Question

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What's the ampersand for when used after class name like ostream& operator <<(…)?

I am new to C++ and I have probably a very noobish question. I have seen a thing like this:

Vector3f & operator = (Vector3f & obj)
{
    _item[0] = obj[0];
    _item[1] = obj[1];
    _item[2] = obj[2];

    return *this;
}

And I was wondering why is there an ampersand (&) after Vector3f. What kind of magic is it doing? I couldn’t find any explanations anywhere. Most importantly, what is the difference between the thing above and

Vector3f operator = (Vector3f obj)
{
    _item[0] = obj[0];
    _item[1] = obj[1];
    _item[2] = obj[2];

    return *this;
}

Answer 1

It’s C++’s syntax for pass-by-reference & return-by-reference. It just means that the parameter is an alias of the object from the calling context, and not a copy, and, simillary, the returned object is actually *this, and not a copy.