operator >> return value

[Tool]

I'm a bit confused about what the expression belove in the code returns.

cin >> buf;

And how can it be used in a while loop:

string buf;
while(cin >> buf) //extracts until eof
   ;

I checked cpp reference:

Return Value
The object itself (*this).
When a value is being "extracted" it is not returned, but directly stored in the variable used as parameter.

It doesn't say anywhere that the return value is zero in case of EOF.

Does the "cin" object in our case becomes something like a NULL and then the while terminates?

I'm confused about this pointer aswell...

[laserlight]

Does the "cin" object in our case becomes something like a NULL and then the while terminates?

Yes. The result, which is a reference to std::istream, is converted to void*. The void pointer is a null pointer when the stream is no longer in a good state (if I remember correctly).

[Elysia]

The stream operator will always return a reference to itself (ie *this).
What it doesn't say is that the class has an overloaded operator void* which is invoked in your comparison.
Ie -> operator >> is called, a std:stream is returned, std:stream's void* operator is called -> expression is converted to true/false (this operator is invoked on the object that is returned from the >> operator, but since it's the same object that you used the >> operator on in the first place...).