registry.view<> on subset of entities without using Tags

[ostorek]

What would be the best way to limit view to subset of entities without using Tags.
We have entities linked in parent-children relationship and suppose i want to do view only on children of the entity:

for (const auto&& [e, c] : viewChildren<Component1,..>(reg, entity) {}

the obvious primitive solution would be emplace every child with ChildTag and then add the tag to the view. However tags would have to be added, cleared and calling this function from multiple threads would add another level of complexity.

Can i somehow create temporary storage, fill it with children entities and create on it view that would do the trick and then return them both?

auto viewChildren(..)
{
   auto onlyChildrenView = ...
   return reg.view<Component...> | onlyChildrenView ;
}

[skypjack]

Can i somehow create temporary storage, fill it with children entities and create on it view that would do the trick and then return them both?

Yeah, definitely. You can create the storage as a standlone member of your class or within the registry with its own name:

struct foo {
    // as member
    entt::storage_type_t<void> storage{};
};

// as managed by the registry
auto &storage = registry.storage<void>("my_name"_hs);

Consider that in the first case you have to manually cleanup it while ie destroy would work on the storage managed by the registry too.
Anyway, when you have it, you can slap it into a view and combine the latter with a second view as you like. For example:

for(auto [entt, my_t]: (entt::basic_view{storage} | registry.view<T>()).each()) {
    // here you get only the entities that are in storage and also have a T
}

Is this what you're looking for?

[ostorek]

That's looks like exactly what i need. Just for clarification:

If i create storage as member, i still need to set it up right, like call bind?
storage.bind(...) // to registry
However seems like bind is there only to later on publish updates to the registry which is kind of unwanted for me.

[skypjack]

If it's unwanted, you can disable the mixin as a whole for all component types. It's also slightly faster in general.
Look at the documentation. It's a matter of setting the storage traits. I plan to make it a compilation flag soon too.
If you disable signals, then you don't need to bind the registry. Otherwise, yeah, it's kinda required, although useless if you never bind a listener.