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

boost, boost-mpl, c++

Solution

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;

Problem

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

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