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std::cout, std::wcout

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Revision as of 21:50, 23 March 2022 by 181.105.133.55 (Talk)

 
 
 
 
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 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)

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.

Example

#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

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]