On 25/05/2011 11:58, Arno Schödl wrote:
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) \ ); \ }
An alternative could also have been to use boost::addressof