std::cerr, std::wcerr

 

C++

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basic_streambuf

basic_filebuf

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ios_base

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basic_spanstream (C++23)

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ostrstream (deprecated in C++98)

strstream (deprecated in C++98)

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basic_osyncstream (C++20)

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streamoff

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Error category interface

iostream_category (C++11)

io_errc (C++11)

 

std::basic_ostream

Global objects
coutwcout	cerrwcerr	clogwclog
Member functions
basic_ostream::basic_ostream
basic_ostream::~basic_ostream
basic_ostream::operator= (C++11)
Formatted output
basic_ostream::operator<<
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basic_ostream::put
basic_ostream::write
Positioning
basic_ostream::tellp
basic_ostream::seekp
Miscellaneous
basic_ostream::flush
basic_ostream::swap (C++11)
Member classes
basic_ostream::sentry
Non-member functions
operator<<(std::basic_ostream)

 

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

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()

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)