std::cerr, std::wcerr

 

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std::basic_ostream

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coutwcout	cerrwcerr	clogwclog
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operator<<(std::basic_ostream)
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vprint_nonunicode(std::ostream) (C++23)

 

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)