std::cerr, std::wcerr
| 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.
#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
| initializes standard stream objects (public member class of std::ios_base) | |
| writes to the standard C error stream stderr (global object) | |
| writes to the standard C output stream stdout (global object) | |
| 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) |