id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
10847	Offset overloads of next() / prior() have stopped compiling for iterators that previously worked	Tony Lewis <tonyelewis@…>	No-Maintainer	"=== Summary ===

In Boost v1.57.0, the offset overloads of `boost::next()` / `boost::prior()` have stopped compiling for iterators that previously worked in <= v1.56.0.

I think this is because commit 651a869d4f9479dd3dfdd0293305a60b8c8d0e1c (Boost Trac won't let me paste link without passing some non-existent captcha) has replaced a call to `std::advance()` with a custom approach using type_traits (such as `has_plus_assign`). 

Unfortunately some iterators indicate their non-provision of random-access through their iterator category (eg `forward_iterator_tag`) but provide `operator+=` and `operator+` methods/functions that don't compile.

Examples:
 * iterators returned by boost::ptr_set and 
 * iterators returned by GCC's std::list<> when `_GLIBCXX_DEBUG` is defined

Importantly, these iterators work without problem under:
 * std::advance(),
 * Boost v1.56.0's `boost::next()` / `boost::prior()` and
 * C++11's `std::next()` / `std::prev()` (at least under GCC v4.9.1's implementation)

...so Boost v1.57.0 `next()` / `prior()` are out of step with the standard (or `std::`) approach.

=== Example ===

Attempting to compile:

{{{
#include <boost/next_prior.hpp>
#include <boost/ptr_container/ptr_set.hpp>

#include <iostream>
#include <list>

int main() {
	boost::ptr_set<int> the_ptr_set;
	the_ptr_set.insert( new int( 1 ) );
	the_ptr_set.insert( new int( 2 ) );
	std::cerr << *boost::next( the_ptr_set.begin(), 1 ) << std::endl;
	
	std::list<int> the_list;
	the_list.push_back( 1 );
	the_list.push_back( 2 );
	std::cerr << *boost::next( the_list.begin(), 1 ) << std::endl;

	return 0;
}
}}}

...with `g++ -D_GLIBCXX_DEBUG -o next_prior_prob next_prior_prob.cpp` (GCC v4.9.1) produces the following compiler errors:

{{{
In file included from /usr/include/c++/4.9/debug/set.h:33:0,
                 from /usr/include/c++/4.9/debug/set:33,
                 from /usr/include/c++/4.9/set:66,
                 from /usr/include/boost/ptr_container/ptr_set.hpp:21,
                 from next_prior_prob.cpp:2:
/usr/include/c++/4.9/debug/safe_iterator.h: In instantiation of ‘__gnu_debug::_Safe_iterator<_Iterator, _Sequence>& __gnu_debug::_Safe_iterator<_Iterator, _Sequence>::operator+=(const difference_type&) [with _Iterator = std::__cxx1998::_List_iterator<int>; _Sequence = std::__debug::list<int>; __gnu_debug::_Safe_iterator<_Iterator, _Sequence>::difference_type = int]’:
/usr/include/boost/next_prior.hpp:73:11:   required from ‘static T boost::next_prior_detail::next_impl1<T, Distance, true>::call(T, Distance) [with T = __gnu_debug::_Safe_iterator<std::__cxx1998::_List_iterator<int>, std::__debug::list<int> >; Distance = int]’
/usr/include/boost/next_prior.hpp:151:67:   required from ‘T boost::next(T, Distance) [with T = __gnu_debug::_Safe_iterator<std::__cxx1998::_List_iterator<int>, std::__debug::list<int> >; Distance = int]’
next_prior_prob.cpp:16:49:   required from here
/usr/include/c++/4.9/debug/safe_iterator.h:357:13: error: no match for ‘operator+=’ (operand types are ‘std::__cxx1998::_List_iterator<int>’ and ‘const difference_type {aka const int}’)
  _M_current += __n;
             ^
/usr/include/c++/4.9/debug/safe_iterator.h: In instantiation of ‘__gnu_debug::_Safe_iterator<_Iterator, _Sequence>& __gnu_debug::_Safe_iterator<_Iterator, _Sequence>::operator+=(const difference_type&) [with _Iterator = std::_Rb_tree_const_iterator<void*>; _Sequence = std::__debug::set<void*, boost::void_ptr_indirect_fun<std::less<int>, int, int>, std::allocator<void*> >; __gnu_debug::_Safe_iterator<_Iterator, _Sequence>::difference_type = int]’:
/usr/include/boost/ptr_container/detail/void_ptr_iterator.hpp:104:23:   required from ‘boost::void_ptr_iterator<VoidIter, T>& boost::void_ptr_iterator<VoidIter, T>::operator+=(boost::void_ptr_iterator<VoidIter, T>::difference_type) [with VoidIter = __gnu_debug::_Safe_iterator<std::_Rb_tree_const_iterator<void*>, std::__debug::set<void*, boost::void_ptr_indirect_fun<std::less<int>, int, int>, std::allocator<void*> > >; T = int; boost::void_ptr_iterator<VoidIter, T>::difference_type = int]’
/usr/include/boost/next_prior.hpp:73:11:   required from ‘static T boost::next_prior_detail::next_impl1<T, Distance, true>::call(T, Distance) [with T = boost::void_ptr_iterator<__gnu_debug::_Safe_iterator<std::_Rb_tree_const_iterator<void*>, std::__debug::set<void*, boost::void_ptr_indirect_fun<std::less<int>, int, int>, std::allocator<void*> > >, int>; Distance = int]’
/usr/include/boost/next_prior.hpp:151:67:   required from ‘T boost::next(T, Distance) [with T = boost::void_ptr_iterator<__gnu_debug::_Safe_iterator<std::_Rb_tree_const_iterator<void*>, std::__debug::set<void*, boost::void_ptr_indirect_fun<std::less<int>, int, int>, std::allocator<void*> > >, int>; Distance = int]’
next_prior_prob.cpp:11:52:   required from here
/usr/include/c++/4.9/debug/safe_iterator.h:357:13: error: no match for ‘operator+=’ (operand types are ‘std::_Rb_tree_const_iterator<void*>’ and ‘const difference_type {aka const int}’)
}}}

(but has no problems if `boost::next()` is replaced with `std::next()` and `-std=c++11` is used)"	Bugs	closed	Boost 1.58.0	utility	Boost 1.57.0	Regression	fixed	next,prior,advance,iterator,category,type_traits,has_plus_assign,forward_iterator	raad@… Andrey.Semashev@…