[boost] Re: Imminent Code Breakage

From: Roland Richter (roland_at_[hidden])
Date: 2003-06-07 09:32:52

>>  + 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*.  

  Very well, here is it: (and of course I read it):

==========

\boost-sandbox\libs\view\test\dereference_equality_test.cpp(15) : error C2676: binary '==' : 
'boost::iterator_facade<Derived,Value,AccessCategory,TraversalCategory,Reference,Difference>::reference' 
does not define this operator or a conversion to a type acceptable to the predefined operator
         with
         [
             Derived=boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,
 
Value=boost::mpl::apply_if<boost::is_same<boost::use_default,boost::use_default>,boost::iterator_value<_STL::vector<int,_STL::allocator<int>>::iterator 
 >,boost::mpl::identity<boost::use_default>>::type,
 
AccessCategory=boost::mpl::apply_if<boost::detail::is_new_iterator_tag<boost::detail::iterator_adaptor_base<boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,_STL::vector<int,_STL::allocator<int>>::iterator 
,boost::use_default,boost::use_default,boost::use_default,boost::use_default>::category>,boost::detail::new_category_to_access<boost::detail::iterator_adaptor_base<boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,_STL::vector<int,_STL::allocator<int>>::iterator 
,boost::use_default,boost::use_default,boost::use_default,boost::use_default>::category,boost::detail::iterator_adaptor_base<boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,_STL::vector<int,_STL::allocator<int>>::iterator 
,boost::use_default,boost::use_default,boost::use_default,boost::use_default>::reference>,boost::detail::iter_category_to_access<boost::detail::iterator_adaptor_base<boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,_STL::vector<int,_STL::allocator<int>>::iterator 
,boost::use_default,boost::use_default,boost::use_default,boost::use_default>::category,boost::detail::iterator_adaptor_base<boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,_STL::vector<int,_STL::allocator<int>>::iterator 
,boost::use_default,boost::use_default,boost::use_default,boost::use_default>::reference>>::type,

 
TraversalCategory=boost::mpl::apply_if<boost::detail::is_new_iterator_tag<boost::detail::iterator_adaptor_base<boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,_STL::vector<int,_STL::allocator<int>>::iterator 
,boost::use_default,boost::use_default,boost::use_default,boost::use_default>::category>,boost::detail::get_traversal_category<boost::detail::iterator_adaptor_base<boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,_STL::vector<int,_STL::allocator<int>>::iterator 
,boost::use_default,boost::use_default,boost::use_default,boost::use_default>::category>,boost::detail::iter_category_to_traversal<boost::detail::iterator_adaptor_base<boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,_STL::vector<int,_STL::allocator<int>>::iterator 
,boost::use_default,boost::use_default,boost::use_default,boost::use_default>::category>>::type,
 
Reference=boost::detail::iterator_adaptor_base<boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,_STL::vector<int,_STL::allocator<int>>::iterator 
,boost::use_default,boost::use_default,boost::use_default,boost::use_default>::reference,
 
Difference=boost::mpl::apply_if<boost::is_same<boost::use_default,boost::use_default>,boost::iterator_difference<_STL::vector<int,_STL::allocator<int>>::iterator 
 >,boost::mpl::identity<boost::use_default>>::type
         ]
\boost-sandbox\libs\view\test\dereference_equality_test.cpp(18) : error C2677: binary '==' : no 
global operator found which takes type 
'boost::iterator_facade<Derived,Value,AccessCategory,TraversalCategory,Reference,Difference>::reference' 
(or there is no acceptable conversion)
         with
         [
             Derived=boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,
 
Value=boost::mpl::apply_if<boost::is_same<boost::use_default,boost::use_default>,boost::iterator_value<_STL::vector<int,_STL::allocator<int>>::iterator 
 >,boost::mpl::identity<boost::use_default>>::type,
 
AccessCategory=boost::mpl::apply_if<boost::detail::is_new_iterator_tag<boost::detail::iterator_adaptor_base<boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,_STL::vector<int,_STL::allocator<int>>::iterator 
,boost::use_default,boost::use_default,boost::use_default,boost::use_default>::category>,boost::detail::new_category_to_access<boost::detail::iterator_adaptor_base<boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,_STL::vector<int,_STL::allocator<int>>::iterator 
,boost::use_default,boost::use_default,boost::use_default,boost::use_default>::category,boost::detail::iterator_adaptor_base<boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,_STL::vector<int,_STL::allocator<int>>::iterator 
,boost::use_default,boost::use_default,boost::use_default,boost::use_default>::reference>,boost::detail::iter_category_to_access<boost::detail::iterator_adaptor_base<boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,_STL::vector<int,_STL::allocator<int>>::iterator 
,boost::use_default,boost::use_default,boost::use_default,boost::use_default>::category,boost::detail::iterator_adaptor_base<boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,_STL::vector<int,_STL::allocator<int>>::iterator 
,boost::use_default,boost::use_default,boost::use_default,boost::use_default>::reference>>::type,
             TraversalCa
tegory=boost::mpl::apply_if<boost::detail::is_new_iterator_tag<boost::detail::iterator_adaptor_base<boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,_STL::vector<int,_STL::allocator<int>>::iterator 
,boost::use_default,boost::use_default,boost::use_default,boost::use_default>::category>,boost::detail::get_traversal_category<boost::detail::iterator_adaptor_base<boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,_STL::vector<int,_STL::allocator<int>>::iterator 
,boost::use_default,boost::use_default,boost::use_default,boost::use_default>::category>,boost::detail::iter_category_to_traversal<boost::detail::iterator_adaptor_base<boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,_STL::vector<int,_STL::allocator<int>>::iterator 
,boost::use_default,boost::use_default,boost::use_default,boost::use_default>::category>>::type,
 
Reference=boost::detail::iterator_adaptor_base<boost::reverse_iterator<_STL::vector<int,_STL::allocator<int>>::iterator>,_STL::vector<int,_STL::allocator<int>>::iterator 
,boost::use_default,boost::use_default,boost::use_default,boost::use_default>::reference,
 
Difference=boost::mpl::apply_if<boost::is_same<boost::use_default,boost::use_default>,boost::iterator_difference<_STL::vector<int,_STL::allocator<int>>::iterator 
 >,boost::mpl::identity<boost::use_default>>::type
         ]

> If you manually specialize boost::detail::iterator_traits it should work.
> 

  The problem is the reference type. Yes, it works if you manually pass
  all (or some) of the types; then again, this would require to include
  thse types into the definition of reverse_iterator etc.

- Roland