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# 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 (as long as `<iostream>` is included).

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] 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()

### [edit] See also

Init | initializes standard stream objects (public member class of `std::ios_base`) [edit]
clog wclog | writes to the standard C error stream stderr (global object) [edit]
cout wcout | writes to the standard C output stream stdout (global object) [edit]

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