30 Input/output library [input.output]

30.4 Standard iostream objects [iostream.objects]

30.4.3 Narrow stream objects [narrow.stream.objects]

istream cin;
The object cin controls input from a stream buffer associated with the object stdin, declared in <cstdio>.
After the object cin is initialized, cin.tie() returns &cout.
Its state is otherwise the same as required for basic_­ios<char>​::​init.
ostream cout;
The object cout controls output to a stream buffer associated with the object stdout, declared in <cstdio>.
ostream cerr;
The object cerr controls output to a stream buffer associated with the object stderr, declared in <cstdio>.
After the object cerr is initialized, cerr.flags() & unitbuf is nonzero and cerr.tie() returns &cout.
Its state is otherwise the same as required for basic_­ios<char>​::​init.
ostream clog;
The object clog controls output to a stream buffer associated with the object stderr, declared in <cstdio>.