[entt::meta] How do I safely insert a component instance into a registry with only its entt::meta_type info?

[JuanDiegoMontoya]

For my game, I'm writing a reflection system with entt::meta and a simple editor. With the reflected type info, I can make a list of components that an entity doesn't have and present them under an "Add Component" drop-down menu. We can easily get the component storage (entt::sparse_set) with this info using registry.storage(id_type).

entt::entity entity = selectedEntity;
for (auto [id, meta] : entt::resolve())
{
    if (meta.traits<Traits>() & Traits::COMPONENT)
    {
        // Ignore if entity already has component.
        if (registry.storage(id)->contains(entity)) // <---- Crashes if storage doesn't exist, but easy to avoid by checking if storage is null.
        {
            continue;
        }

        if (ImGui::Selectable(std::string(meta.info().name()).c_str()))
        {
            registry.storage(id)->push(entity); // <---- Crashes if storage doesn't exist.
        }
    }
}

Since the storage can be nonexistent, causing registry.storage to return nullptr, the above code will crash if an instance of the component hasn't yet been inserted directly through the registry itself with one of the function templates.

The more I think about it, the more it seems like the only way to go about solving this is to register an "add component" function on every reflected component type (as in, I think this is a C++ limitation). So something like this:

// Side note: I don't know how to forward references to these, so I just use pointers.
entt::meta<Foo>().func<[](entt::registry* registry, entt::entity entity) { registry->emplace(entity);  }>("EmplaceComponent"_hs);

Then I can invoke this function in my editor:

if (ImGui::Selectable(std::string(meta.info().name()).c_str()))
{
    if (auto emplaceFunc = meta.func("EmplaceComponent"_hs))
    {
        emplaceFunc.invoke({}, &registry, entity);
    }
}

I hope there is a better way to do this, but if not, I can automate it away with macros.

[skypjack]

This question has been asked many times and I think there are several discussions with different examples. Have you seen those already?
The main problem is that the registry cannot create a storage for you if it doesn't know the C++ type, and it's unknown when you invoke storage with a type info object.
Briefly, you've literally two options here: attach to the meta type a let's create the storage function to which you pass the registry, or set up a sort of emplace-or-replace function that returns the element wrapped in a meta_any by reference.
Let me know if you can't find a proper answer to your doubts and need more details. 👍

[JuanDiegoMontoya]

Thanks, that's as I suspected. It makes sense.

And sorry for making a duplicate question. I tried looking, but apparently not hard enough.

[skypjack]

No worries at all. I'm glad to help in any case. 🙂