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).
The 'c' in the name refers to "character" (stroustrup.com FAQ); cerr means "character error (stream)" and wcerr means "wide character error (stream)".
Output to stderr via std::cerr flushes out the pending output on std::cout, while output to stderr via std::clog does not.
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()[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior LWG 455 C++98 std::cerr.tie() andstd::ios_base) [edit] writes to the standard C error stream stderr
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