What is a "valid entity type"? (Question)

[Batres3]

According to the documentation: "User defined identifiers are allowed as enum classes and class types that define an entity_type member of type std::uint32_t or std::uint64_t."

I'm quite a beginner when it comes to C++ so I don't really understand what this means. In the faq the example given is:

enum class entity: std::uint32_t {};

What would a valid example look like for a custom entity that is not an enum class, but a regular class?

I'm trying to make it work with the following class:

class UUID
{
public:
	using entity_type = uint64_t;
	static constexpr uint64_t null = 0;
	UUID();
	UUID(uint64_t id);
	UUID(const UUID&) = default;

	constexpr operator entity_type() const noexcept { return m_UUID; }
private:
	entity_type m_UUID;
};

This is obviously not correct since it raises the error

error C2127: 'value': illegal initialization of 'constexpr' entity with a non-constant expression

But I don't understand what has to be constant here. Sorry if this has been asked before but I couldn't quite find it.

[skypjack]

Is this what you are looking for?

[Batres3]

That's exactly it, thank you and sorry for the late response.