STL iterator: "dereferencing" iterator to a temporary. Is it possible?

Question

I'm writing a 3D grid for my scientific software and I need to iterate through the nodes of the grid to get their coordinates. Instead of holding each node object in the container I'd rather like to just calculate the coordinates on the fly while iterating. The problem is that stl::iterator requires to return reference to a value as a result of operator*(), or pointer for operator->().

Some of the code below:

class spGridIterator {
public:
    typedef forward_iterator_tag iterator_category;
    typedef spVector3D value_type;
    typedef int difference_type;
    typedef spVector3D* pointer;
    typedef spVector3D& reference;

    spGridIterator(spGrid* gr, int index);

    spGridIterator& operator++();
    spGridIterator& operator++(int);

    reference operator*() const;
    pointer operator->() const;

private:
    spGrid* m_grid;
    int m_idx;
};

spGridIterator::reference spGridIterator::operator*() const {
    // return m_grid->GetPoint(m_idx);
}

spGridIterator::pointer spGridIterator::operator->() const {
    // return m_grid->GetPoint(m_idx);
}

This method queries the node coordinates by index provided

spVector3D spGrid::GetPoint(int idx) const {
    // spVector3D vec = ... calculate the coordinates here ...
    return vec;
}

Any input on this?

Thanks in advance, Ilya

Answer 1

You could use a member variable to hold the grid point it is currently pointing to:

class spGridIterator {
public:
    typedef forward_iterator_tag iterator_category;
    typedef spVector3D value_type;
    typedef int difference_type;
    typedef spVector3D* pointer;
    typedef const spVector3D* const_pointer;
    typedef const spVector3D& const_reference;
    typedef spVector3D& reference;

    spGridIterator(spGrid* gr, int index);

    spGridIterator& operator++();
    spGridIterator& operator++(int);

    reference operator*();
    const_reference operator*() const;

    pointer operator->();
    const_pointer operator->() const;

private:
    spGrid* m_grid;
    int m_idx;
    mutable spVector3D CurrentPoint;
};

Then the dereference operator could look like this:

spGridIterator::const_reference spGridIterator::operator*() const {
    CurrentPoint = m_grid->GetPoint(m_idx);
    return CurrentPoint;
}

Thanks to @greg for pointing out that CurrentPoint needs to be mutable for this to work. This would be a lazy implementation (only retrieving the point when the iterator is actually dereferenced). An eager implementation would update the CurrentPoint member in the mutator methods of the iterator (the operator++ variants in this example), making the mutable superfluous.