by the functions declared in[<cstdio>](cstdio.syn#header:%3ccstdio%3e "31.13.1 Header <cstdio> synopsis [cstdio.syn]"), and includes all the headers necessary to use these objects[.](#overview-2.sentence-1)

The dynamic types of the stream buffers
initially associated with these objects are unspecified,
but they have the behavior specified forstd​::​basic_filebuf<char> orstd​::​basic_filebuf<wchar_t>[.](#overview-2.sentence-2)

[3](#overview-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L484)

The objects are constructed and the associations are established at some
time prior to or during the first time an object of classios_base​::​Init is constructed, and in any case before the body
of main ([[basic.start.main]](basic.start.main "6.10.3.1 main function")) begins execution[.](#overview-3.sentence-1)

The objects are not destroyed during program execution[.](#overview-3.sentence-2)[258](#footnote-258 "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.")

[4](#overview-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L501)

*Recommended practice*: If it is possible for them to do so, implementations should
initialize the objects earlier than required[.](#overview-4.sentence-1)

[5](#overview-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L506)

The results of including [<iostream>](#header:%3ciostream%3e "31.4.1 Header <iostream> synopsis [iostream.syn]") in a translation unit shall be as if[<iostream>](#header:%3ciostream%3e "31.4.1 Header <iostream> synopsis [iostream.syn]") defined an instance of ios_base​::​Init with static
storage duration[.](#overview-5.sentence-1)

Each C++ library module ([[std.modules]](std.modules "16.4.2.4 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]](basic.start.dynamic "6.10.3.3 Dynamic initialization of non-block variables"))[.](#overview-5.sentence-2)

[*Note [1](#overview-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[.](#overview-5.sentence-3)

Whether such a definition exists is unobservable by a program that
does not reference any of the standard iostream objects[.](#overview-5.sentence-4)

— *end note*]

[6](#overview-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L522)

Mixing operations on corresponding wide- and narrow-character streams
follows the same semantics as mixing such operations onFILEs,
as specified in the C standard library[.](#overview-6.sentence-1)

[7](#overview-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L528)

Concurrent access to a synchronized ([[ios.members.static]](ios.members.static "31.5.2.5 Static members")) standard iostream object's formatted and unformatted input ([[istream]](istream "31.7.5.2 Class template basic_­istream")) and output ([[ostream]](ostream "31.7.6.2 Class template basic_­ostream")) functions or a standard C stream by multiple threads does not result in a data race ([[intro.multithread]](intro.multithread "6.10.2 Multi-threaded executions and data races"))[.](#overview-7.sentence-1)

[*Note [2](#overview-note-2)*:

Unsynchronized concurrent use of these objects and streams by multiple threads
can result in interleaved characters[.](#overview-7.sentence-2)

— *end note*]

See also: ISO/IEC 9899:2024, 7.21.2

[258)](#footnote-258)[258)](#footnoteref-258)

Constructors and destructors for objects with
static storage duration can
access these objects to read input fromstdin or write output tostdout orstderr[.](#footnote-258.sentence-1)

### [31.4.3](#narrow.stream.objects) Narrow stream objects [[narrow.stream.objects]](narrow.stream.objects)

[🔗](#lib:cin)

`istream cin;
`

[1](#narrow.stream.objects-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L545)

The object cin controls input from a stream buffer associated with the object stdin,
declared in [<cstdio>](cstdio.syn#header:%3ccstdio%3e "31.13.1 Header <cstdio> synopsis [cstdio.syn]")[.](#narrow.stream.objects-1.sentence-1)

[2](#narrow.stream.objects-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L550)

After the objectcin is initialized,cin.tie() returns&cout[.](#narrow.stream.objects-2.sentence-1)

Its state is otherwise the same as required for[basic_ios<char>​::​init](istream.cons#lib:basic_ios,init "31.7.5.2.2 Constructors [istream.cons]")[.](#narrow.stream.objects-2.sentence-2)

[🔗](#lib:cout)

