How to correctly setup for different entity type sizes

[Ramito]

Hello! I am trying to define a special entity registry that uses a smaller entity type size (uint16_t, specifically).

I am trying this:

I declare the traits like so:

template <>
struct entt::entt_traits<uint16_t>
{
    using value_type = uint16_t;

    using entity_type = uint16_t;
    using version_type = uint8_t;

    static constexpr entity_type entity_mask = (1u << 14) - 1;
    static constexpr entity_type version_mask = (1u << 2) - 1;
};

(Yes, 14 bits for id and merely 2 bits for version is enough for my purposes)

And then try to use my registry given by:

typedef entt::basic_registry<uint16_t> SpecialRegistry;
typedef SpecialRegistry::entity_type SpecialEntity;

However if I try to compile I get this:
error C2039: 'to_entity': is not a member of 'entt::entt_traits<uint16_t>'

I assume I do not need to implement every single member needed for the entity traits, but I don't understand what is the correct way to set this up.

Can someone point me to how am I supposed to create a registry with such entity traits?

Thank you!

[skypjack]

Unless you want the same traits for all entity types based on uint16_t, this is what you are looking for:

enum my_own_entity : std::uint16_t {};

struct my_own_entity_traits {
    using value_type = my_own_entity;
    using entity_type = std::uint16_t;
    using version_type = std::uint8_t;
    static constexpr entity_type entity_mask = (1u << 14) - 1;
    static constexpr entity_type version_mask = (1u << 2) - 1;
};

template<>
struct entt::entt_traits<my_own_entity>: entt::basic_entt_traits<my_own_entity_traits> {};

[Ramito]

Thanks for the reply!

I tried a couple things going from your guidance but I could still not make it to work. It seems to me that overall using short types is not supported by the library out of the box.

For example after trying this which is the simplest I could do exemplify the issues I run into:

struct short_traits
{
    using value_type = uint16_t;
    using entity_type = uint16_t;
    using version_type = uint8_t;
    static constexpr entity_type entity_mask = (1u << 14) - 1;
    static constexpr entity_type version_mask = (1u << 2) - 1;
};

template <>
struct entt_traits<uint16_t> : basic_entt_traits<short_traits>
{};

typedef entt::basic_registry<uint16_t> SpecialRegistry;

I got this a type cast error in construct in entity.hpp:
error C2397: conversion from 'int' to 'unsigned short' requires a narrowing conversion

I then went ahead and hacked the code in there to do type casts for the bitwise operators (which return an int type), but then I just hit:
'page_size': is not a member of 'entt::entt_traits<uint16_t>'

If I do something more inline to what you suggested I get a complain that traits for uint16_t are not defined, which is how I got to try the above.

I'll grant that generally using a short type for entity is not super useful in general, but I am still surprised I am the first to try it!

[skypjack]

It seems to me that overall using short types is not supported by the library out of the box.

Uhm... 🤔 Now that you mention it, it's very likely.
I remember they were a real hassle and had no use, but I don't remember if or when support was discontinued.
I guess I should give them a try...

[skypjack]

Oh, yeah, 2020, here we go: removed support for 16b identifiers.
It's been too long to remember the reason, but I think it's something along the lines of too much trouble, no benefit.
With a slightly suboptimal version, the number of entities was still negligible for any application.

[skypjack]

The doc is also clear on this point actually:

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.

So, yeah, 32b and up is the way. Sorry for that, no guarantees on 16b. You're out of luck,

[Ramito]

It's all good, it's probably fine as it is, but it was worth a try!

Thanks for your help and thanks for the awesome library!

Been using entt for ~10 years, and it's on any and every project I have been involved with.

[skypjack]

Wow, that makes me so happy and kinda proud! 🙂
Given your long experience, feel free to throw suggestions for improvements and/or new features at me.
I'm always looking for ways to improve. 👍