Calling a function for each variadic template argument and an array

You could refactor or wrap f to return a new X instead of having it passed, since this would play pack expansion into the hand and make the function really concise:

template<class T>
X fw(T const& t){ X x; f(x, t); return x; }

template<class... Args>
void h(Args... args){
  X xs[] = { fw(args)... };
  g(xs, sizeof...(Args));
}

Live example.

And if you could change g to just accept an std::initializer_list, it would get even more concise:

template<class... Args>
void h(Args... args){
  g({f(args)...});
}

Live example. Or (maybe better), you could also provide just a wrapper g that forwards to the real g:

void g(X const*, unsigned){}

void g(std::initializer_list<X> const& xs){ g(xs.begin(), xs.size()); }

template<class... Args>
void h(Args... args){
  g({f(args)...});
}

Live example.
Edit: Another option is using a temporary array:

template<class T>
using Alias = T;

template<class T>
T& as_lvalue(T&& v){ return v; }

template<class... Args>
void h(Args... args){
  g(as_lvalue(Alias<X[]>{f(args)...}), sizeof...(Args));
}

Live example. Note that the as_lvalue function is dangerous, the array still only lives until the end of the full expression (in this case g), so be cautious when using it. The Alias is needed since just X[]{ ... } is not allowed due to the language grammar.

If all of that’s not possible, you’ll need recursion to access all elements of the args pack.

#include <tuple>

template<unsigned> struct uint_{}; // compile-time integer for "iteration"

template<unsigned N, class Tuple>
void h_helper(X (&)[N], Tuple const&, uint_<N>){}

template<unsigned N, class Tuple, unsigned I = 0>
void h_helper(X (&xs)[N], Tuple const& args, uint_<I> = {}){
  f(xs[I], std::get<I>(args));
  h_helper(xs, args, uint_<I+1>());
}

template<typename... Args>
void h(Args... args)
{
    static constexpr unsigned nargs = sizeof...(Args);
    X xs[nargs];

    h_helper(xs, std::tie(args...));

    g(xs, nargs);
}

Live example.

Edit: Inspired by ecatmur’s comment, I employed the indices trick to make it work with just pack expansion and with f and g as-is, without altering them.

template<unsigned... Indices>
struct indices{
  using next = indices<Indices..., sizeof...(Indices)>;
};
template<unsigned N>
struct build_indices{
  using type = typename build_indices<N-1>::type::next;
};
template <>
struct build_indices<0>{
  using type = indices<>;
};
template<unsigned N>
using IndicesFor = typename build_indices<N>::type;

template<unsigned N, unsigned... Is, class... Args>
void f_them_all(X (&xs)[N], indices<Is...>, Args... args){
  int unused[] = {(f(xs[Is], args), 1)...};
  (void)unused;
}

template<class... Args>
void h(Args... args){
  static constexpr unsigned nargs = sizeof...(Args);
  X xs[nargs];
  f_them_all(xs, IndicesFor<nargs>(), args...);
  g(xs, nargs);
}

Live example.