Entt: How to clone registry?

[OverShifted]

Hello every one

I want to basically duplicate a registry and maybe run some physics simulation so the main registry is not modified.
Is it possible with snapshots? + can someone explain what a snapshot really is?

thanks for the awesome library btw

[skypjack]

Hi, thank you for using EnTT!

A registry isn't meant for copies, because it may be very expensive.
Though, you can copy entities and components between registries easily:

// all entities
dst.assign(src.data(), src.data() + src.size(), src.destroyed());

// everything of a given type
auto view = registry.view<T>();
dst.insert(view.data(), view.data() + view.size(), view.raw(), view.raw() + view.size());

A snapshot is the result of a paid work 😄 and tbh I wouldn't use it nowadays, since range-functionalities are way faster.

[OverShifted]

what that src.destroyed() means?

[skypjack]

It returns the _head_ of the list of destroyed entities.
This is how EnTT manages its entities: https://skypjack.github.io/2019-05-06-ecs-baf-part-3/
So, destroyed literally returns an integral value that contains enough information to know _where_ this list is. In a sense, this value is the full list.

[OverShifted]

I am a student and right now I don't have much time reading that and I am going to close this issue.
I will definitely read that link and play around with this solution you gave me later.
Thank you very much 😀.

[skypjack]

Ahah no worries. It was only for reference.
If you have any other question, I invite you to join the gitter channel or the discord server. There are many people there who can help. :+1:

[OverShifted]

I am using an older version of EnTT this is the entt.hpp used in my engine and registries, don't have any public destroyed method. Is it a new feature? when I tried to update EnTT, I got a lot of weird errors.

[skypjack]

Ah, interesting. I've just realized that the single include file doesn't contain the version!
By the way, destroyed is pretty recent, yeah. Previous versions work without it:

// all entities
dst.assign(src.data(), src.data() + src.size());

Long story short, it's a matter of performance. With the value returned by destroyed, assign performs only a raw copy of the vector of entities. Previous versions iterated the vector to reconstruct the implicit list of destroyed entities instead.

[OverShifted]

Ok. I don't care about performance in this situation. Thanks 🙏🏻.

[skypjack]

You're welcome. 🙂

[OverShifted]

I made a copy-constructor for my Scene class like so:

Scene::Scene(const Scene& other)
    : m_Registry(), m_ViewportWidth(other.m_ViewportWidth), m_ViewportHeight(other.m_ViewportHeight),
      m_RootHandles(other.m_RootHandles), m_ComponentList(other.m_ComponentList)
{
    const auto& reg = other.m_Registry;
    m_Registry.assign(reg.data(), reg.data() + reg.size());
}

looks like entities are copied fine but there is no component attached! (or at least my TransformComponent which is attached to all entities)

and one more thing:
all my components, store a Scene* and an entt::entity and when they are copied, I want to change that Scene* to point to the new scene. Is there any better way to do that than looping through all components after copy?

[skypjack]

assign copies entities between registries, it doesn't copy their components. 🙂
Long story short, most of the time I don't want to do a blind copy of all types, because it doesn't make muche sense. For example, I use components to dispatch messages and they shouldn't be copied in any case.
Take a look at insert to also copy components of interest if needed. 👍

Also, can I suggest you to join the gitter channel or the discord server for your questions?
I'm fine with using GH, but it may be somewhat better with a chat?

[OverShifted]

OK, It makes more sense to copy different component types one by one. I can also apply my modifications to them.
+I personally prefer GH but I will give gitter and discord a try.

[skypjack]

+I personally prefer GH but I will give gitter and discord a try.

Oh, no, don't worry, I'm fine with GH. I suggested it only in case you're interested in a more direct chat. 👍