The predefined object clog is an instance of ostream class. The clog object is said to be attached to the standard error device, which is also a display screen but the object clog is buffered. This means that each insertion to clog could cause its output to be held in a buffer until the buffer is filled or until the buffer is flushed.
The clog is also used in conjunction with the stream insertion operator as shown in the following example.
int main( )
char str = “Unable to read….”;
clog << “Error message : ” << str << endl;
When the above code is compiled and executed, it produces the following result:
Error message : Unable to read….
You would not be able to see any difference in cout, cerr and clog with these small examples, but while writing and executing big programs then difference becomes obvious. So this is good practice to display error messages using cerr stream and while displaying other log messages then clog should be used.