Returning range of different containers

Question

I have multiple containers with the same element type T. I would like to select one of the containers depending on a enum.

I tried something like this:

auto range = [category,system]() -> auto { switch(category) { case sound_category::voice: return std::ranges::views::all (system->dialogue_voices); // could be std::array case sound_category::mono: return std::ranges::views::all (system->mono_voices); // could be std::vector case sound_category::music: return std::ranges::views::all (system->music_voices); // could be std::list default: return std::ranges::views::all (system->sfx_voices); // could be std::deque } }();

but that will result in a compiler error, since the deduced type in the cases is different.

Is there some way to achieve that?

@KamilCuk The value type of the containers (sfx_voices, mono_voices etc) is the same and I am only interested in the values/references to these values — Raildex
– Raildex, Commented Oct 7, 2022 at 14:45
std::span is the way to go. Changing the return type of the lambda to std::span<T> and return std::span(system->...) should work for the purpose. — kotatsuyaki
– kotatsuyaki, Commented Oct 7, 2022 at 14:49
@NathanOliver while a span could potentially solve my issue now. It won't work if I want to switch out the container in a later development stage (to a non-contiguous one). This is not a question about "performance" or "array vs vector vs list vs deque". My goal is to have a generic way to get a view from a collection selected at runtime. — Raildex
– Raildex, Commented Oct 7, 2022 at 15:03
@Raildex I have as well, feel free to roll back, but I figured removing all mention that these are arrays currently isn't needed since you want a generic solution. — NathanOliver
– NathanOliver, Commented Oct 7, 2022 at 15:09

Jarod42 · Accepted Answer · 2022-10-08 00:22:23Z

ranges-v3 has any_view for type erasure view.

So it would be something like:

auto range = [category,system]() -> ranges::v3::any_view<Voice> { switch(category) { case sound_category::voice: return std::ranges::views::all (system->dialogue_voices); // could be std::array case sound_category::mono: return std::ranges::views::all (system->mono_voices); // could be std::vector case sound_category::music: return std::ranges::views::all (system->music_voices); // could be std::list default: return std::ranges::views::all (system->sfx_voices); // could be std::deque } }();

KamilCuk · Accepted Answer · 2022-10-07 15:51:05Z

1

Use a subrange from two iterators.

#include <ranges> #include <array> int main() { std::array<int, 2> a; std::array<int, 3> b; auto c = std::ranges::subrange<int*>(a.begin(), a.end()); c = std::ranges::subrange<int*>(b.begin(), b.end()); }

edited Oct 7, 2022 at 15:51

answered Oct 7, 2022 at 14:57

KamilCuk

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3 Comments

Raildex Over a year ago

That does not compile. godbolt.org/z/8fTbTc9dW

KamilCuk Over a year ago

Compiles on gcc, no idea about msvc. Doing explicit template type std::ranges::subrange<int*> works fine. Could be std::range<>::iterator is more special on msvc, dunno

Richard Over a year ago

Doesn't compile on clang and msvc.

Artyer · Accepted Answer · 2022-10-07 15:54:31Z

Instead of something like:

auto range = [category,system]() -> auto { switch(category) { case sound_category::voice: // ... } }(); // stuff with range

You can use a visitor pattern:

auto visitor = [&](const auto& range) { // stuff with range }; switch (category) { case sound_category::voice: return visitor(system->dialogue_voices); case sound_category::mono: return visitor(system->mono_voices); // ... }

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