boost::mpl::map of type and char string

Question

Is something like following

typedef boost::mpl::map<
      pair<int,"int">
    , pair<long,"long">
    , pair<bool,"bool">
    > m;

possible? If not, what are the alternatives ?

Answer 1

If you can use a C++14 compiler (currently only Clang >= 3.5), you can use Boost.Hana:

#include <boost/hana.hpp>
namespace hana = boost::hana;

auto m = hana::make_map(
    hana::make_pair(hana::type_c<int>, BOOST_HANA_STRING("int")),
    hana::make_pair(hana::type_c<long>, BOOST_HANA_STRING("long")),
    hana::make_pair(hana::type_c<bool>, BOOST_HANA_STRING("bool"))
);

// These assertions are done at compile-time
BOOST_HANA_CONSTANT_ASSERT(m[hana::type_c<int>] == BOOST_HANA_STRING("int"));
BOOST_HANA_CONSTANT_ASSERT(m[hana::type_c<long>] == BOOST_HANA_STRING("long"));
BOOST_HANA_CONSTANT_ASSERT(m[hana::type_c<bool>] == BOOST_HANA_STRING("bool"));

If you're also willing to use a non-standard GNU extension supported (at least) by Clang and GCC, you can even do the following and drop the ugly BOOST_HANA_STRING macro:

#define BOOST_HANA_CONFIG_ENABLE_STRING_UDL

#include <boost/hana.hpp>
namespace hana = boost::hana;
using namespace hana::literals;

constexpr auto m = hana::make_map(
    hana::make_pair(hana::type_c<int>, "int"_s),
    hana::make_pair(hana::type_c<long>, "long"_s),
    hana::make_pair(hana::type_c<bool>, "bool"_s)
);

static_assert(m[hana::type_c<int>] == "int"_s, "");
static_assert(m[hana::type_c<long>] == "long"_s, "");
static_assert(m[hana::type_c<bool>] == "bool"_s, "");