Safe reference implementation. C++ part. See https://eliasdaler.github.io/game-object-references for details

safe_reference.cpp

#include <sol.hpp>
#include <string>
#include <iostream>

using EntityId = int;

class Entity {
public:
    explicit Entity(EntityId id) :
        name("John"), id(id)
    {}

    const std::string& getName() const { return name; }
    void setName(const std::string& n) { name = n; }
    EntityId getId() const { return id; }
private:
    std::string name;
    EntityId id;
};

sol::state lua; // globals are bad, but we'll use it for simpler implementation

class EntityManager {
public: 
    EntityManager() : idCounter(0) {}

    Entity& createEntity()
    {
        auto id = idCounter;
        ++idCounter;

        auto inserted = entities.emplace(id, std::make_unique<Entity>(id));
        auto it = inserted.first; // iterator to created id/Entity pair
        auto& e = *it->second; // created entity
        lua["createHandle"](e);
        return e;
    }

    void removeEntity(EntityId id)
    {
        lua["onEntityRemoved"](id);
        entities.erase(id);
    }
private:
    std::unordered_map<EntityId, std::unique_ptr<Entity>> entities;
    EntityId idCounter;
};

int main()
{
    lua.open_libraries();

    lua.new_usertype<Entity>("Entity",
        "getName", &Entity::getName,
        "setName", &Entity::setName,
        "getId", &Entity::getId);

    lua.do_file("safe_reference.lua");

    EntityManager entityManager;
    auto& entity = entityManager.createEntity();

    lua["test"](entity);

    std::cout << "Testing bad reference" << std::endl;
    lua["testBadReference"](entity);
}