on Tue Sep 09 2008, "Kris Rousey" <krousey-AT-gmail.com> wrote:
I just tried to make a zip iterator out of two vector back_insert_iterators and got a very curious compile failure:
/usr/include/boost/iterator/iterator_facade.hpp:653: error: invalid parameter type 'void' /usr/include/boost/iterator/iterator_facade.hpp:653: error: in declaration 'typename boost::detail::operator_brackets_result<Derived, Value, Reference>::type boost::iterator_facade<I, V, TC, R, D>::operator[](Difference) const' /usr/include/boost/iterator/iterator_facade.hpp:693: error: invalid parameter type 'void' /usr/include/boost/iterator/iterator_facade.hpp:693: error: in declaration 'Derived& boost::iterator_facade<I, V, TC, R, D>::operator+=(Difference)' /usr/include/boost/iterator/iterator_facade.hpp:699: error: invalid parameter type 'void' /usr/include/boost/iterator/iterator_facade.hpp:699: error: in declaration 'Derived& boost::iterator_facade<I, V, TC, R, D>::operator-=(Difference)' /usr/include/boost/iterator/iterator_facade.hpp:705: error: invalid parameter type 'void' /usr/include/boost/iterator/iterator_facade.hpp:705: error: in declaration 'Derived boost::iterator_facade<I, V, TC, R, D>::operator-(Difference) const'
It seems that even though back_insert_iterator has an output_iterator tag, iterator_facade still trying to define random access operators.
That's not exactly true; the operator[] is only supposed to be instantiated if used, but we never considered that someone would try to use Difference=void.
If assignment and incrementing are the only valid operations on an iterator (as is the case with output iterators), then doesn't it make sense to support a difference_type of void?
I guess. It couldn't hurt, though, to leave the Difference parameter as std::ptrdiff_t. If you really want std::iterator_traits<I>::difference_type to be void, you'd normally just typedef void difference_type; inside your derived iterator body. However, you're using zip_iterator.
Is this a curiosity of my stdlib implementation? A Boost.Iterator library oversight?
Probably the latter.
A Boost.Iterator conscious design decision? Or a loose nut behind the keyboard trying something he shouldn't?
Heh, maybe both. We didn't test the library too well on output iterators. Try this patch to boost/iterator/iterator_facade.hpp; does it fix the problem for you? -- Dave Abrahams BoostPro Computing http://www.boostpro.com