extern usage problem

[C++ Programming]

when i review showing somewhat like this,
namespace std {
extern istream cin;
extern ostream cout;
extern ostream cerr;
extern ostream clog;

extern wistream wcin;
extern wostream wcout;
extern wostream wcerr;
extern wostream wclog;
};

Such globals like cin,cout,cerr are considered to be defined in other files accoring to "extern" keyword.But I have not got where they are defined from MSDN.http://www.barakasoft.com/script/Forums/Images/smiley_confused.gifSo i wonder if they are defined in just ,although with extern keyword included.

Thanks very much for your reply.Appreciating.http://www.barakasoft.com/script/Forums/Images/rose.gif