How to convert std::array to std::tuple?

Question

I am working on a aux module to pass values between polymorphic objects and at some point I have

std::array<void*, N>

and need to send forward

std::tuple<void*, void*, /* N times */>

I can figure some solution with the use of index_sequence or/and recursions, but all of those look bulky and difficult to read.
Is there any more straightforward way to do this by the means of the standard library?

Also, just in case - am I right that the layout of std::array is a dense set of respective objects, thus equal to, lets say, void** of respective length, whereas the layout of tuple allows gaps?

Not actually a duplicate. The former question is about more specific problem and for some reason has no satisfying answer (so asking it again anyway was useful). — Glib, Aug 15 '19 at 18:17

StoryTeller - Unslander Monica · Accepted Answer · 2019-08-15T15:18:04.010

21

If your implementation supports it, you can use std::tuple_cat. Under some implementations it concatenates any number of objects that respect the tuple interface (which std::array does) into a single flat tuple. Concatenating a single tuple like object will just produce a tuple that holds copies of the members of said source "tuple".

std::array<void*, N> a;
auto b = std::tuple_cat(a);

Also, just in case - am I right that the layout of std::array is a dense set of respective objects, thus equal to, lets say, void** of respective length, whereas the layout of tuple allows gaps?

A std::array is an aggregate the will hold a void*[N] internally. So yes, the elements will be without padding in between them. The layout of the tuple's elements is not specified to such an extent.

edited Aug 15 '19 at 15:18

answered Aug 15 '19 at 13:30

StoryTeller - Unslander Monica

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Thanks! This is just what I was looking for... – Glib Aug 15 '19 at 13:33
you're an `std::tuple_dog`. – Hatted Rooster Aug 15 '19 at 13:34
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@SombreroChicken - Said the `std::tuple_chicken`! – StoryTeller - Unslander Monica Aug 15 '19 at 13:35
I was looking for this myself but cppreference mentioned that it only concatenates tuples. Never thought that std::array was usable as well (though the example shows a std::make_pair case) – gast128 Aug 15 '19 at 13:40
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@gast128 Yeah, but, from the other end: ["An array can also be used as a tuple of N elements of the same type."](https://en.cppreference.com/w/cpp/container/array) – Lightness Races in Orbit Aug 15 '19 at 13:41
@gast128 - Usually, anything that goes by `std::get(...)` can work for a `std::array` as well. There are overloads and type traits for it that are meant to make it behave like a tuple. – StoryTeller - Unslander Monica Aug 15 '19 at 13:42
This is formally UB (you break a *Requires*), and not guaranteed to be supported. @gast128 it no longer does. – T.C. Aug 15 '19 at 15:00

How to convert std::array to std::tuple?

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