return value of getline ?

In books and websites you frequently see examples where the getline function from <string> is used for example like this:

Code:

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while (getline(istream, str))
    cout << str << endl;

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I was wondering what value getline actually returns so that the loop stops? All references I can find say that it returns the stream it was reading, but how can a stream suddenly become 0 or false or anything like that?

It returns a reference to the istream object.

Quote:

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but how can a stream suddenly become 0 or false or anything like that

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In C++, you can program an object to behave anyway you want. The ostream class is programmed so that when on ostream object is in a boolean context, it evaluates to false if any error flags have been set for the object, like eof. Otherwise, it evalutes to true.

Ahh, that's interesting, thanks.

>> it evaluates to false if any error flags have been set for the object, like eof.

Technically, it evaluates to false if failbit or badbit is set. It does not return false if eof is set by itself. In many circumstances the eofbit and failbit are set at the same time when one final attempt to read fails because the end of the file is reached.

However, in some cases, especially with getline above, the last call to getline will retrieve some data and consume the newline at the end of the file. This will cause the eofbit to be set, but because data was consumed and getline succeeded the failbit won't be set yet. This is what you'd want though, because you want to process that last piece of data. The next time through the read will fail completely because there is nothing left in the stream, and so both failbit and eofbit will be set causing the loop to end.

The details are confusing, but in practice just using the return value of the stream as the example above does is usually the best way to go.