An input range iteration question

[ref2401 via Digitalmars-d-learn]

If Iterator is a struct then Iterator.input won't be adjusted 
properly when the foreach loop ends.
Iterator.input still holds "abcde" value.
If i mark Iterator as class then Iterator.input will be an empty 
string after the foreach loop.
Could anyone explain me the difference please?
Thank you.

struct Iterator {
        string input;

        this(string input) {
                this.input = input;
        }


        bool empty() { return input.empty; }

        dchar front() { return input.front; }

        void popFront() {
                if (empty) {
                        return;
                }

                input.popFront();
                writeln("(iterator.popFront)", input);
        }
}

void main(string[] args) {
        Iterator iterator = Iterator("abcde");

        foreach (dchar c; iterator) {
                writefln("%s", c);
        }

        writefln("(the end): %s", iterator.input);
}