std::cerr,std::wcerr(3)

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)