boost::mpl::map fails boost::mpl::equal?

Question

Boost MPL documentation states that boost::map::equal

"Returns a true-valued Integral Constant if the two sequences Seq1 and Seq2 are identical when compared _element_ by _element_ .

but it seems that the associative sequence map is not checked for equality element _wise_:

The following demo will show this: Map2 should equal Map3, which both increment the 'int_<1>' value_type at 'key'. Look at the typedef defining Map3. Size and the only element is dumped n the demo:

#include<iostream>
#include<boost/mpl/map.hpp>
#include<boost/mpl/at.hpp>
#include<boost/mpl/insert.hpp>
#include<boost/mpl/erase_key.hpp>
#include<boost/mpl/pair.hpp>
#include<boost/mpl/int.hpp>
#include<boost/mpl/plus.hpp>
#include<boost/mpl/equal.hpp>
#include<boost/mpl/size.hpp>
#include<boost/mpl/front.hpp>

namespace mpl = boost::mpl;
using mpl::int_;

using std::cout;
using std::endl;
using std::is_same;

int main(int argc, char *argv[])
{

    typedef int key;

    typedef typename mpl::map<mpl::pair<key, int_<1>>> Map;
    typedef typename mpl::map<mpl::pair<key, int_<2>>> Map2;

    typedef typename mpl::insert<
    typename mpl::erase_key<Map,
                            key>::type, 
        mpl::pair<key, 
                  typename mpl::plus<int_<1>, 
                                     typename mpl::at<Map, key>::type>::type
        >::type
    >::type Map3;

    cout << "equal? " << mpl::equal<Map2,Map3>::type::value << endl;
    cout << "size? " << mpl::size<Map3>::value << endl;
    cout << "key type at front? " << typeid(mpl::front<Map3>::type::first).name() << endl;
    cout << "value type at front? " << mpl::front<Map3>::type::second::value << endl;

    cout << "expected size? " << mpl::size<Map2>::value << endl;
    cout << "expected key type at front? " << typeid(mpl::front<Map2>::type::first).name() << endl;
    cout << "expected value type at front? " << mpl::front<Map2>::type::second::value << endl;

    return 0;
}

I am using gcc 4.8.1 with boost 1.51

Answer 1

The reason it does not work as expected is that mpl::plus<int_<1>, mpl::at<Map, key>::type is mpl::integral_constant<int, 2>, whereas mpl::int_<2> is a different type.

A working version:

typedef mpl::map<mpl::pair<key, mpl::integral_c<int, 1>>> Map;
typedef mpl::map<mpl::pair<key, mpl::integral_c<int, 2>>> Map2;

typedef mpl::insert<
    mpl::erase_key<Map, key>::type,
    mpl::pair<key, mpl::plus<mpl::integral_c<int, 1>, mpl::at<Map, key>::type>::type>
>::type Map3;