RE: [Boost-Users] Problems with iterator_adaptors

From: Ben Hutchings (ben.hutchings_at_[hidden])
Date: 2003-04-11 10:08:31

Olaf Petzold wrote:
> I try to write a cyclic Iterator using boost::iterator_adaptors.
> Unfortunally, the advance function doesn't work for negative distances
> correct:

> template<class Iterator>
> class cycle_iterator_policies
> {
> public:
>   cycle_iterator_policies(const Iterator& begin, const Iterator&  end)
>     : m_begin( begin ), m_end( end )
>     {
>       assert( m_begin != m_end );
>     }
>  ....
>   template<class IteratorAdaptor, class DifferenceType>
>   void advance( IteratorAdaptor& x, DifferenceType n ) {
>     n %= std::distance( m_begin, m_end );
<snip>

The problem is that the result of % and / (and by implication %= and
/=) is implementation defined if either operand is negative.  Most
implementations round the quotient towards zero and produce a
remainder with the same sign as the quotient.  Evidently you want
to have a non-negative remainder always.

The simplest solution, I think, is:

      DifferenceType length = std::distance( m_begin, m_end );
      n %= length;
      if (n < 0)
          n += length;