C++ provides four standard I/O streams that are automatically instantiated before program’s outset. They are defined in the standard header
cin // standard input stream of charcout // standard output stream of char cerr // standard unbuffered output stream for error messagesclog // standard output stream for error messages
The difference between cerr and clog is that cerr is unbuffered, whereas clog is buffered. In addition, each of these four streams has a corresponding wide character version:
wcin wcout wcerr wclog