std::cerr,std::wcerr (3) - Linux Manuals

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

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 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 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 cerr flushes out the pending output on cout, while output to stderr via 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();
  }

Output:

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