std::cerr, std::wcerr

< cpp‎ | io
Defined in header <iostream>
extern std::ostream cerr;
extern std::wostream wcerr;

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)

[edit] Notes

The 'c' in the name refers to "character" ( FAQ); cerr means "character error (stream)" and wcerr means "wide character error (stream)"

[edit] Example

output to stderr via cerr flushes out the pending output on cout, while output to stderr via clog does not

#include <thread>
#include <iostream>
#include <chrono>
void f()
    std::cout << "Output from thread...";
    std::cout << "...thread calls flush()" << std::endl;
int main()
    std::thread t1(f);
    std::clog << "This output from main is not tie()'d to cout\n";
    std::cerr << "This output is tie()'d to cout\n";


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

[edit] See also

initializes standard stream objects
(public member class of std::ios_base) [edit]
writes to the standard C error stream stderr
(global object) [edit]
writes to the standard C output stream stdout
(global object) [edit]