Base

#include <iostream>
#include <memory>
#include <vector>
#include <algorithm>
#include <functional>
#include <stdexcept>

using EntityID_t = unsigned long;

class Entity;

class GameWorld {
public:
    GameWorld() : nextEntityID(0) {};
    EntityID_t advanceEntityID() {return ++nextEntityID;}

    bool add(Entity* pEntity) {
        return false;
    }

    bool contains(Entity* pEntity) {
        findOneByPred([&](const std::shared_ptr<Entity>& collEnt) {
            return collEnt.get()==pEntity;
        });
        return true;
    }

    std::shared_ptr<Entity> findOneByPred(std::function<bool(const std::shared_ptr<Entity>&)> predicate) {
        auto result = std::find(entities.begin(), entities.end(), predicate);
        if(result!=entities.end()) {
            return *result;
        }
        throw std::runtime_error("Not found!");
    }

private:
    EntityID_t nextEntityID;
    std::vector<std::shared_ptr<Entity>> entities;
};

class Entity {
public:
    Entity(GameWorld& pGameWorld) : gameWorld(pGameWorld) {
        id = gameWorld.advanceEntityID();
        gameWorld.add(this);
    }

    EntityID_t getID() {return id;}

private:
    EntityID_t id;
    GameWorld& gameWorld;
};


int main() {

    return 0;
}