How to unpack tuple with unknown size?

Question

Given,

template<typename T>
void foo(T t)
{
    std::tie(/*xxx*/)=t;
    auto &[/*yyy*/]=t;
}
int main()
{
    foo(forward_as_tuple(1,2,3));
    foo(forward_as_tuple(1,2,3,4,5));
}

I want foo() to unpack the tuple that's passed to itself. Can decomposition declarations with auto or std::tie(), handle the unknown tuple sizes like xxx - yyy up there?
If yes, how?

I'm trying to think of another ways, maybe all elements could be pushed back to a vector of that type, once they're got.

std::vector<T> bar;
size_t baz=std::tuple_size<T>::value; //useless because it can't be used as:
for(int i=0; i<baz; i++)
    bar.push_back(std::get<i>(t)); //since i isn't constant

Using vectors was just a bad idea that failed.
How else can it be done?

What I'm trying to, shortly is; I want to get tuple elements in a for loop. That's why I think I need them to be extracted somehow.

Relevant question: [Convert std::tuple to std::array C++11](https://stackoverflow.com/q/10604794/580083), [C++ Transform a std::tuple to a std::vector or std::deque](https://stackoverflow.com/q/42494715/580083). — Daniel Langr, May 11 '20 at 14:58
what would that be good for? What do you want to do with the unpacked references? — 463035818_is_not_an_ai, May 11 '20 at 14:58
First, why would you want to process a tuple of unknown size and unknown types? If the arguments list is homogeneous, then it's probably not a use case for a tuple. — Piotr Skotnicki, May 11 '20 at 15:15
@PiotrSkotnicki I'm trying to find a way and then understand it. The argument list isn't homogeneous. — , May 11 '20 at 15:19
@idclev463035818 I plan to use some of them in recursive function calls, and other arguments will show which ones and how. — , May 11 '20 at 15:19
@Xemdocia: [Demo](https://coliru.stacked-crooked.com/a/bfd015499abeb36c) for your deleted question. — Jarod42, May 11 '20 at 20:53
@Jarod42 Thank you, but how can I map these all into one single name of "foo" and let it be chosen among overloads? Also why it gives a set of errors when I change it from `std::map` to `std::multimap`? — , May 11 '20 at 21:30

score 3 · Answer 1 · answered May 11 '20 at 15:02

What I'm trying to, shortly is; I want to get tuple elements in a for loop.

You don't need to unpack a tuple to iterate over it. Here's how I'd do it:

template <typename Integer, Integer ...I, typename F>
constexpr void constexpr_for_each(std::integer_sequence<Integer, I...>, F &&func)
{
    (func(std::integral_constant<Integer, I>{}) , ...);
}

template <auto N, typename F>
constexpr void constexpr_for(F &&func)
{
    if constexpr (N > 0)
    {
        constexpr_for_each(std::make_integer_sequence<decltype(N), N>{}, std::forward<F>(func));
    }
}

template <typename T>
void foo(T t)
{
    constexpr_for<std::tuple_size_v<T>>([&](auto index)
    {
        constexpr auto i = index.value;

        std::cout << std::get<i>(t) << '\n';
    });
}

This works perfectly but I can't understand the logic behind it. I haven't learned about _constant expressions_ yet. I'll check it about it, thanks. — , May 11 '20 at 15:11

score 3 · Accepted Answer · edited Jun 20 '20 at 09:12

3

How to unpack tuple with unknown size?
auto &[/*yyy*/]=t;

You can't.

maybe all elements could be pushed back to a vector of that type

Use std::apply:

std::vector<int> baz;
std::apply([&baz](auto... args) {
    (baz.push_back(args), ...);
}, t);

edited Jun 20 '20 at 09:12

Community

1
1

answered May 11 '20 at 15:04

eerorika

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Can you show me a small example on how it's done? I tried to use your solution but can't compile. It says `expansion pattern 'int' contains no argument packs` – May 11 '20 at 15:09
2

`int...` doesn't work here. It has to be `auto...`, or `std::same_as auto...` if you want to show off. :P – HolyBlackCat May 11 '20 at 15:12
@HolyBlackCat Good catch. I didn't verify correctness. – eerorika May 11 '20 at 15:16
I couldn't compile it also with `auto...`. Can you please verify it and show how it will compile well? – May 11 '20 at 15:21
@Xemdocia When program does not compile, first step is to read the error message. – eerorika May 11 '20 at 15:23
@eerorika Now, it compiles well after your edit. Thank you very much – May 11 '20 at 15:27

How to unpack tuple with unknown size?

2 Answers2