std::cerr, std::wcerr

 

C++

Language

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

 

Input/output library

I/O manipulators

C-style I/O

Buffers

basic_streambuf

basic_filebuf

basic_stringbuf

strstreambuf (deprecated)

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Abstractions

ios_base

basic_ios

basic_istream

basic_ostream

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basic_ifstream

basic_ofstream

basic_fstream

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basic_istringstream

basic_ostringstream

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istrstream (deprecated)

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strstream (deprecated)

Types

streamoff

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Error category interface

iostream_category (C++11)

io_errc (C++11)

 

std::basic_ostream

Global objects
cout wcout	cerr wcerr	clog wclog
Member functions
basic_ostream::basic_ostream
basic_ostream::~basic_ostream
basic_ostream::operator= (C++11)
Formatted input
basic_ostream::operator<<
Unformatted input
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 constructed before the first constructor of a static object is called and they are guaranteed to outlive the last destructor of a static object, so that it is always possible to write to std::cerr in user code.

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)

Example

output to stderr via cerr flushes out the pending output on cout, while output to stderr via clog does not

#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()

See also

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