In what state should my overridden >> operator leave my object when the input stream contains invalid data?

Question

Say I have overridden the >> operator for a class like below:

friend istream& operator>>(istream& in, MyObject& obj)
{
    ...
    if (in.fail())
        // What do I do here?  Should obj be returned unmodified?
    ...
}

and say that the input stream is in a fail state because the data there means that I cannot get a valid MyObject out of it. How should I leave the obj object?

Answer 1

Ideally, you just need to manipulate the state bits when your operator>> can't read what it is supposed to be read. In other words, the user should check if the stream is in a valid state before using your object.

Answer 2

I would recommend leaving it unmodified and throwing an exception, since the client code violates an essential assumption. To leave the object unmodified even if the stream fails half-way, read the contents into a temporary MyObject, and swap them when complete.

Answer 3

The way that most code that performs stream I/O is written is:

MyObject obj;

if ( ! ( cin >> obj ) ) {
   // handle error
}

In other words, it is up to the user of operator >> to check that the operation worked, and the way to do that is to test the state of the stream, not the object. I'd also observe that except for low-level "value" objects like strings and integers, stream input via operator >> is not normally very useful because it requires that the object being read is default constructible, and most higher level objects are not.