Get boost::variant's type index with boost::mpl

Question

boost::variant has member types which is some kind of boost::mpl structure. Is there a way to get an index of type in that structure at compile time, so late in in runtime i could do

if(myVariantInstance.which() == typeIndex)
{
   /*...*/
}

Instead of

if(myVariantInstance.type() == typeid(ConcreteType))
{
  /*...*/
}

Answer 1

It's a bit convoluted, and there might be a better way, but you could use boost::mpl::copy. Here's something that ought to work, based off of the example from your comment:

#include <boost/variant.hpp>
#include <boost/mpl/copy.hpp>
#include <boost/mpl/find.hpp>
#include <boost/mpl/vector.hpp>

typedef boost::mpl::vector<int, long, char> MyMplVector;
typedef boost::mpl::find<MyMplVector, long>::type MyMplVectorIter;
static_assert(MyMplVectorIter::pos::value == 1, "Error");

typedef boost::variant<int, long, char> MyVariant;
typedef boost::mpl::vector<> EmptyVector;
typedef boost::mpl::copy<
  MyVariant::types,
  boost::mpl::back_inserter<EmptyVector>>::type ConcatType;

typedef boost::mpl::find<ConcatType, long>::type MyVariantTypesIter;
static_assert(MyVariantTypesIter::pos::value == 1, "Error");