std::clog,std::wclog (3) - Linux Man Pages
Defined in header <iostream>
extern std::ostream clog; (1)
extern std::wostream wclog; (2)
The global objects std::clog and std::wclog control output to a stream buffer of implementation-defined type (derived from std::streambuf), associated with the standard C output stream stderr, but, unlike std::cerr/std::wcerr, these streams are not automatically flushed and not automatically tie()'d with cout.
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 sync_with_stdio(false) has been issued, it is safe to concurrently access these objects from multiple threads for both formatted and unformatted output.
The 'c' in the name refers to "character" (stroustrup.com_FAQ); clog means "character log" and wclog means "wide character log"
// Run this code
Init (public member class of std::ios_base)
cerr (global object)
cout (global object)