variant-fused.cpp

#include <boost/mpl/at.hpp>
#include <boost/mpl/size.hpp>
#include <boost/variant/variant.hpp>
#include <type_traits>

namespace {

template <size_t N>
struct emplace_nth_helper {
    template <typename Variant, typename Transform>
    Variant
    emplace (int const n, bool* const success, Transform&& transform) {
        using type_vector = typename Variant::types;
        using nth_type    = typename boost::mpl::at_c<type_vector, N>::type;
        if (n == N) {
            *success = true;
            return Variant (transform (nth_type ()));
        } else {
            return emplace_nth_helper<N - 1> ().template emplace<Variant> (
                n, success, std::forward<Transform> (transform));
        }
    }
};

template <>
struct emplace_nth_helper<0> {
    template <typename Variant, typename Transform>
    Variant
    emplace (int const n, bool* const success, Transform&& transform) {
        using type_vector = typename Variant::types;
        using nth_type    = typename boost::mpl::at_c<type_vector, 0>::type;
        if (n) {
            *success = false;
            return Variant ();
        }
        *success = true;
        return Variant (transform (nth_type ()));
    }
};

template <typename Variant, typename Transform>
inline Variant
emplace_nth (int n, bool* success, Transform&& transform) {
    using type_vector = typename Variant::types;
    auto const max    = boost::mpl::size<type_vector>::value - 1;

    if ((n < 0) || (n > max)) {
        *success = false;
        return Variant ();
    }

    emplace_nth_helper<boost::mpl::size<type_vector>::value - 1> next;
    return next.template emplace<Variant> (
        n, success, std::forward<Transform> (transform));
}

} // namespace

struct SomeOperation {
    template <typename T>
    T
    operator() (T x) {
        return x;
    }
};

int
main (int argc, char** argv) {
    bool success;
    auto var = emplace_nth<boost::variant<int, float>> (
        argc, &success, SomeOperation ());
    return var.which ();
}

Result of g++ -std=c++14 -DNDEBUG -O3 variant.cpp under GCC 7.2.0

 510:	31 c0                	xor    %eax,%eax
 512:	83 ff 01             	cmp    $0x1,%edi
 515:	0f 94 c0             	sete   %al
 518:	c3                   	retq   
 519:	0f 1f 80 00 00 00 00 	nopl   0x0(%rax)