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 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)