on Thu Mar 22 2007, Scott Meyers <usenet-AT-aristeia.com> wrote:
I expect the following code to compile cleanly. It does not.
#include <boost/mpl/set.hpp> #include <boost/mpl/equal.hpp> #include <boost/mpl/erase_key.hpp>
namespace mpl = boost::mpl; using mpl::_1; using mpl::_2;
struct A {}; struct B {};
typedef mpl::set<A, B> set1; typedef mpl::set<B, A> set2;
BOOST_MPL_ASSERT(( mpl::equal< mpl::erase_key<set1, A>::type, mpl::erase_key<set2, A>::type
));
I'm simply trying to assert that the sets {A,B} and {B,A} have the same elements after A has been removed from both. What am I doing wrong?
Nothing; this is clearly a bug in MPL, better demonstrated by: #include <boost/mpl/set.hpp> #include <boost/mpl/equal.hpp> #include <boost/mpl/erase_key.hpp> namespace mpl = boost::mpl; struct A {}; struct B {}; typedef mpl::set<A, B> ab_set; typedef mpl::erase_key<ab_set, A>::type b_set; BOOST_MPL_ASSERT(( boost::is_same< mpl::deref< mpl::begin<b_set>::type >::type , B >)); which yields: fu.cpp:17: error: No match for ‘assertion_failed( mpl_::failed ************ boost::is_same< boost::mpl::set2<A, B> , B>::************)’ In other words, erasing A from set<A,B> results in a sequence whose first element is set2<A,B>. I'll have to defer to Aleksey at this point, since he is the implementor. Aleksey? -- Dave Abrahams Boost Consulting www.boost-consulting.com