NAME

std::cerr,std::wcerr - std::cerr,std::wcerr

Synopsis

Defined in header <iostream>
extern std::ostream cerr; (1)
extern std::wostream wcerr; (2)

The global objects std::cerr and std::wcerr control output to a stream buffer of
implementation-defined type (derived from std::streambuf and std::wstreambuf,
respectively), associated with the standard C error output stream stderr.

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.

Once initialized, (std::cerr.flags() & unitbuf) != 0 (same for wcerr) meaning that
any output sent to these stream objects is immediately flushed to the OS (via
std::basic_ostream::sentry's destructor).

In addition, std::cerr.tie() returns &std::cout (same for wcerr and std::wcout),
meaning that any output operation on std::cerr first executes std::cout.flush() (via
std::basic_ostream::sentry's constructor) (since C++11).

Notes

The 'c' in the name refers to "character" (stroustrup.com FAQ); cerr means
"character error (stream)" and wcerr means "wide character error (stream)".

Example

output to stderr via std::cerr flushes out the pending output on std::cout, while
output to stderr via std::clog does not.

// Run this code

#include <thread>
#include <iostream>
#include <chrono>
void f()
{
std::cout << "Output from thread...";
std::this_thread::sleep_for(std::chrono::seconds(2));
std::cout << "...thread calls flush()" << std::endl;
}

int main()
{
std::thread t1(f);
std::this_thread::sleep_for(std::chrono::seconds(1));
std::clog << "This output from main is not tie()'d to cout\n";
std::cerr << "This output is tie()'d to cout\n";
t1.join();
}

Possible output:

This output from main is not tie()'d to cout
Output from thread...This output is tie()'d to cout
...thread calls flush()

See also

Init initializes standard stream objects
(public member class of std::ios_base)
clog writes to the standard C error stream stderr
wclog (global object)
cout writes to the standard C output stream stdout
wcout (global object)
stdin expression of type FILE* associated with the input stream
stdout expression of type FILE* associated with the output stream
stderr expression of type FILE* associated with the error output stream
(macro constant)