[boost] iterator_facade triggers ADL, fails to compile

Subject: [boost] iterator_facade triggers ADL, fails to compile
From: Arno Schödl (aschoedl_at_[hidden])
Date: 2011-05-25 05:58:53

Hello,

the attached file arg_names.cpp fails to compile (VS2008) when using boost::lambda::_1 or boost::phoenix::arg_names::_1. The compilation succeeds when using boost::bind. All three variations are in the attached code for illustration.

The compilation fails due to the presence of a templated operator& definition which is introduced by the instantiation of a class that is declared "andable" (from boost/operators.hpp).

The compilation fails because iterator_facade triggers ADL in BOOST_ITERATOR_FACADE_INTEROP and finds the aforementioned operator& when comparing iterators that carry boost::lambda::_1 or boost::phoenix::arg_names::_1 as part of their type specification. In the example, this is illustrated by using filter_iterator.

It is not clear to me what exactly the mechanism is that causes this ADL problem. But the static_cast in BOOST_ITERATOR_FACADE_INTEROP can be implemented using references instead of pointers, as shown below, which makes the problem disappear. In any case, using operator& on the iterator may be problematic if the iterator overloads operator&.
 
--- C:/Program Files (x86)/Microsoft Visual Studio 9.0/VC/boost_1_46_1/boost/iterator/iterator_facade.hpp	Wed May 25 11:42:14 2011
+++ C:/Program Files (x86)/Microsoft Visual Studio 9.0/VC/boost_1_46_1/boost/iterator/iterator_facade - Copy.hpp	Wed May 25 11:41:49 2011
@@ -819,8 +819,8 @@
           is_interoperable< Derived1, Derived2 >::value                         \
       ));                                                                       \
       return_prefix iterator_core_access::base_op(                              \
-          *static_cast<Derived1 const*>(&lhs)                                   \
-        , *static_cast<Derived2 const*>(&rhs)                                   \
+          static_cast<Derived1 const&>(lhs)                                     \
+        , static_cast<Derived2 const&>(rhs)                                     \
         , BOOST_ITERATOR_CONVERTIBLE(Derived2,Derived1)                         \
       );                                                                        \
   }

Does anyone understand better what is happening? Or can we simply adopt the change?

Regards,

Arno

--
Dr. Arno Schödl | aschoedl_at_[hidden]
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