Difference between revisions of "cpp/io/cout"

Revision as of 14:08, 14 September 2023

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)

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() :
        n(0)
    {
        std::cout << "static constructor\n";
        std::cout << '\n';
    }
    ~Foo()
    {
        std::cout << "static destructor\n";
        std::cout << '\n';
    }
};
 
Foo f; // static object
 
int main()
{
    std::cout << "main function\n";
    std::cout << '\n';
}

static constructor
main function
static destructor

See also