std::clog,std::wclog (3) Linux Manual Page

std::clog,std::wclog – std::clog,std::wclog

Synopsis

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.

Notes

The ‘c’ in the name refers to "character" (stroustrup.com_FAQ); clog means "character log" and wclog means "wide character log"

Example

// Run this code

#include <iostream>
struct Foo {
    int n;
    Foo()
    {
        std::clog << "static constructor\n";
    }
    ~Foo()
    {
        std::clog << "static destructor\n";
    }
};

  Foo f; // static object

int main()

{
    std::clog << "main function\n";
}

Output:

  static constructor
  main function
  static destructor

See also

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