[boost] Re: Imminent Code Breakage

From: David Abrahams (dave_at_[hidden])
Date: 2003-06-06 16:34:46

Roland Richter <roland_at_[hidden]> writes:

> David Abrahams wrote:
>> This is going to break every boost library and every user which is
>> depending on the current CVS version of iterator_adaptors.  The new
>> adaptors are so much better designed and easier to use that making the
>> transition should be cake, but before I do anything drastic I want to
>> hear how loud the pre-emptive screaming is ;-)
>> Objections?
>> 
>
>   No objections, Your Honour :-) ... just two remarks.
>
>   I tried to update the view library (uses iterator_adaptors a lot),
>   and generally it turned out to be straight-forward. However:
>
>   +  permutation_iterator is missing - when will it be due?

I guess there's no plan.

If you found it easy to use the new iterator adaptors, I'm sure you
could write it in a jiffy.

Or you could petition Toon to do a new one.

>      (It would be easier to "port" derived iterator types if there was
>       a detailed how-to somewhere.)

Really?  There's almost nothing to it, unless they need to calculate
complicated default types.  The permutation iterator in particular is
*really* easy.

Off-the-cuff:

  template <class ElementIterator, IndexIterator>
  class permutation_iterator
      : public iterator_adaptor< 
            permutation_iterator<ElementIterator, IndexIterator>
          , ElementIterator >
  {
      typedef iterator_adaptor< 
            permutation_iterator<ElementIterator, IndexIterator>
          , ElementIterator> super_t;

      friend class iterator_core_access;

   public:
      permutation_iterator() : order_it_() {}

      explicit permutation_iterator(ElementIterator x, OrderIterator y) 
          : super_t(x), order_it_(y) {}

      template<class OtherElementIterator, OtherIndexIterator>
      permutation_iterator(
          permutation_iterator<OtherIterator> const& r
          , typename enable_if_convertible<OtherElementIterator, ElementIterator>::type* = 0
          , typename enable_if_convertible<OtherIndexIterator, IndexIterator>::type* = 0
          )
          : super_t(r.base())
      {}

   private:
      typename super_t::reference dereference() const
          { return *(this->base() + *this->order_it_); }
    
      void increment() { ++this->order_it_; }
      void decrement() { --this->order_it_; }

      void advance(typename super_t::difference_type n)
      {
          this->order_it_ += -n;
      }

      template<class OtherElementIterator, OtherIndexIterator>
      typename super_t::difference_type
      distance_to(reverse_iterator<OtherElementIterator, OtherIndexIterator> const& y) const
      {
          return y.order_it_ - this->order_it_;
      }

      IndexIterator order_it_;
  };

I'm just guessing.  Why don't you put this through its paces, then
contribute revised documentation and tests for the new library?

>   + Problems with MSVC++ 7.0 (as ever) and dereference (I guess this is
>     a problem with default types). Demonstrated with reverse_iterator:
>
>
>    int numbers[] = { 1,2,3,4,5,6,7 };
>    const int N = sizeof(numbers)/sizeof(int);
>    std::vector<int> v( numbers, numbers + N );
>
>    typedef boost::reverse_iterator< std::vector<int>::iterator >
>            reverse_iterator;
>    reverse_iterator rit( v.begin() );
>
>    if( *rit == v[0] ) // Does not compile
>    { }
>
>    if( v[0] == *rit ) // Does not compile
>    { }

What is the error message you get?  ** Did you read it? **

The problem is most likely that vc7 has no partial specialization and
thus can't deduce the value_type of an int*.  If you manually
specialize boost::detail::iterator_traits it should work.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com