Re: [Boost-users] [mpl] erase_key confusion

on Thu Mar 22 2007, Scott Meyers wrote:

I expect the following code to compile cleanly. It does not.

#include #include #include

namespace mpl = boost::mpl; using mpl::_1; using mpl::_2;

struct A {}; struct B {};

typedef mpl::set set1; typedef mpl::set set2;

BOOST_MPL_ASSERT(( mpl::equal< mpl::erase_key::type, mpl::erase_key::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 #include #include namespace mpl = boost::mpl; struct A {}; struct B {}; typedef mpl::set ab_set; typedef mpl::erase_key::type b_set; BOOST_MPL_ASSERT(( boost::is_same< mpl::deref< mpl::begin::type >::type , B >)); which yields: fu.cpp:17: error: No match for ‘assertion_failed( mpl_::failed ************ boost::is_same< boost::mpl::set2 , B>::************)’ In other words, erasing A from set results in a sequence whose first element is set2. I'll have to defer to Aleksey at this point, since he is the implementor. Aleksey? -- Dave Abrahams Boost Consulting www.boost-consulting.com