c++ getline()的返回值

string line
ifstream  inf("file.txt");
while(getline(inf, string))
{
   cout << string << endl;
}

一直不明白  getline的返回值是什么， 为什么可以用来 判断
因为  它的原型是

istream& getline (char* s, streamsize n );
istream& getline (char* s, streamsize n, char delim );

是一个引用，引用不会为空，它应该永远为真阿， 这是怎么实现的？问题先记到这里，
等有机会看源码，看它是怎么实现这个函数的
下面是网上摘过来的
istream&   getline   (istream&   is,   string&   str,   char   delim   =   '\n ');

Reads   characters   from   an   input   stream   into   a   string,  stopping   when   one   of   the   following   things   happens:
An   end-of-file   condition   occurs   on   the   input   stream   When   the   maximum  
 number   of   characters   that   can   fit   into   a   string   have   been   read
When   a   character   read   in   from   the   string   is   equal   to   the   specified  
 delimiter   (newline   is   the   default delimiter);   the   delimiter   character   is  
removed   from   the   input   stream,   but   not   appended   to   the   string.  
The   return   value   is   a   reference   to   the   input   stream.   If  
the   stream   is   tested   as   a   logical   value   (as   in   an  
if   or   while),   it   is   equivalent   to   true   if   the   read  
was   successful   and   false   otherwise   (e.g.,   end   of   file).