head file including problem

Noticing that when include ,ifstream can be introducd like
ifstream source(fileName);

After checking the file in nodepad,i can find no class ifstream.
Then includes other head files,I check them one by one recursively
but find nothing.

From MSDN,A type basic_ifstream specialized on char template parameters.
typedef basic_ifstream > ifstream;
and the required head file is

Then i was confused.

Thanks for your reply.