Getting a list of member types from a boost fusion adapted struct

Question

I have boost fusion adapted structs like this one:

struct A {
    int x;
    double y;
    std::string z;
};
BOOST_FUSION_ADAPT_STRUCT(
    A,
    x,
    y,
    z
)

I'd like to iterate over the types of the adaptation at compile time. E.g. if I have a class that wraps a type:

template <typename T> class Foo { ... };

then I'd like to be able to get type std::tuple<Foo<int>, Foo<double>, Foo<std::string>> given my struct A. I use std::tuple here just as an example; it can be another variadic type template class.

A c++17 solution would be welcome.

Answer 1

Helper for transforming adapted fusion struct to something like std::tuple:

template<class Adapted, template<class ...> class Tuple = std::tuple>
struct AdaptedToTupleImpl
{
    using Size = boost::fusion::result_of::size<Adapted>;

    template<size_t ...Indices>
    static Tuple<typename boost::fusion::result_of::value_at_c<Adapted, Indices>::type...> 
        Helper(std::index_sequence<Indices...>);

    using type = decltype(Helper(std::make_index_sequence<Size::value>()));
};

template<class Adapted, template<class ...> class Tuple = std::tuple>
using AdaptedToTuple = typename AdaptedToTupleImpl<Adapted, Tuple>::type;

Validation:

using AsTuple = AdaptedToTuple<A>;
static_assert(std::is_same_v<std::tuple<int, double, std::string>, AsTuple>);

Helper which applies metafunction to each type in tuple:

template<class List, template<class> class Func> struct ForEachImpl;

template<class ...Types, template<class ...> class List, template<class> class Func>
struct ForEachImpl<List<Types...>, Func>
{
    using type = List<Func<Types>...>;
};

template<class List, template<class> class Func>
using ForEach = typename ForEachImpl<List, Func>::type;

Validation:

static_assert(std::is_same_v<ForEach<AsTuple, std::add_pointer_t>, std::tuple<int*, double*, std::string*>>);

Also take a look at Boost.MP11 library. It has mp_transform metafunction which is equivalent to ForEach function described above.