Pass/forward a selected number of parameter pack to another function

Question

Coming from this earlier question on finding the number of unique values of a parameter pack, has made me think, if it is feasible to pass/forward a selected number of variadic parameter pack to another function or to return them, for instance, assume we have a struct that takes a homogeneous pack

template<size_t ... Rest>
struct my_struct;

I would like to have a function foo that takes this parameter pack and returns another struct with unique values of the parameter pack passed to it, something like

template<size_t N, size_t ... Rest>
my_struct<uniques<N,Rest...>> foo(const my_struct<N,Rest...> &a) {
// do stuff
}

As an example:

my_struct<0,5,2,0,4,2> a;
my_struct<0,5,2,4> b = foo(a);

Sorting the values, is not of any interest here. Is this functionality possible to achieve using only the standard library?

score 3 · Accepted Answer · edited May 23 '17 at 11:44

Just wrap uniques in another metafunction. Just building off of my last answer:

template <class T, template <T...> class Z>
struct quote_c {
    template <class... Ts>
    using apply = Z<Ts::value...>;
};

template <class MFC, class TL>
struct apply_typelist;

template <class MFC, class TL>
using apply_typelist_t = typename apply_typelist<MFC, TL>::type;


template <class MFC, class... Ts>
struct apply_typelist<MFC, typelist<Ts...>> {
    using type = typename MFC::template apply<Ts...>;
};

Now we have a metafunction class version of my_struct, and a metafunction that takes a metafunction class and a typelist and jams them together.

So now it's just:

template <class T, T... Vals>
using typelist_c = typelist<std::integral_constant<T, Vals>...>;

template <size_t N,
          size_t ... Rest,
          class R = apply_typelist_t<quote_c<size_t, my_struct>,
                        uniq_t<typelist_c<size_t, N, Rest...>>>
          >
R foo(const my_struct<N,Rest...> &a) {
    // ...
}

To work through your example of <0,5,2,0,4,2>.

First, we wrap all those numbers into types to get a typelist<0i,5i,2i,0i,4i,2i> (I'm just using i as shorthand to mean that it's a std::integral_constant).
Next we wrap that in uniq_t, which produces typelist<0i,5i,2i,4i>.
Next, we pass that into apply_typelist which gives us the type quote_c<size_t, my_struct>::apply<0i, 5i, 2i, 4i>
Which itself is an alias for my_struct<0, 5, 2, 4>, as desired.

I think the bit in the signature for function `foo `should be modified to read `quote_c` instead of just `quote_c`. — romeric, May 17 '16 at 11:56

Pass/forward a selected number of parameter pack to another function

1 Answers1