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Difference between revisions of "cpp/io/cout"

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< cpp‎ | io
m
m (Reverted edits by $HOME (talk) to last revision by Andreas Krug)
 
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{{cpp/io/basic_ostream/navbar}}
 
{{cpp/io/basic_ostream/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl header | iostream }}
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{{dcl header|iostream}}
−
{{dcl | num=1 | 1=
+
{{dcl|num=1|1=
 
extern std::ostream cout;
 
extern std::ostream cout;
 
}}
 
}}
−
{{dcl | num=2| 1=
+
{{dcl|num=2|1=
 
extern std::wostream wcout;
 
extern std::wostream wcout;
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
  
−
The global objects {{lc|std::cout}} and {{lc|std::wcout}} control output to a stream buffer of implementation-defined type (derived from {{lc|std::streambuf}}), associated with the standard C output stream {{lc|stdout}}.
+
The global objects {{tt|std::cout}} and {{tt|std::wcout}} control output to a stream buffer of implementation-defined type (derived from {{lc|std::streambuf}}), associated with the standard C output stream {{lc|stdout}}.
  
−
These objects are guaranteed to be initialized during or before the first time an object of type {{lc|std::ios_base::Init}} is constructed and are available for use in the constructors and destructors of static objects (as long as {{tt|<iostream>}} is included).
+
These objects are guaranteed to be initialized during or before the first time an object of type {{lc|std::ios_base::Init}} is constructed and are available for use in the constructors and destructors of static objects with [[cpp/language/initialization#Non-local_variables|ordered initialization]] (as long as {{header|iostream}} is included before the object is defined).
  
−
Unless {{c|sync_with_stdio(false)}} has been issued, it is safe to concurrently access these objects from multiple threads for both formatted and unformatted output.
+
Unless {{c|std::ios_base::sync_with_stdio(false)}} has been issued, it is safe to concurrently access these objects from multiple threads for both formatted and unformatted output.
  
−
Once initialized, {{lc|std::cout}} is {{c|tie()}}'d to {{lc|std::cin}} and {{lc|std::wcout}} is tie()'d to {{lc|std::wcin}}, meaning that any input operation on {{lc|std::cin}} executes {{c|std::cout.flush()}} (via {{lc|std::basic_istream::sentry}}'s constructor).
+
By specification of {{lc|std::cin}}, {{c|std::cin.tie()}} returns {{c|&std::cout}}. This means that any input operation on {{tt|std::cin}} executes {{c|std::cout.flush()}} (via {{lc|std::basic_istream::sentry}}'s constructor). Similarly, {{c|std::wcin.tie()}} returns {{c|&std::wcout}}.
  
−
Once initialized, {{lc|std::cout}} is also {{cc|tie()}}'d to {{lc|std::cerr}} and {{lc|std::wcout}} is tie()'d to {{lc|std::wcerr}}, meaning that any output operation on {{lc|std::cerr}} executes {{c|std::cout.flush()}} (via {{lc|std::basic_ostream::sentry}}'s constructor) {{mark since c++11}}
+
By specification of {{lc|std::cerr}}, {{c|std::cerr.tie()}} returns {{c|&std::cout}}. This means that any output operation on {{tt|std::cerr}} executes {{c|std::cout.flush()}} (via {{lc|std::basic_ostream::sentry}}'s constructor). Similarly, {{c|std::wcerr.tie()}} returns {{c|&std::wcout}}. {{mark since c++11}}
 +
 
 +
===Notes===
 +
The 'c' in the name refers to "character" ([https://www.stroustrup.com/bs_faq2.html#cout stroustrup.com FAQ]); {{tt|cout}} means "character output" and {{tt|wcout}} means "wide character output".
 +
 
 +
Because [[cpp/language/initialization#Dynamic_initialization|dynamic initialization]] of [[cpp/language/templates#Templated_entity|templated]] variables are unordered, it is not guaranteed that {{tt|std::cout}} has been initialized to a usable state before the initialization of such variables begins, unless an object of type {{lc|std::ios_base::Init}} has been constructed.
  
