Why is it okay to have a system that takes a entity and a const component, but not okay to take an entity and non-const component?

[TypeDefinition]

[SanderMertens]

My guess is that root is an empty type, and empty types are passed by value (try root p).

The reason for this is that empty types are not actually stored by the ECS, and if they are part of a system/query signature a temporary object of the empty type is created to pass to the each function.

The reason this works with const but not for non-const, is that while C++ does let you pass a temporary object to a const T&, you can't pass a temporary object to a T&.

Since an empty type can't actually store values, a cleaner way is sometimes to use with:

world.system("Move")
  .with<root>()
  .each([](flecs::entity e) { // component passed by `with` doesn't become part of signature
  });

Hope that helps!

[TypeDefinition]

Thank you. root indeed is an empty type.