How to check if a template type is one of the types of a variant type?

Question

Considering a variant type and a template function, how can I check the template type is one of the types of the variant ? Is there a more elegant way than the following ?

typedef boost::variant<Foo,Bar> Var;

template <typename T>
void f(const T& x)
{
  BOOST_STATIC_ASSERT(
       boost::is_same<T,Foo>::value
    || boost::is_same<T,Bar>::value
  );
}

Note : I use Boost 1.57 and gcc 4.8.3. I don't use C++11 for compatibility with old gcc versions.

Answer 1

Use MPL:

#include <boost/variant/variant.hpp>
#include <boost/mpl/contains.hpp>

typedef boost::variant<Foo, Bar> Var;

template <typename T>
void f(const T& x)
{
    BOOST_STATIC_ASSERT(boost::mpl::contains<Var::types, T>::value);
}

DEMO

or manually iterate over boost:::variant types:

#include <boost/variant/variant_fwd.hpp>
#include <boost/type_traits.hpp>

template <typename T, typename V>
struct variant_has_type;

template <typename T, BOOST_VARIANT_ENUM_SHIFTED_PARAMS(typename Ts)>
struct variant_has_type<T, boost::variant<T, BOOST_VARIANT_ENUM_SHIFTED_PARAMS(Ts)> >
    : boost::true_type {};

template <typename T, typename U, BOOST_VARIANT_ENUM_SHIFTED_PARAMS(typename Ts)>
struct variant_has_type<T, boost::variant<U, BOOST_VARIANT_ENUM_SHIFTED_PARAMS(Ts)> >
    : variant_has_type<T, boost::variant<BOOST_VARIANT_ENUM_SHIFTED_PARAMS(Ts), void> > {};

template <typename T, BOOST_VARIANT_ENUM_SHIFTED_PARAMS(typename Ts)>
struct variant_has_type<T, boost::variant<void, BOOST_VARIANT_ENUM_SHIFTED_PARAMS(Ts)> >
    : boost::false_type {};

DEMO 2