 
===Example===
 
===Example===
 
{{example
 
{{example
−
| code=
+
|code=
 
#include <iostream>
 
#include <iostream>
−
struct Foo {
+
 
 +
struct Foo
 +
{
 
     int n;
 
     int n;
−
     Foo() {
+
     Foo()
−
      std::cout << "static constructor\n";
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    {
 +
        std::cout << "static constructor\n";
 
     }
 
     }
−
     ~Foo() {
+
     ~Foo()
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      std::cout << "static destructor\n";
+
    {
 +
        std::cout << "static destructor\n";
 
     }
 
     }
 
};
 
};
 +
 
Foo f; // static object
 
Foo f; // static object
 +
 
int main()
 
int main()
 
{
 
{
 
     std::cout << "main function\n";
 
     std::cout << "main function\n";
 
}
 
}
−
| output=
+
|output=
 
static constructor
 
static constructor
 
main function
 
main function
Line 47: Line 58:
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
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{{dsc inc | cpp/io/ios_base/dsc Init}}
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{{dsc inc|cpp/io/ios_base/dsc Init}}
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{{dsc inc | cpp/io/dsc clog}}
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{{dsc inc|cpp/io/dsc cerr}}
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{{dsc inc | cpp/io/dsc cout}}
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{{dsc inc|cpp/io/dsc clog}}
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{{dsc inc|cpp/io/c/dsc std streams}}
 
{{dsc end}}
 
{{dsc end}}
  
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[[de:cpp/io/cout]]
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{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
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[[es:cpp/io/cout]]
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[[fr:cpp/io/cout]]
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[[it:cpp/io/cout]]
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[[ja:cpp/io/cout]]
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[[pt:cpp/io/cout]]
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[[ru:cpp/io/cout]]
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[[zh:cpp/io/cout]]
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Latest revision as of 18:32, 14 September 2023

 
C++
 
Input/output library
 
std::basic_ostream
 
Defined in header <iostream>
extern std::ostream cout;
 (1)
extern std::wostream wcout;
 (2)

The global objects std::cout and std::wcout control output to a stream buffer of implementation-defined type (derived from std::streambuf), associated with the standard C output stream stdout.

These objects are guaranteed to be initialized during or before the first time an object of type std::ios_base::Init is constructed and are available for use in the constructors and destructors of static objects with ordered initialization (as long as <iostream> is included before the object is defined).

Unless std::ios_base::sync_with_stdio(false) has been issued, it is safe to concurrently access these objects from multiple threads for both formatted and unformatted output.

By specification of std::cin, std::cin.tie() returns &std::cout. This means that any input operation on std::cin executes std::cout.flush() (via std::basic_istream::sentry's constructor). Similarly, std::wcin.tie() returns &std::wcout.

By specification of std::cerr, std::cerr.tie() returns &std::cout. This means that any output operation on std::cerr executes std::cout.flush() (via std::basic_ostream::sentry's constructor). Similarly, std::wcerr.tie() returns &std::wcout. (since C++11)

[edit] Notes

The 'c' in the name refers to "character" (stroustrup.com FAQ); cout means "character output" and wcout means "wide character output".

Because dynamic initialization of templated variables are unordered, it is not guaranteed that std::cout has been initialized to a usable state before the initialization of such variables begins, unless an object of type std::ios_base::Init has been constructed.

[edit] Example

Run this code
#include <iostream>
 
struct Foo
{
    int n;
    Foo()
    {
        std::cout << "static constructor\n";
    }
    ~Foo()
    {
        std::cout << "static destructor\n";
    }
};
 
Foo f; // static object
 
int main()
{
    std::cout << "main function\n";
}

Output: 

static constructor
main function
static destructor

[edit] See also

 initializes standard stream objects
(public member class of std::ios_base) [edit]
writes to the standard C error stream stderr, unbuffered
(global object)[edit]
writes to the standard C error stream stderr
(global object)[edit]
expression of type FILE* associated with the input stream
expression of type FILE* associated with the output stream
expression of type FILE* associated with the error output stream
(macro constant) [edit]
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