Easiest way to make a cyclic iterator (circulator)?

Question

I have an object that I want to travel in a continuous loop in a game. I have a series of coordinates in a std::vector that I want to use as waypoints.

Answer 1

Ok, now your problem is clearer :-)

Take a look at boost::iterator_facade and boost::iterator adaptor. They implement the full iterator interface and your cycle_iterator only as to implement a few methods like increment(), decrement():

template
class cycle_iterator 
     : public boost::iterator_adaptor< 
          cycle_iterator,     // the derived class overriding iterator behavior
          IteratorBase,       // the base class providing default behavior
          boost::use_default, // iterator value type, will be IteratorBase::value_type
          std::forward_iterator_tag, // iterator category
          boost::use_default  // iterator reference type
       > 
{
  private:
     IteratorBase m_itBegin;
     IteratorBase m_itEnd;

  public:
     cycle_iterator( IteratorBase itBegin, IteratorBase itEnd ) 
       : iterator_adaptor_(itBegin), m_itBegin(itBegin), m_itEnd(itEnd)
     {}

     void increment() {
        /* Increment the base reference pointer. */
        ++base_reference();

        /* Check if past-the-end element is reached and bring back the base reference to the beginning. */
        if(base_reference() == m_itEnd)
            base_reference() = m_itBegin;
     }

     // implement decrement() and advance() if necessary
  };

This probably doesn't compile but should get you started.

Edit:

boost::iterator_adaptor implements the full iterator interface in terms of few functions. It provides default implementations for increment(), decrement(), advance(), distance_to(), equal_to() and dereference() using the base iterator passed down to the iterator_adaptor base class.

If all you need is a forward iterator, only the increment() method must be implemented to wrap around once you reach the end iterator. A cyclical iterator can be bidirectional if you implement decrement() in a similar fashion. If IteratorBase is itself a random access iterator, the cycle iterator can also be random access and method advance and distance_to must be implemented using modulo operations.