Duplication of unnamed entities with recycled ids when using world.from_json()

[Terzalo]

When an entity does not have a name assigned and is reusing an id of a previously destroyed entity, serializing and deserializng it with world.to_json() creates a copy of it with a different id instead of overwriting the existing entity.

Example code:

#include "flecs.h"
#include <cstdio>

int main()
{
    flecs::world world;

    auto e1 = world.entity().add(flecs::Prefab);
    printf("e1 id: %u\n", e1.id());
        // e1 id: 540
    e1.destruct();

    auto e2 = world.entity().add(flecs::Prefab);
    printf("e2 id: %u\n", e2.id());
        // e2 id: 540

    auto json = world.to_json();

    printf("world.to_json(): %s\n", json.c_str());
        // world.to_json(): {"results":[{"name":"#540", "id":540, "tags":["flecs.core.Prefab"]}]}

    world.from_json(json);

    json = world.to_json();
    printf("world.to_json(): %s\n", json.c_str());
        // world.to_json(): {"results":[{"name":"#540", "id":540, "tags":["flecs.core.Prefab"]}, {"name":"#541", "id":541, "tags":["flecs.core.Prefab"]}]}

    world.from_json(json);

    json = world.to_json();
    printf("world.to_json(): %s\n", json.c_str());
        // world.to_json(): {"results":[{"name":"#540", "id":540, "tags":["flecs.core.Prefab"]}, {"name":"#541", "id":541, "tags":["flecs.core.Prefab"]}, {"name":"#542", "id":542, "tags":["flecs.core.Prefab"]}]}

    return 0;
}

[Terzalo]

A workaround is to set the name to something other than #<id>

// set the name to be "e540" instead of "#540"
e2.set_name( std::format( "e{}", uint32_t(e2.id()) ).c_str() );

[SanderMertens]

Ah, the reason for this is that the entity version isn't stored in the serialized data. Will take a look.