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 std::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 std::wcerr and
std::wcout), meaning that any output operation on std::cerr first executes
std::cout.flush() (via std::basic_ostream::sentry's constructor).

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 <chrono>
#include <iostream>
#include <thread>
using namespace std::chrono_literals;

void f()
{
std::cout << "Output from thread...";
std::this_thread::sleep_for(2s);
std::cout << "...thread calls flush()" << std::endl;
}

int main()
{
std::jthread t1{f};
std::this_thread::sleep_for(1000ms);
std::clog << "This output from main is not tie()'d to cout\n";
std::cerr << "This output is tie()'d to cout\n";
}

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()

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
std::cerr.tie() and they return &std::cout and
LWG 455 C++98 std::wcerr.tie() returned null &std::wcout respectively
pointers

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)