operator >> return value

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

Code:

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cin >> buf;

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And how can it be used in a while loop:

Code:

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

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I checked cpp reference:

Code:

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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.

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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...

Quote:

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Originally Posted by Tool

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

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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).

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::ostream is returned, std::ostream'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...).