I'm trying to implement a compile-time topological sort of the DAG representing a certain dependency relationship between my classes.

One of the data structures I can use to implement topological sort is a map, so I'm trying to use mpl::map using mpl::size_t<> as key. The documentation doesn't define key equality for mpl::map, but I assume it's effectively std::is_same<>. If so, I think I have a problem. In the following code, the last two lines fail static_assert().

The essence of the problem is that my attempt to construct/build my keys using copy with back_inserter does not give me a mpl::vector<> of mpl::size_t<> that is_same to Count, the expected value. It does give me a sequence, and the elements have the right value and value_type but they are not is_same to the expected size_t<> types. So, if I use the elements together with the size_t<> in an mpl::map, won't they be considered to be different keys?

Josh

#include <type_traits>

#include <boost/mpl/at.hpp>

#include <boost/mpl/back_inserter.hpp>

#include <boost/mpl/copy.hpp>

#include <boost/mpl/equal.hpp>

#include <boost/mpl/equal_to.hpp>

#include <boost/mpl/range_c.hpp>

#include <boost/mpl/size_t.hpp>

#include <boost/mpl/vector.hpp>

using namespace std;

namespace mpl = boost::mpl;

using Empty = typename mpl::vector<>;

using Count = mpl::vector<mpl::size_t<0>, mpl::size_t<1>>;

using ConstructedCount = typename mpl::copy<

typename mpl::range_c<size_t, 0, 2>, mpl::back_inserter<Empty>>::type;

using The0 = typename mpl::at<ConstructedCount, mpl::size_t<0>>::type;

using The1 = typename mpl::at<ConstructedCount, mpl::size_t<1>>::type;

static_assert(mpl::equal<Count, ConstructedCount, mpl::equal_to<mpl::_1, mpl::_2>>::value, "succeeds");

static_assert(The1::value == 1, "succeeds");

static_assert(std::is_same<typename The1::value_type, size_t>::value, "succeeds");

static_assert(std::is_same<Count, ConstructedCount>::value, "fails");

static_assert(std::is_same<The1, mpl::size_t<1>>::value, "fails");