c++ - Can I convert a reverse iterator to a forward iterator?

Question

I have a class called Action, which is essentially a wrapper around a deque of Move objects.

Because I need to traverse the deque of Moves both forward and backwards, I have a forward iterator and a reverse_iterator as member variables of the class. The reason for this is becuase I need to know when I have gone one past the "end" of the deque, both when I am going forwards or backwards.

The class looks like this:

class Action
{
public:
    SetMoves(std::deque<Move> & dmoves) { _moves = dmoves; }
    void Advance();
    bool Finished() 
    {
        if( bForward )
            return (currentfwd==_moves.end());
        else
            return (currentbck==_moves.rend());
    }
private:
    std::deque<Move> _moves;
    std::deque<Move>::const_iterator currentfwd;
    std::deque<Move>::const_reverse_iterator currentbck;
    bool bForward;
};

The Advance function is as follows:

void Action::Advance
{
    if( bForward)
        currentfwd++;
    else
        currentbck++;
}

My problem is, I want to be able to retrieve an iterator to the current Move object, without needing to query whether I am going forwards or backwards. This means one function returning one type of iterator, but I have two types.

Should I forget returning an iterator, and return a const reference to a Move object instead?

Answer 1

Reverse iterators have a member base() which returns a corresponding forward iterator. Beware that this isn't an iterator that refers to the same object - it actually refers to the next object in the sequence. This is so that rbegin() corresponds with end() and rend() corresponds with begin().

So if you want to return an iterator, then you would do something like

std::deque<Move>::const_iterator Current() const
{
    if (forward)
        return currentfwd;
    else
        return (currentbck+1).base();
}

I would prefer to return a reference, though, and encapsulate all the iteration details inside the class.