`ostream cout;
`

[3](#narrow.stream.objects-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L567)

The object cout controls output to a stream buffer associated with the object stdout,
declared in [<cstdio>](cstdio.syn#header:%3ccstdio%3e "31.13.1 Header <cstdio> synopsis [cstdio.syn]")[.](#narrow.stream.objects-3.sentence-1)

[🔗](#lib:cerr)

`ostream cerr;
`

[4](#narrow.stream.objects-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L579)

The object cerr controls output to a stream buffer associated with the object stderr,
declared in [<cstdio>](cstdio.syn#header:%3ccstdio%3e "31.13.1 Header <cstdio> synopsis [cstdio.syn]")[.](#narrow.stream.objects-4.sentence-1)

[5](#narrow.stream.objects-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L584)

After the objectcerr is initialized,cerr.flags() & unitbuf is nonzero and cerr.tie() returns &cout[.](#narrow.stream.objects-5.sentence-1)

Its state is otherwise the same as required for[basic_ios<char>​::​init](istream.cons#lib:basic_ios,init "31.7.5.2.2 Constructors [istream.cons]")[.](#narrow.stream.objects-5.sentence-2)

[🔗](#lib:clog)

`ostream clog;
`

[6](#narrow.stream.objects-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L600)

The object clog controls output to a stream buffer
associated with the object stderr,
declared in [<cstdio>](cstdio.syn#header:%3ccstdio%3e "31.13.1 Header <cstdio> synopsis [cstdio.syn]")[.](#narrow.stream.objects-6.sentence-1)

### [31.4.4](#wide.stream.objects) Wide stream objects [[wide.stream.objects]](wide.stream.objects)

[🔗](#lib:wcin)

`wistream wcin;
`

[1](#wide.stream.objects-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L615)

The object wcin controls input from a stream buffer associated with the object stdin,
declared in [<cstdio>](cstdio.syn#header:%3ccstdio%3e "31.13.1 Header <cstdio> synopsis [cstdio.syn]")[.](#wide.stream.objects-1.sentence-1)

[2](#wide.stream.objects-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L620)

After the objectwcin is initialized,wcin.tie() returns&wcout[.](#wide.stream.objects-2.sentence-1)

Its state is otherwise the same as required for[basic_ios<wchar_t>​::​init](istream.cons#lib:basic_ios,init "31.7.5.2.2 Constructors [istream.cons]")[.](#wide.stream.objects-2.sentence-2)

[🔗](#lib:wcout)

`wostream wcout;
`

[3](#wide.stream.objects-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L637)

The object wcout controls output to a stream buffer associated with the object stdout,
declared in [<cstdio>](cstdio.syn#header:%3ccstdio%3e "31.13.1 Header <cstdio> synopsis [cstdio.syn]")[.](#wide.stream.objects-3.sentence-1)

[🔗](#lib:wcerr)

`wostream wcerr;
`

[4](#wide.stream.objects-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L649)

The object wcerr controls output to a stream buffer associated with the object stderr,
declared in [<cstdio>](cstdio.syn#header:%3ccstdio%3e "31.13.1 Header <cstdio> synopsis [cstdio.syn]")[.](#wide.stream.objects-4.sentence-1)

[5](#wide.stream.objects-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L654)

After the objectwcerr is initialized,wcerr.flags() & unitbuf is nonzero and wcerr.tie() returns &wcout[.](#wide.stream.objects-5.sentence-1)

Its state is otherwise the same as required for[basic_ios<wchar_t>​::​init](istream.cons#lib:basic_ios,init "31.7.5.2.2 Constructors [istream.cons]")[.](#wide.stream.objects-5.sentence-2)

[🔗](#lib:wclog)

`wostream wclog;
`

[6](#wide.stream.objects-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L670)

The object wclog controls output to a stream buffer associated with the object stderr,
declared in [<cstdio>](cstdio.syn#header:%3ccstdio%3e "31.13.1 Header <cstdio> synopsis [cstdio.syn]")[.](#wide.stream.objects-6.sentence-1)