Correct use of boost::mpl::contains and static_assert

Question

I've just started to use boost::variant and I've run into a situation where I'd like to ensure that a template parameter is only one of the types allowed in the variant. After reading the boost documentation it seems that a combination of static_assert, boost::variant::types and boost::mpl::contains should do exactly what I want. I've managed to get a solution working but it looks rather verbose and I wondered if there was a better way to do this. Anyway, my solution is:

typedef boost::variant<int, double, std::string> my_variant;

template<typename T>
void Func()
{
    typedef boost::mpl::contains<my_variant::types, T>::type query_t;
    typedef boost::mpl::bool_<true>::type query_result_t;

    static_assert(boost::is_same<query_t, query_result_t>::value, "T isn't a valid my_variant type.");

    // ...
}

Is it possible to get the same effect without boost::mpl::bool_ and/or boost::is_same?

Answer 1

You can get rid of is_same and query_result_t, since contains returns an MPL Integral Constant:

typedef boost::variant<int, double, std::string> my_variant;

template<typename T>
void Func()
{
    static_assert(boost::mpl::contains<my_variant::types, T>::type::value, "T isn't a valid my_variant type.");

    // ...
}

I believe that's as concise as it gets - you need some predicate to query the type sequence, and boost::mpl::contains is the correct one for that.