On Tue, Jun 12, 2012 at 9:11 AM, <dariomt@gmail.com> wrote:

Hi list,

I'm trying to use the version of mpl::at with three template parameters, as specified here (http://www.boost.org/doc/libs/1_46_0/libs/mpl/doc/refmanual/at.html)

But I'm getting a compiler error saying 'boost::mpl::at' : too many template arguments.

What am I doing wrong?


#include <boost/mpl/map.hpp>
#include <boost/mpl/at.hpp>
#include <boost/mpl/size.hpp>
#include <boost/mpl/empty.hpp>
#include <boost/type_traits/is_same.hpp>

using namespace boost;
using namespace boost::mpl;

typedef map<
      pair<int,unsigned>
    , pair<char,unsigned char>
    , pair<long_<5>,char[17]>
    , pair<int[42],bool>
    > m;

BOOST_MPL_ASSERT_RELATION( size<m>::value, ==, 4 );
BOOST_MPL_ASSERT_NOT(( empty<m> ));

BOOST_MPL_ASSERT(( is_same< at<m,int>::type, unsigned > ));
BOOST_MPL_ASSERT(( is_same< at<m,long_<5> >::type, char[17] > ));
BOOST_MPL_ASSERT(( is_same< at<m,int[42]>::type, bool > ));
BOOST_MPL_ASSERT(( is_same< at<m,long>::type, void_ > ));

BOOST_MPL_ASSERT(( is_same< at<m,long,void_>::type, void_ > )); // fails!

It seems there is a discrepancy between the documentation and the code; if you take a look at boost/mpl/at_fwd.hpp, it forward declares boost::mpl::at with only 2 template parameters :(

Please file a trac ticket, and I'll try to find out what the correct resolution is on the developer's list.

- Jeff