# std::cerr, std::wcerr

From cppreference.com
< cpp‎ | io

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
--- | --- | --- | ---
LWG 455 | C++98 | std::cerr.tie() and std::wcerr.tie() returned null pointers | they return & std::cout and & std::wcout respectively

### See also

Init | initializes standard stream objects (public member class of `std::ios_base`)
clogwclog | writes to the standard C error stream stderr (global object)
coutwcout | writes to the standard C output stream stdout (global object)
stdinstdoutstderr | expression of type FILE* associated with the input stream expression of type FILE* associated with the output stream expression of type FILE* associated with the error output stream (macro constant)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/io/cerr&oldid=158808"