std::clog, std::wclog

From cppreference.com

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 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::clog 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.

[edit] Example

#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

[edit] See also


Init	initializes standard stream objects (public member class of `std::ios_base`)

cerr wcerr	writes to the standard C error stream stderr, unbuffered (global object)

cout wcout	writes to the standard C output stream stdout (global object)

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)