std:: cerr, std:: 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 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

#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";
}

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.

See also