std::inserter with set - insert to begin() or end()?

Question

I have some code that looks like this:

std::set<int> s1, s2, out;

// ... s1 and s2 are populated ...

std::set_intersection(s1.begin(), s1.end(),
                      s2.begin(), s2.end(),
                      std::inserter(out, out.end()));

I've read inserts can be done in amortized constant time if the value being inserted to the set immediately follows the iterator given as a "hint". This would obviously be beneficial when running the set intersection, especially since everything being written to out is already in sorted order.

How do I guarantee this optimal performance? When creating the std::inserter, out is empty so out.begin() == out.end() so I can't see it makes any difference whether I specify out.begin() or out.end() as the hint. However, if this is interpreted at inserting every element at begin(), it doesn't seem that I would get the optimum algorithmic performance. Can this be done better?

Answer 1

You could use a custom functor instead of std::inserter and re-call out.end() every time a new element is inserted.

Alternatively, if your values are sorted descendingly, out.begin() will be fine.

Answer 2

I've chosen Alexander Gessler's answer as the 'correct' answer, because it led me to this solution, which I thought I would post anyway. I've written a last_inserter(), which guarantees that the insert position is always an iterator to the last element (or begin() if empty), because set wants an iterator to the element preceding the actual insert position for best performance (so not end() - that would be one after the actual insert position).

The usage as per the original example is like this:

std::set<int> s1, s2, out;

// ... s1 and s2 are populated ...

std::set_intersection(s1.begin(), s1.end(),
                      s2.begin(), s2.end(),
                      last_inserter(out));  // note no iterator provided

This guarantees that the insert hint is always an iterator to the last element, hopefully providing best-case performance when using an output iterator to a set with a sorted range, as above.

Below is my implementation. I think it's platform specific to Visual C++ 2010's STL implementation, because it's based heavily on the existing insert_iterator, and I can only get it working by deriving from std::_Outit. If anyone knows how to make this portable, let me know:

// VC10 STL wants this to be a checked output iterator.  I haven't written one, but
// this needs to be defined to silence warnings about this.
#define _SCL_SECURE_NO_WARNINGS

template<class Container>
class last_inserter_iterator : public std::_Outit {
public:
    typedef last_inserter_iterator<Container> _Myt;
    typedef Container container_type;
    typedef typename Container::const_reference const_reference;
    typedef typename Container::value_type _Valty;

    last_inserter_iterator(Container& cont)
        : container(cont)
    {
    }

    _Myt& operator=(const _Valty& _Val)
    {
        container.insert(get_insert_hint(), _Val);
        return (*this);
    }

    _Myt& operator=(_Valty&& _Val)
    {
        container.insert(get_insert_hint(), std::forward<_Valty>(_Val));
        return (*this);
    }

    _Myt& operator*()
    {
        return (*this);
    }

    _Myt& operator++()
    {
        return (*this);
    }

    _Myt& operator++(int)
    {
        return (*this);
    }

protected:
    Container& container;

    typename Container::iterator get_insert_hint() const
    {
        // Container is empty: no last element to insert ahead of; just insert at begin.
        if (container.empty())
            return container.begin();
        else
        {
            // Otherwise return iterator to last element in the container.  std::set wants the
            // element *preceding* the insert position as a hint, so this should be an iterator
            // to the last actual element, not end().
            return (--container.end());
        }
    }
};

template<typename Container>
inline last_inserter_iterator<Container> last_inserter(Container& cont)
{
    return last_inserter_iterator<Container>(cont);
}