If I want to iterate through a tuple using std::apply
but not apply one function to the entire thing, how can I separate the tuple, i.e. apply one function to the first n
values and another to all values after it?
some_values
would be a tuple which could have any length and types and length_of_first_part
(likely named a little less verbosely) would known at compile-time.
std::tuple<char, long long, double, long double, float> some_values(33, 2, 3.4, 5.6, 7.8);
const size_t length_of_first_part = 2;
std::apply(
[](auto&&... current_val) {
((std::cout << "(Should be first part) " << current_val << "\n"), ...); //Would obviously do a litle more than cout, but this is just a minimal example
}, some_values
);
std::apply(
[](auto&&... current_val) {
((std::cout << "(Should be second part) " << current_val << "\n"), ...);
}, some_values
);
Gave this a shot. Looking at the "possible implementation" here: https://en.cppreference.com/w/cpp/utility/apply
It passes an std::index_sequence
to a helper function to call std::invoke
. I changed it to pass std::make_sequence<N>
(N being the split index) instead of the tuple size. Then I passed the inverse (tuple size - N) to call std::invoke
on the second function:
template <int N, typename F1, typename F2, typename Tuple, size_t... I1s, size_t... I2s>
decltype(auto) split_apply_impl(F1&& f1, F2&& f2, Tuple&& t, std::index_sequence<I1s...>, std::index_sequence<I2s...>) {
std::invoke(std::forward<F1>(f1), std::get<I1s>(std::forward<Tuple>(t))...);
return std::invoke(std::forward<F2>(f2), std::get<(I2s+N)>(std::forward<Tuple>(t))...);
}
template <size_t N, typename F1, typename F2, typename Tuple>
decltype(auto) split_apply(F1&& f1, F2&& f2, Tuple&& t)
{
return split_apply_impl<N>(
std::forward<F1>(f1),
std::forward<F2>(f2),
std::forward<Tuple>(t),
std::make_index_sequence<N>{},
std::make_index_sequence<(std::tuple_size_v<std::remove_reference_t<Tuple>>-N)>{});
}
Usage:
std::tuple<char, long long, double, long double, float> some_values(33, 2, 3.4, 5.6, 7.8);
constexpr size_t length_of_first_part = 2;
split_apply<length_of_first_part>(
[](auto&&... current_val) {
((std::cout << "(Should be first part) " << current_val << "\n"), ...);
},
[](auto&&... current_val) {
((std::cout << "(Should be second part) " << current_val << "\n"), ...);
},
some_values);
Proof of concept: https://godbolt.org/z/6dvMre